Posted: June 19th, 2022
Module 7 – Google Slides
Please see attached.
CHAPTER 23 Urinary System and Fluid Balance
STUDENT LEARNING OBJECTIVES
At the completion of this chapter, you should be able to do the following:
1.Describe the overall anatomy of the urinary system, listing the primary structures.
2.Explain how the components of the urinary system work together to maintain fluid homeostasis.
3.Describe the basic structure of the kidney.
4.List the microscopic structures of the nephron and give their functions.
5.Give an overview description of the following terms: filtration, tubular reabsorption, tubular secretion.
6.Discuss the mechanism of filtration and the formation of urine.
7.Discuss the importance of fluid and electrolyte balance in the body.
8.List the ways in which water enters and leaves the body.
9.Discuss the significance of water and electrolyte regulation in the ICF.
10.Discuss the significance of the regulation of sodium and potassium levels in the body fluids.
11.List the mechanisms that control pH of body fluids.
LANGUAGE OF SCIENCE AND MEDICINE
Before reading the chapter, say each of these terms out loud. This will help you avoid stumbling over them as you read.
acid-base balance (ASS-id bays BAL-ents)
[acid- sour, -base foundation]
[acid- sour, -ity state]
[acid- sour, -osis condition]
aldosterone (AL-doh-steh-rohn or al-DAH-stair-ohn)
[aldo- aldehyde, -stero- solid or steroid derivative, -one chemical]
[alkal- ashes, -ine relating to]
[alkal- ashes, -in- relating to, -ity state]
[alkal- ashes, -osis condition]
[ana- up, -ion go (ion)]
antidiuretic hormone (ADH) (an-tee-dye-yoo-RET-ik HOR-mohn)
[anti- against, -dia- through, -uret- urination, -ic relating to, hormon- excite]
atrial natriuretic hormone (ANH) (AY-tree-al nay-tree-yoo-RET-ik HOR-mohn)
[atrium entrance courtyard (atrium of heart), natri- sodium, -ure- urine, -ic relating to, hormon- excite]
Bowman capsule (BOH-men KAP-sul)
[William Bowman English anatomist]
[calyx cuplike] pl., calyces (KAY-lih-seez)
[cat- down, -ion go (ion)]
collecting duct (CD)
cortical nephron (KOHR-tih-kal NEF-ron)
[cortic- bark (cortex), -al relating to, nephro- kidney, -on unit]
countercurrent mechanism (kown-ter-KER-ent MEK-ah-nih-zem)
[counter- against, -current flow, mechan- machine, -ism state]
[de- remove, -hydro- water, -ation process]
[detrus- thrust, -or agent]
[dis- apart, -socia- unite, -ate action]
distal convoluted tubule (DCT) (DIS-tall KON-vo-LOO-ted TOO-byool)
[dist- distance, -al relating to, con- together, -volut- roll, tub- tube, -ul little]
[electro- electricity, -lyt loosening]
extracellular fluid (ECF) (eks-trah-SELL-yoo-lar FLOO-id)
[extra- outside, -cell- storeroom, -ular relating to, fluid flow]
[filtr- filter, -ate characterized by]
filtration slit (fil-TRAY-shen slit)
[filtr- filter, -at- characterized by, -tion process, slit split]
[glomer- ball, -ulus little] pl., glomeruli (gloh-MAIR-yoo-leye)
Henle loop (HEN-lee)
[Friedrich Gustave Henle German anatomist]
[hilum least bit] pl., hila (HYE-lah)
[hypo- deficient, -chlor- green (chlorine), -emia blood condition]
[hypo- deficient, -kal- potassium, -emia blood condition]
intracellular fluid (ICF) (in-trah-SELL-yoo-lar FLOO-id)
[intra- occurring within, -cell- storeroom, -ular relating to, fluid flow]
juxtaglomerular apparatus (juks-tah-gloh-MER-yoo-lar app-ah-RAT-us)
[juxta- near or adjoining, -glomer- ball, -ul- little, -ar relating to]
juxtamedullary nephron (juks-tah-MED-oo-lair-ee NEF-ron)
[juxta- near or adjoining, -medulla- middle, -ary relating to, nephro- kidney, -on unit]
[mictur- urinate, -tion process]
[nephro- kidney, -on unit]
[non- not, -electro- electricity, -lyt loosening]
[osmos- impulse, -recept- receive, -or agent]
peritubular capillary (pair-ee-TOOB-yoo-lar KAP-ih-lair-ee)
[peri- around, -tub- tube, -ul- little, -ar relating to, capill- hair, -ary relating to]
[abbreviation for potenz power, hydrogen hydrogen]
pitting edema (eh-DEE-mah)
[pod- foot, -cyte cell]
proximal convoluted tubule (PCT) (PROK-sih-mal KON-voh-LOO-ted TOO-byool)
[proxima- near, -al relating to, con- together, -volut- roll, tub- tube, -ul little]
[re- back again, -absorp- swallow, -tion process]
renal clearance (REE-nal)
[ren- kidney, -al relating to]
renal column (REE-nal KOH-lum)
[ren- kidney, -al relating to]
renal corpuscle (REE-nal KOR-pus-ul)
[ren- kidney, -al relating to, corpus- body, -cle little]
renal cortex (REE-nal KOR-teks)
[ren- kidney, -al relating to, cortex bark] pl., cortices (KOR-tih-sees)
renal medulla (REE-nal meh-DUL-ah)
[ren- kidney, -al relating to, medulla middle] pl., medullae or medullas (meh-DUL-ee, meh-DUL-ahz)
renal pelvis (REE-nal PEL-vis)
[ren- kidney, -al relating to, pelvis basin]
renal pyramid (REE-nal PIR-ah-mid)
[ren- kidney, -al relating to]
[ren- kidney, -in substance]
sodium cotransport (SOH-dee-um koh-TRANZ-port)
[sod- soda, -ium chemical ending, co- with, -trans- across, -port carry]
thick ascending limb (TAL) (thik ah-SEND-ing lim)
[a[d]- toward, -scend- climb]
thin ascending limb of Henle (tALH) (thin ah-SEND-ing lim ov HEN-lee)
[a[d]- toward, -scend- climb, Friedrich Gustave Henle German anatomist]
[tri- three, -gon corner]
[ure- urine, -ter agent or channel]
[ure- urine, -thr agent or channel]
vasa recta (VAH-sah REK-tah)
[vasa vessels, recta straight] sing., vas rectum
THE southern sun beat down on Whitney’s head as she and her classmates cleared rubble around what used to be their college’s courtyard before the hurricane. Her shirt was soaked with sweat, and she kept having to wipe her forehead as sweat dripped into her eyes. Whitney knew she should be drinking more water, but the cooler was on the bus—two blocks away. She just didn’t have the energy to face that walk. “We’ll be finished in just another hour,” she thought. As she bent down to pick up a cement block, she suddenly felt dizzy and sat down quickly.
From what you know from your life experiences, what is your best diagnosis for Whitney’s condition? Based on what you think you know, what would you do for Whitney? Try to answer the questions at the end of this chapter and see if you are right on target (or need to review!).
Now that you have read this chapter, see if you can answer these questions about Whitney from the Introductory Story.
1.What is your best diagnosis for Whitney’s condition?
a.Allergic reaction to the sun
d.Too much extracellular water
Whitney’s teacher rushed over, felt her skin and pulse and said, “You haven’t been drinking any water today, have you?”
2.What would be the best fluid to give Whitney now?
d.A sports drink
3.What two hormones will be released to compensate for Whitney’s condition?
a.ADH and aldosterone
b.Estrogen and renin
c.Aldosterone and TSH
d.Renin and angina
To solve these questions, you may have to refer to the glossary or index, other chapters in this textbook, A&P Connect, Mechanisms of Disease, and other resources.
The urinary system not only produces urine, it also balances the composition of our blood plasma. The water content is adjusted, and important ions circulating in the blood (such as sodium and potassium) are maintained at appropriate levels. Even the pH of the blood can be controlled. In these ways, the urinary system regulates the content of blood plasma so that the homeostasis of the entire internal fluid environment is maintained continuously within normal limits.
In this chapter, we will explore the basic principles of urinary structure and function. We will also look at the role of the kidneys and other organs that maintain fluid and ion homeostasis. Finally, we will briefly tour the mechanisms of homeostasis of body fluid and electrolyte levels. You’ll see that fluid balance is critical to our survival. The volume of fluid and the electrolyte concentrations inside our cells, in the interstitial spaces, and in our blood vessels must all remain relatively constant.
Last, we discuss the importance of acid-base balance and pH control of body fluids and the mechanisms that provide for homeostatic control of these processes.
ANATOMY OF THE URINARY SYSTEM
The principal organs of the urinary system are the kidneys, which process blood and form urine as a waste to be excreted—that is, removed from the body (Box 23-1). The excreted urine travels from the kidneys to the outside of the body via accessory organs: the ureters, urinary bladder, and urethra.
Our kidneys really do resemble kidney beans. As you can see from Figure 23-1, they are roughly oval and about the size of a mini-football. Usually the right kidney is slightly smaller and a little lower than the left, because the liver takes up some of the space above the right kidney.
BOX 23-1 FYI
The urinary system’s chief function is to regulate the volume and composition of body fluids and excrete unwanted material. However, it is not the only system in the body that is able to excrete unneeded substances.
The table below compares the excretory functions of several systems. Although all of these systems contribute to the body’s effort to remove wastes, only the urinary system can finely adjust the water and electrolyte balance to the degree required for normal homeostasis of body fluids.
The medial surface of each kidney has a concave notch called the hilum (Figure 23-2, A). Vessels and other structures enter or leave the kidney through this notch.
The kidneys lie in a retroperitoneal position (meaning behind the parietal peritoneum) against the posterior wall of the abdomen (see Figure 23-1, B). Note in Figure 23-1, A, that the superior or upper portions (poles) of both kidneys extend above the level of the twelfth rib and the lower edge of the thoracic parietal pleura. In addition to being partly protected by the ribs, this anatomical relationship has important clinical implications (Box 23-2).
A heavy cushion of fat—the renal fat pad—encases each kidney and helps to hold it in position. Connective tissue, the renal fascia, anchors the kidneys to surrounding structures and also helps maintain their normal position. A tough, white fibrous capsule encases each kidney (see Figure 23-2, A).
The coronal section dividing the right kidney in Figure 23-2, A, into anterior and posterior sections shows you the major
FIGURE 23-1 Location of urinary system organs. A, Anterior view of the urinary organs with the peritoneum and visceral organs removed. B, Horizontal (transverse) section of the abdomen showing the retroperitoneal position of the kidneys. C, Sagittal view of the female and male urinary tract. Each shows a distended (stretched) bladder.
FIGURE 23-2 Internal structure of the kidney. A, Coronal section of the right kidney in an artist’s rendering. B, Circulation of blood through the kidney. Diagram showing the major arteries and veins of the renal circulation.
internal structures of the kidney. Stop for a moment and identify the renal cortex, or outer region, and the renal medulla, or inner region. Note that a dozen or so distinct triangular wedges, the renal pyramids, make up much of the medullary tissue. The base of each pyramid faces outward, and the narrow papilla of each faces toward the hilum. Each renal papilla has multiple openings that release urine. Notice that the cortical tissue dips into the medulla between the pyramids, forming areas known as renal columns.
Each renal papilla (point of a pyramid) juts into a cuplike structure called a calyx. The calyces are considered the beginnings of the “plumbing system” of the urinary system. Urine (leaving the renal papilla) is collected by the calyces for transport out of the kidney. The cups that drain the renal papillae directly are called minor calyces. These minor calyces are stemlike branches that join together to form larger branches called major calyces. The major calyces join together to form a large collection “basin” called the renal pelvis. The pelvis of the kidney narrows as it exits the hilum to become the tubelike ureter, which then transports urine to the bladder.
Blood Vessels of the Kidneys
Use Figure 23-2, B, to help you as you read the following paragraph. First, notice that a large branch of the abdominal aorta—the renal artery—brings blood into each kidney. As it nears the kidney, it divides into segmental arteries, which then divide to become lobar arteries. Between the pyramids of the kidney’s medulla, the lobar arteries branch further to form interlobar arteries. These arteries extend out toward the cortex, and then arch over the bases of the pyramids to form the arcuate arteries. From the arcuate arteries, interlobular arteries penetrate the cortex. (Note that the interlobar and interlobular arteries are different arteries.)
BOX 23-2 Health Matters
Suspected disease of the kidney, such as renal cancer, often requires a needle biopsy to confirm the diagnosis. In these procedures, a hollow needle is inserted through the skin surface and then guided into the diseased organ to withdraw a tissue sample for analysis. In a renal biopsy, tissue is removed from the lower rather than the upper or superior end of the diseased kidney. This avoids the possibility of the biopsy needle damaging the pleura and thereby causing a pneumothorax (air or gas collects in the pleural cavity).
Needle biopsy of kidney. Magnetic resonance image (MRI) of biopsy needle (arrow) inserted into the left kidney to remove tissue. K, Kidney.
Branches of the interlobular arteries called afferent arterioles carry blood directly to the tiny functional units of the kidney called nephrons. We will discuss the structure and function of nephrons later in this chapter.
Knowing the pathway of blood flow in the kidney is important for understanding how the kidneys work. We continue the story later in the chapter. We put all the pieces together in an overview in Tracing Blood Flow in the Kidney online at A&P Connect.
1. Name the basic components of the urinary system.
2. Name two general functions of the urinary system.
3. Distinguish between the renal cortex and the renal medulla.
4. Which branch of the aorta brings blood to the kidney?
The two ureters (about 28 to 34 cm in length) are tubes that transport urine from the kidneys to the urinary bladder (see Figure 23-1). Each ureter begins on the medial side of the kidney, at the narrow outlet of the renal pelvis. The ureters are retroperitoneal (they lie behind or dorsal to the peritoneum) and pass downward until they attach to the bottom of the bladder (see Figure 23-1, C). Each ureter runs at an oblique angle for about 2 cm through the bladder wall and then opens at the outside angles of the trigone (floor) of the bladder (Figure 23-3, A). Because of its oblique passage through the bladder wall, the end of the tube closes and acts as a valve when the bladder is full, thus preventing backflow of urine (Figure 23-3, B).
The ureter is lined with transitional epithelium, which permits significant stretching without damage to the epithelial lining. This feature also permits either high or low rates of flow through the ureters.
In females, the ureters are in close proximity to the ovaries and cervix of the uterus. In males, the ureters are in close proximity to the seminal vesicles and near the prostate gland (see Figure 23-1, C). Each ureter is composed of three layers of tissue: a mucous lining, a muscular middle layer, and a fibrous outer layer (Figure 23-4).
The urinary bladder is a muscular, collapsible bag located directly behind the pubic symphysis and in front of the rectum (see Figure 23-1, C). It lies below the parietal peritoneum, which covers only its superior surface (see Figure 23-3, A). In women, the bladder sits on the anterior surface of the vagina and in front of the uterus, whereas in men, it rests on the prostate.
FIGURE 23-3 Structure of the urinary bladder. A, Frontal view of a dissected urinary bladder (male) in a fully distended state. B, The oblique path of the ureter through wall of the bladder, seen in frontal section, permits the junction to act as a valve—both reducing flow into the bladder as it becomes full and eliminating backflow into the kidney.
FIGURE 23-4 Ureter (cross section). Low-power micrograph. Note the convoluted folds, covered by a transitional mucous lining, that almost fill the lumen. The thick muscular layer is surrounded by a tough fibrous coat.
The wall of the bladder is made mostly of smooth muscle tissue collectively called the detrusor (see Figure 23-3, A). This smooth muscle layer is formed by a crisscrossing network of circular, oblique, and longitudinal bundles. The bladder is lined with mucous transitional epithelium that forms folds called rugae (see Figure 23-3, A). Because of the folds and the extensibility of transitional epithelium, the bladder can distend considerably. There are three openings in the floor of the bladder—two from the incoming ureters and one from the outgoing urethra. The ureter openings lie at the posterior corners of the triangle-shaped floor—the trigone. The urethral opening lies at the anterior, lower corner.
The bladder performs two major functions:
1.It serves as a reservoir for urine before it leaves the body.
2.Aided by the urethra, it expels urine from the body.
The urethra is a small tube lined with mucous membrane. It leads from the floor of the bladder (trigone) to the exterior of the body. In females, the urethra extends downward and forward from the bladder for a distance of about 3 cm (1.2 inches), and ends at the external urinary meatus (see Figure 23-1, C). The female urethral tract is separate from the lower reproductive tract (vagina), which lies just behind the urethra.
The male urethra, on the other hand, extends along a winding path for about 20 cm (7.9 inches) (see Figure 23-1, C). It passes through the center of the prostate gland just after leaving the bladder. Within the prostate, it is joined by two ejaculatory ducts. After leaving the prostate, the urethra first extends downward, then forward, then upward to enter the base of the penis. The urethra then travels through the center of the penis and ends as a urinary meatus at the tip of the penis.
Because the male urethra is joined by the ejaculatory ducts, it also serves as a pathway for semen (fluid containing sperm) as it is ejaculated out of the body through the penis. In reality, then, the male urethra is a part of two different systems: the urinary system (when it is used to void urine) and the reproductive system (when it is used to ejaculate semen). Urine is prevented from mixing with semen during ejaculation by a reflex closure of sphincter muscles guarding the bladder’s opening (see Figure 23-3, A).
The mechanism for micturition (urination) begins with involuntary contractions of the detrusor muscle. As the pressure of urine against the inside of the bladder wall increases with urine volume, involuntary micturition contractions develop. This rapid succession of involuntary contractions gets stronger and stronger as the bladder fills and the urine volume and pressure increase. At the same time, the internal urethral sphincter muscles relax.
The internal urethral sphincters include a ringlike part of the detrusor muscle of the bladder wall, as you can see in Figure 23-3, A. The relaxation of these internal sphincters along with the micturition contractions of the bladder wall can force urine out of the bladder and through the urethra. Note that the skeletal muscles of the pelvic floor, including the levator ani muscle, act as external urethral sphincters.
The urinary tract can be examined using medical imaging techniques. Check out Visualizing the Urinary Tract online at A&P Connect to find out how.
5. Where does the ureter enter the bladder? Why is its angle through the wall significant?
6. What type of mucous epithelium lines most of the urinary tract? What is the functional advantage of this type of epithelium?
7. What are the two major functions of the bladder?
Voluntary control of micturition is not possible until the nervous system matures sufficiently. For this reason, voluntary control of urination is not possible during infancy and very early childhood.
Over a million microscopic functional units called nephrons make up the bulk of each kidney. The shape of the nephron is uniquely suited to its function of blood plasma processing and
FIGURE 23-5 Nephron. The nephron is the basic functional unit of the kidney. Arrows show the direction of flow within the nephron.
urine formation (Figure 23-5). It resembles a tiny funnel with a long, winding stem about 3 cm (1.2 inches) long. Notice that each nephron is made up of two main regions: the renal corpuscle and the renal tubule. Fluid is filtered out of the blood plasma in the renal corpuscle. Then the filtrate (filtered fluid) flows through the renal tubule and collecting duct—where much of the filtrate is returned to the blood. The remaining filtrate leaves the collecting duct as urine. We will come back to the processing of filtrate later. For now, let’s explore the structure of the nephron and collecting duct. Here are the main structures, listed in the order in which fluid flows through them:
▪ Renal corpuscle
▪ Glomerulus (capillaries)
▪ Bowman capsule
▪ Renal tubule
▪ Proximal convoluted tubule
▪ Henle loop
▪ Distal convoluted tubule
As you read the brief description of each of these microscopic structures, refer often to Figure 23-5.
For our purposes, the simplified scheme of microscopic renal anatomy shown in Figure 23-5 works well. However, some renal biologists prefer the more elaborate scheme sketched out in a Detailed Map of Nephron online at A&P Connect.
The renal corpuscle, comprising the Bowman capsule and the glomerulus, is the first part of the nephron. Formation of a renal corpuscle is sometimes compared to pushing your fist into the end of an inflated balloon, as illustrated for you in Figure 23-6, A. Note that as the glomerular tuft of capillaries pushes into the balloon, it becomes surrounded by a double-walled cup with parietal (outer) and visceral (inner) walls—the Bowman capsule (Figure 23-6, B). Fluid from the blood plasma first filters out of the glomerulus and then into the Bowman capsule.
The Bowman capsule (glomerular capsule) is the cup-shaped mouth of a nephron. The capsule is formed by two layers of epithelial cells with a space, called the Bowman space (capsular space), between them. Fluids, waste products, and electrolytes that pass through the porous glomerular capillaries and enter this space constitute the filtrate. These substances will be processed in the nephron to form urine.
The parietal wall is composed of simple squamous epithelium. It plays no role in the production of glomerular filtrate. However, the visceral wall is quite different. It is composed of special epithelial cells called podocytes (meaning “cells with feet”). Look at the scanning electron micrograph in Figure 23-6, C. Notice that the primary branches extending from the cell bodies divide into a network of smaller branches that end in little “feet” called pedicels. The pedicels are packed so closely together that only narrow slits of space lie between them. These spaces are called filtration slits. The slits are not merely open spaces, however. Within them is a mesh of fine connective tissue fibers collectively called the slit diaphragm that prevents the slits from enlarging under pressure and at the same time maintains permeability of the slit. The slit diaphragm is vital to filtration because it prevents many large macromolecules, such as proteins, from passing through.
FIGURE 23-6 Renal corpuscle. A, Model of the anatomical relationship of the glomerulus to the Bowman capsule. B, Structure of the renal corpuscle. C, Scanning electron micrograph (SEM) of the glomerular capillaries. A podocyte cell body (CB) and pedicels (P) are shown on the outer surface of a capillary endothelial cell.
The glomerulus capillary network is vital to our survival. Its relationship to the Bowman capsule is clearly visible in Figure 23-6, A and B. Notice in both figures that an afferent arteriole leads into the glomerular network and an efferent arteriole leads out.
Like all capillaries, glomerular capillaries have thin, membranous walls composed of a single layer of endothelial cells. Many pores, or fenestrations (“windows”), are present in the glomerular capillary endothelium. These pores are not present in regular capillaries. This increased porosity is necessary for filtration to occur at the rate required for normal kidney function.
The renal tubule is a winding, hollow tube extending from the renal corpuscle to the end of the nephron. There it joins a collecting duct shared in common with other nearby nephrons. The renal tubule is divided into different regions: the proximal convoluted tubule, the Henle loop, and the distal convoluted tubule. Follow along in Figure 23-5 as we briefly explore these regions.
Proximal Convoluted Tubule
The proximal convoluted tubule (PCT), or more simply proximal tubule, is the second part of the nephron. It is also the first segment of the renal tubule. As its name suggests, the proximal convoluted tubule is proximal, or nearest, to the Bowman capsule. Because it follows a winding, convoluted course, it is called a convoluted tubule. Its wall consists of one layer of epithelial cells that have a brush border facing the lumen of the tubule. Thousands of microvilli form the brush border and greatly increase its luminal surface area—a structural fact of importance to its function, as we shall see.
The Henle loop, or nephron loop, is the segment of renal tubule just beyond the proximal tubule. It consists of a thin descending limb, a sharp turn, and an ascending limb. Note in Figure 23-5 that the ascending limb has two regions of different wall thickness: the thin ascending limb of Henle (tALH) and the thick ascending limb (TAL). The length of the Henle loop is important in the production of highly concentrated or very dilute urine.
Distal Convoluted Tubule
The distal convoluted tubule (DCT) is a convoluted portion of the tubule beyond (distal to) the Henle loop. The distal convoluted tubule conducts filtrate out of the nephron and into a collecting duct.
The juxtaglomerular apparatus (“structure near the glomerulus”) is found at the point where the afferent arteriole brushes past the distal convoluted tubule (see Figure 23-6, B). This structure is important in maintaining homeostasis of blood flow. This is because the juxtaglomerular complex quickly secretes renin when blood pressure in the afferent
FIGURE 23-7 Blood supply of nephrons. Two types of nephrons (cortical and juxtaglomerular) are shown surrounded by the peritubular blood supply.
arteriole drops. Renin triggers a mechanism that produces angiotensin. This substance causes vasoconstriction and results in an increase in blood pressure (see Figure 23-10, p. 547).
The collecting duct (CD) is formed by the joining of renal tubules of several nephrons. All the collecting ducts of one renal pyramid converge at a renal papilla and release urine through their openings into one of the minor calyces. Note that Bowman capsules and both convoluted tubules lie entirely within the cortex of the kidney, whereas the Henle loops and collecting ducts extend into the medulla (Figure 23-7).
Blood Supply of the Nephron
Earlier in this chapter, we traced renal blood flow from the renal artery and its branches to the afferent arteriole. Blood then flows from the afferent arteriole into the glomerular capillaries and exits through an efferent arteriole (see Figure 23-7). The efferent arteriole then enters another capillary network that runs alongside the renal tubule. The capillaries of this network are called peritubular capillaries. Some of the blood from the efferent arteriole flows through long hairpin-shaped loops that follow the nephron loop. These long, looping arterioles are called vasa recta (singular, vas rectum) or straight arteriole. Blood flows very slowly through the vasa recta, a fact that plays an important role in the function of these vessels.
As you can see in Figure 23-7, blood flows through the efferent arteriole to the peritubular capillaries and vasa recta—the peritubular blood supply. It then flows back toward the heart through interlobular veins and arcuate veins that head toward the large renal veins.
Types of Nephrons
About 85% of all nephrons are located almost entirely in the renal cortex and are called cortical nephrons. The remainder, called juxtamedullary nephrons, are found adjoining (juxta) the medulla. Juxtamedullary nephrons have long Henle loops that dip far into the medulla (see Figure 23-7). The special role of these long Henle loops of juxtamedullary nephrons in concentrating urine is discussed later.
Review the overall plan of renal blood flow in Tracing Blood Flow in the Kidney online at A&P Connect.
8. Name the segments of the nephron in the order in which fluid flows through them.
9. What characteristics of the glomerular capsular membrane permit filtration?
10. What is the name of the blood supply that surrounds the nephron?
11. What is the other name for the straight arterioles that follow the nephron loop?
PHYSIOLOGY OF THE URINARY SYSTEM
Overview of Kidney Function
The chief functions of the kidney are to process blood plasma and excrete urine. These functions are vital because they maintain the homeostatic balance of our bodies. For example, the kidneys are the most important organs in the body for maintaining fluid-electrolyte and acid-base balance. The kidneys do this by varying the amount of water and electrolytes leaving the blood in the urine so that they equal the amounts of these substances entering the blood from various other avenues. Nitrogenous wastes from protein metabolism, notably urea, leave the blood by way of the kidneys.
Here are just a few of the blood constituents that cannot be held within their normal concentration ranges if the kidneys fail:
▪Nitrogenous wastes (especially urea)
The kidneys also perform other important functions. They influence the rate of secretion of the hormones antidiuretic hormone (ADH) and aldosterone. They also synthesize the active form of vitamin D, the hormone erythropoietin, and certain prostaglandins.
As you now know, the basic functional unit of the kidney is the nephron. It has two main parts—the renal corpuscle and renal tubule—that form urine by means of three processes:
1.Filtration—movement of water and protein-free solutes from blood plasma in the glomerulus, across the glomerular capsular membrane, and into the capsular space of the Bowman capsule
2.Tubular reabsorption—movement of molecules out of the various segments of the tubule and into the peritubular blood
3.Tubular secretion—movement of molecules out of peritubular blood and into the tubule for excretion
These three mechanisms are used in concert to process blood plasma and form urine. First, a hydrostatic pressure gradient drives the filtration of much of the plasma into the nephron (Figure 23-8). Because the filtrate contains materials that the body must conserve (save), the walls of the tubules start reabsorbing these materials back into the blood. As the filtrate (urine) begins to leave the nephron, the kidney may secrete a few “last minute” items into the urine for excretion. In short, the kidney does not selectively filter out only harmful or excess material. It first filters out much of the plasma, then reabsorbs what should not be “thrown out” before the filtrate reaches the end of the tubule and becomes urine.
Our first step, filtration, is a physical process that occurs in the kidneys’ 2.5 million renal corpuscles (see Figures 23-5 and 23-6, B). As blood flows through the glomerular capillaries, water and small solutes filter out of the blood into Bowman capsules. The only blood constituents that do not move out are the blood cells and most plasma proteins. The result is about 180 liters of glomerular filtrate being formed each day. This filtration takes place through the glomerular capsular membrane.
FIGURE 23-8 Overview of urine formation. The diagram shows the basic mechanisms of urine formation—filtration, reabsorption, and secretion—and where they occur in the nephron. Details of these steps are revealed later in this chapter.
Filtration from glomeruli into Bowman capsules occurs because a pressure gradient exists between them. (Note: This is not the same as the concentration gradient of diffusion.) The main factor establishing the pressure gradient between the blood in the glomeruli and the filtrate in the Bowman capsule is the hydrostatic pressure of glomerular blood. This pressure gradient causes the filtration to occur from the glomerular blood plasma into Bowman capsules. The intensity of glomerular hydrostatic pressure is influenced by systemic blood pressure and the resistance to blood flow through the glomerular capillaries.
Reabsorption, the second step in urine formation, requires passive and active transport mechanisms from all parts of the renal tubules. However, a major portion of water and electrolytes and (normally) all nutrients are reabsorbed from the proximal convoluted tubules. The rest of the renal tubule reabsorbs comparatively little of the filtrate. Researchers are still investigating the exact mechanisms of reabsorption in the various segments of the nephron. We have summarized only the essential principles of some of the current concepts in the following paragraphs.
Reabsorption in the Proximal Convoluted Tubule
More than two thirds of the filtrate is reabsorbed before it reaches the end of the proximal convoluted tubule. Proximal convoluted tubules are thought to reabsorb sodium ions (Na+) by actively transporting them out of the lumen of the tubule and into peritubular blood.
Through the process of osmosis, water diffuses rapidly from the tubule fluid into peritubular blood, thus making the two fluids isotonic to each other. In short, transport of sodium ions out of the proximal convoluted tubules causes osmosis of water out of the tubules as well.
Proximal convoluted tubules reabsorb nutrients from the tubule fluid, notably glucose and amino acids, into peritubular blood by a special type of active transport mechanism called sodium cotransport. Normally, all the glucose that has filtered out of the glomeruli returns to the blood by this sodium cotransport mechanism. Therefore, glucose is not normally lost in urine.
Urea is a nitrogen-containing waste formed as a result of protein catabolism (see Chapter 22). Actually, toxic ammonia is formed first, but much of it is quickly transformed into the less toxic urea. Urea in the tubule fluid remains in the proximal convoluted tubule as sodium, chloride, and water are reabsorbed into blood. Once these are gone, a tubule fluid high in urea is left. Because the urea concentration in the tubule is then greater than its concentration in peritubular blood, urea passively diffuses into the blood. About half the urea present in the tubule fluid leaves the proximal convoluted tubule this way.
Reabsorption in the proximal convoluted tubules can be summarized in the following manner:
1.Sodium is actively transported out of the tubule fluid and into blood.
2.Glucose and amino acids “hitch a ride” with sodium and passively move out of the tubule fluid by means of the sodium cotransport mechanism.
3.About half the urea present in the tubule fluid passively moves out of the tubule, with half the urea thus left to move on to the Henle loop.
4.The total content of the filtrate has been reduced greatly by the time it is ready to leave the proximal convoluted tubule. Most of the water and solutes have been recovered by the blood. Only a small volume of fluid is left to continue to the next portion of the tubule, the Henle loop.
Reabsorption in the Henle Loop
In juxtamedullary nephrons (those lying low in the cortex, near the medulla), the Henle loop and its vasa recta participate in a very unique process called a countercurrent mechanism. A countercurrent structure is any set of parallel passages in which the contents flow in opposite directions (Figure 23-9). The Henle loop is a countercurrent structure because the contents of the ascending limb travel in a direction opposite to the direction of flow in the descending limb. The vasa recta also have a countercurrent structure because arterial blood flows down into the medulla and venous blood flows up toward the cortex. The kidney’s countercurrent mechanism functions to keep the solute concentration of the medulla extremely high.
The primary functions of the Henle loop are summarized as follows:
▪The Henle loop reabsorbs water from the tubule fluid (and picks up urea from the interstitial fluid) in its descending limb. It reabsorbs sodium and chloride from the tubule fluid in the thick ascending limb.
▪By reabsorbing salt from its thick ascending limb, it makes the tubule fluid dilute (hypotonic).
▪Reabsorption of salt in the thick ascending limb also creates and maintains a high osmotic pressure, or high solute concentration, of the medulla’s interstitial fluid.
12. What are the three basic processes a nephron uses to form urine?
13. How is urea removed from the proximal convoluted tubule?
14. What is a countercurrent mechanism?
15. How does the function of the descending limb of the Henle loop differ from the function of the thick ascending limb?
Reabsorption in the Distal Tubules and Collecting Ducts
The distal convoluted tubule is similar to the proximal convoluted tubule in that it also reabsorbs some sodium by active transport, but in much smaller amounts.
Given no other circumstances, the kidney produces and excretes only very dilute (hypotonic) urine. However, if this indeed happened, it would be catastrophic. Why? Because the body would soon dehydrate. In fact, another regulatory mechanism centered outside the kidney normally prevents excessive loss of water. This mechanism involves antidiuretic hormone (ADH), a hormone released by the neurohypophysis (posterior pituitary). ADH targets cells of the distal tubules and collecting ducts and causes them to become more permeable to water. The solute concentration of the urine excreted depends in large part on the amount of ADH present.
In addition to reabsorption, tubule cells also secrete certain substances. Tubular secretion is the movement of substances out of the blood and into tubular fluid. Recall that the descending limb of the Henle loop removes urea by means of diffusion. The distal tubules and collecting ducts secrete potassium, hydrogen, and ammonium ions. They actively transport potassium ions (K+) or hydrogen ions (H+) out of the blood into tubule fluid in exchange for sodium ions (Na+), which diffuse back into the blood. Potassium secretion increases when the blood aldosterone concentration increases. Aldosterone, a hormone of the adrenal cortex, targets distal tubule and collecting duct cells and causes them to increase the activity of the sodium-potassium pumps that move sodium out of the tubule and potassium into the tubule.
FIGURE 23-9 Concept of countercurrent flow. Countercurrent flow simply refers to flow in opposite directions, as the inset shows. Tubule filtrate in the Henle loop flows in a countercurrent manner, as does blood flowing through vasa recta of the peritubular blood supply.
TABLE 23-1 Summary of Nephron Function
PART OF NEPHRON
Smaller solute particles (ions, glucose, etc.)
Proximal convoluted tubule (PCT)
Active transport: Na+
Cotransport: glucose and amino acids
Diffusion: Cl−, PO4−, urea, other solutes
Descending limb (DLH)
Thin ascending Limb (tALH)
Active transport: Na+
Thick ascending limb (TAL)
Distal convoluted tubule (DCT)
Active transport: Na+
Diffusion: Cl−, other anions
Osmosis: water (only in the presence of ADH)
Active transport: K+, H+, some drugs
Collecting duct (CD)
Active transport: Na+
Osmosis: water (only in the presence of ADH)
Active transport: K+, H+, some drugs
Table 23-1 summarizes the functions of the different parts of the nephron in forming urine.
Hormones that Regulate Urine Volume
ADH has a central role in the regulation of urine volume. Control of the solute concentration of urine translates into control of urine volume. If no water is reabsorbed by the distal tubule and collecting ducts, urine volume is relatively high—and water loss from the body is high. As water is reabsorbed under the influence of ADH, the total volume of urine is reduced by the amount of water removed from the tubules. Thus ADH reduces water loss by the body.
Another hormone that tends to decrease urine volume—and thus conserves water—is aldosterone. It increases distal tubule and collecting duct absorption of sodium. The cooperative roles of ADH and aldosterone in regulating urine volume—and thus regulating fluid balance in the whole body—are explained in more detail in Figure 23-10.
Atrial natriuretic hormone (ANH) also influences water reabsorption in the kidney. ANH is secreted by specialized muscle fibers in the atrial wall of the heart. Its name implies its function: ANH promotes natriuresis (loss of Na+ via urine). ANH indirectly acts as an antagonist of aldosterone, by promoting the secretion of sodium into the kidney tubules rather than sodium reabsorption. ANH also inhibits the secretion of aldosterone and opposes the aldosterone-ADH mechanism to reabsorb less water and therefore produce more urine. In short, ANH inhibits the ADH mechanism—thus inhibiting water conservation by the internal environment and increasing urine volume.
Urine volume also relates to the total amount of solutes other than sodium excreted in urine. Generally, the more solutes, the more urine. Probably the best-known example of this principle occurs in untreated diabetes mellitus. The symptom that often brings a person with undiagnosed diabetes to a physician is the voiding of abnormally large amounts of urine. Excess glucose “spills over” into urine, thereby increasing the solute concentration of urine (and decreasing the solute concentration of plasma), which in turn leads to diuresis, the excessive formation and excretion of urine.
The physical characteristics of normal urine are listed in Table 23-2. Notice that normal and abnormal characteristics are listed.
TABLE 23-2 Characteristics of Urine
Transparent yellow, amber, or straw color
Abnormal colors or cloudiness, which may indicate the presence of blood, bile, bacteria, drugs, food pigments, or a high solute concentration
Mineral ions (for example, Na+, Cl, K+)
Nitrogenous wastes: ammonia, creatinine, urea, uric acid
Suspended solids (sediment)*
Bile, bacteria, blood cells, casts (solid matter)
Acetone odor, which is common in diabetes mellitus
4.6-8.0 (freshly voided urine is generally acidic)
High in alkalosis; low in acidosis
High specific gravity can cause precipitation of solutes and the formation of kidney stones
* Occasional trace amounts.
FIGURE 23-10 Cooperative roles of ADH and aldosterone in regulating urine and plasma volume. The drop in blood pressure that accompanies loss of fluid from the internal environment triggers the hypothalamus to rapidly release ADH from the posterior pituitary gland. ADH increases water reabsorption by the kidney by increasing the water permeability of the distal tubules and collecting ducts. The drop in blood pressure is also detected by each nephron’s juxtaglomerular apparatus, which responds by secreting renin. Renin triggers the formation of angiotensin II, which stimulates the release of aldosterone from the adrenal cortex. Aldosterone then slowly boosts water reabsorption by the kidneys by increasing reabsorption of Na+. Because angiotensin II also stimulates the secretion of ADH, it serves as an additional link between the ADH and aldosterone mechanisms.
Urine is approximately 95% water, in which are dissolved several kinds of substances; the most important are discussed below:
Nitrogenous wastes—(resulting from protein catabolism) such as urea (the most abundant solute in urine), uric acid, ammonia, and creatinine (Box 23-3).
Electrolytes—mainly the following ions: sodium, potassium, ammonium, chloride, bicarbonate, phosphate, and sulfate. The amounts and kinds of minerals vary with diet and other factors.
Toxins—during disease, bacterial poisons leave the body in urine. One reason for “forcing fluids” on patients suffering with infectious diseases is the need to dilute the toxins that might damage the kidney cells if eliminated in a concentrated form.
Pigments—especially urochromes, yellowish bile pigments derived from products of the breakdown of old red blood cells in the liver and elsewhere. Various foods and drugs may contain, or be converted to, pigments that are cleared from plasma by the kidneys and are therefore found in the urine.
BOX 23-3Diagnostic Study
Blood Indicators of Renal Dysfunction
Renal clearance is the volume of plasma from which a substance is removed by the kidneys per minute. Elevated urea levels in blood, as measured in a blood urea nitrogen (BUN) test, were one of the earliest clinical measurements of kidney dysfunction. Elevated BUN levels indicate failure of the kidney to clear urea and, therefore, other substances as well (see BUN online at Mechanisms of Disease: Urinary System and Fluid Balance).
Blood levels of creatinine are also used to test renal function. Creatinine levels in blood seldom change significantly because they are determined by skeletal muscle mass—which seldom changes much. Therefore, an increase in the blood level of plasma creatinine is considered to be a reliable indicator of depressed renal function.
Hormones—high hormone levels sometimes result in significant amounts of hormone in the filtrate (and therefore in urine).
Abnormal constituents—such as blood, glucose, albumin (a plasma protein), casts (chunks of dead cellular material covered by secretions that harden inside the urinary passages and then are washed out in urine), or calculi (small kidney stones).
As we will see in the following sections, what the kidneys do is vitally important to maintaining proper fluid and electrolyte balance in our bodies.
16. Does ADH promote water loss from the internal environment or water conservation by the internal environment?
17. How does aldosterone cause the body to conserve water?
18. What gives urine its characteristic yellowish color?
FLUID AND ELECTROLYTE BALANCE
Several of the physical properties of matter discussed in Chapter 2 explain the mechanisms of fluid and electrolyte balance. The concept of chemical bonding is a good example. The type of chemical bonds between molecules of certain chemical compounds, such as sodium chloride (NaCl), permits breakup, or dissociation, into separate particles (Na+ and Cl−). Recall that such compounds are known as electrolytes. The dissociated particles of an electrolyte are called ions and carry an electrical charge. Organic substances such as glucose, however, have a type of bond that does not permit the compound to break up, or dissociate, in solution. These compounds are called nonelectrolytes.
Many electrolytes and their dissociated ions are of critical importance in fluid balance throughout our bodies. Fluid balance and electrolyte balance are so interdependent that, if one deviates from normal, so does the other.
TOTAL BODY WATER
Normal values for total body water expressed as a percentage of total body weight will vary between 45% and 75%. Differences occur because of age, fat content of the body, and gender. In newborn infants, total body water represents about 75% of body weight. This percentage then decreases rapidly during the first 10 years of life. At adolescence, adult values are reached and gender differences, which account for about a 10% variation in body fluid volumes between the sexes, appear. In young, nonobese adults, males weighing 70 kg (154 pounds) will have on average about 60% of their body weight as water (nearly 40 liters) and females about 50% (Figure 23-11 and Table 23-3).
Adipose, or fat, tissue contains the least amount of water of any tissue (including bone) in the body. Therefore,
FIGURE 23-11 Distribution of total body water. Expressed as percentage of total fluid volume of body.
regardless of age, obese individuals, with their high body fat content, have less body water per kilogram of weight than slender people do. In aged individuals of either sex, body water content may decrease to 45% of total body weight. One reason for this is that old age is often accompanied by a decrease in muscle mass (65% water) and an increase in fat (20% water). In addition, with advancing age the kidneys are less able to produce concentrated urine, and sodium-conserving responses become less effective. This affects our body fluid volume.
TABLE 23-3 Volumes of Body Fluid Compartments*
* Percentage of body weight.
BODY FLUID COMPARTMENTS
On a functional level, we can divide our body water into two major fluid compartments: the extracellular and the intracellular fluid compartments. Extracellular fluid (ECF) consists mainly of the plasma found in the blood vessels and the interstitial fluid that surrounds the cells. In addition, the lymph and so-called transcellular fluid—such as cerebrospinal fluid, joint fluids, and humors of the eye—are also considered extracellular fluid. Intracellular fluid (ICF) refers to the water inside the cells. The distribution of body water by compartment is shown in Figure 23-11.
Extracellular fluid makes up the internal environment of the body. It serves the dual vital functions of providing a relatively constant environment for cells and transporting substances to and from them. Intracellular fluid, on the other hand (because it is a solvent) functions to facilitate intracellular chemical reactions that maintain life.
If we were to compare the relative volumes of these fluids, we’d see that intracellular fluid is the largest (25 liters), plasma the smallest (3 liters), and interstitial fluid in between (12 liters). Table 23-3 lists volumes of the body fluid compartments for infants and both sexes as a percentage of body weight.
ELECTROLYTES IN BODY FLUIDS
As we’ve seen, an electrolyte is a compound that will break up or dissociate into charged particles called ions when placed in solution. Sodium chloride, when dissolved in water, provides a positively charged sodium ion (Na+) and a negatively charged chloride ion (Cl−).
If two electrodes charged with a weak current are placed in an electrolyte solution, the ions will move, or migrate, in opposite directions according to their charge. Positive ions such as Na+ will be attracted to the negative electrode (cathode) and are called cations. Negative ions such as Cl− will migrate to the positive electrode (anode) and are called anions. Various anions and cations serve critical nutrient or regulatory roles in the body. Important cations include sodium (Na+), calcium (Ca++), potassium (K+), and magnesium (Mg++). Important anions include chloride (Cl−), bicarbonate (HCO3−), phosphate (HPO4−), and many proteins.
Extracellular vs. Intracellular Fluids
Compared chemically, the two extracellular fluids (plasma and interstitial fluid) are almost identical. Intracellular fluid, on the other hand, shows striking differences when compared to either of the two extracellular fluids. Let us examine first the chemical structure of plasma and interstitial fluid, as shown in Figure 23-12.
In Figure 23-12 you can see that blood plasma contains a slightly larger total of electrolytes (ions) than interstitial fluid. Take a moment to compare the two fluids, ion for ion, to discover for yourself the most important difference between blood plasma and interstitial fluid. Look at the anions (negative ions) in these two extracellular fluids. Note that blood contains an appreciable amount of protein anions. Interstitial fluid, in contrast, contains hardly any protein anions. This is the only functionally important difference between blood and interstitial fluid. It exists because the normal capillary membrane is practically impermeable to proteins. For this reason, almost all protein anions remain behind in the blood instead of filtering out into the interstitial fluid. Because proteins remain in the blood, certain other differences also exist between blood and interstitial fluid—notably, blood contains more sodium ions and fewer chloride ions than interstitial fluid does.
Extracellular fluids and intracellular fluid are more unlike than alike chemically. Considerable chemical differences exist between the extracellular and intracellular fluids.
FIGURE 23-12 Electrolyte and protein concentrations in body fluid compartments. This illustration compares individual electrolyte and protein concentration in the three fluid compartments.
Study Figure 23-12 and see if you can make some generalizations about the main chemical differences between the extracellular and intracellular fluids. For example: What is the most abundant cation in extracellular fluids? In intracellular fluid? What is the most abundant anion in extracellular fluids? In intracellular fluid? What about the relative concentrations of protein anions in extracellular fluids and intracellular fluid?
HOW WATER ENTERS AND LEAVES THE BODY
Water enters the body, as you undoubtedly know, by way of the digestive tract—in the liquids one drinks and in the foods one eats (Figure 23-13). But, in addition, water is added to the body’s total fluid volume by metabolic activity of its billions of cells. Each cell produces water as it catabolizes foods, and this water enters the bloodstream. Water normally leaves the body in four basic ways: kidneys (urine), lungs (water in expired air), skin (by diffusion and by sweat), and intestines (feces). In accord with the cardinal principle of fluid balance, the total volume of water entering the body normally equals the total volume leaving. In short, fluid intake normally equals fluid output. Figure 23-13 illustrates the portals of water entry and exit, and their normal volumes. These volumes, however, can vary considerably and still be considered normal.
19. List three important cations and anions that serve critical nutrient or regulatory roles in the body.
20. Name the most abundant chemical constituent in blood plasma, interstitial fluid, and intracellular fluid.
21. Explain why extracellular fluids and intracellular fluids are not alike chemically.
22. List the major “portals” of water entry and exit from the body.
GENERAL PRINCIPLES OF FLUID BALANCE
Obviously, if more or less water leaves the body than enters it, an imbalance will result. Total fluid volume will increase or decrease but cannot remain constant under these conditions.
Mechanisms for varying output so that it equals intake are the most important means of maintaining fluid balance. However, mechanisms for adjusting intake to output also operate. Figure 23-14 summarizes for you the role that the renin-angiotensin-aldosterone system (RAAS) has in decreasing
FIGURE 23-13 Sources of fluid intake and output.
FIGURE 23-14 Role of aldosterone in ECF homeostasis. Aldosterone tends to restore normal extracellular fluid (ECF) volume when it decreases below normal. Excess aldosterone, however, leads to excess ECF volume—that is, excess blood volume (hypervolemia) and excess interstitial fluid volume (edema)—as well as an excess of the total Na+ content of the body.
fluid output (urine volume) in order to compensate for decreased water intake.
Mechanisms for controlling water movement between the fluid compartments of the body are the most rapidacting fluid balance processes. They serve first of all to maintain normal blood volume at the expense of interstitial fluid volume.
HOMEOSTASIS OF TOTAL FLUID VOLUME
Under normal conditions, homeostasis of the total volume of water in the body is maintained or restored primarily by mechanisms that adjust output (urine volume) to intake and secondarily by mechanisms that adjust fluid intake.
Regulation of Fluid Intake
A detailed explanation of the mechanism for controlling fluid intake and output is still beyond our understanding. However, research has shown that nerve cells located in the roof of the third ventricle of the brain act as critical regulators of fluid homeostasis. Cells in the hypothalamus are involved in antidiuretic hormone (ADH) production, which is important in conservation of body water when fluid intake is restricted.
Physiologists now identify these ventricles and hypothalamic cells as important osmoreceptors, which together make up the functional thirst center of the brain. Osmoreceptors are cells able to detect an increase in solute concentration in extracellular fluid caused by water loss. Signals generated by osmoreceptors in the ventricular roof and hypothalamus stimulate ADH secretion. They also affect a number of other body functions, including a decrease in the secretion of saliva.
Signals from the nuclei are sent to the cerebrum, where they trigger a conscious sense of dry mouth and thirst. This in turn initiates complex behaviors and thought processes. In most individuals these behaviors include a perceived need to increase the consumption of water in particular. Have you ever heard someone say, “Soda pop tastes good but nothing satisfies my thirst like a glass of water”? The end result is an overall increase in fluid intake to offset increased loss, regardless of cause, and this tends to restore fluid balance (Figure 23-15). If, however, an individual takes nothing by mouth for several days, fluid balance cannot be maintained despite every effort of homeostatic mechanisms to compensate for the zero intake. Obviously, under this
FIGURE 23-15 Homeostasis of the total volume of body water. A basic mechanism for adjusting intake to compensate for excess output of body fluid is diagrammed.
condition, the only way balance could be maintained would be for fluid output to also decrease to zero. But this cannot occur. Some output is obligatory. Why? Because as long as respirations continue, some water leaves the body by way of expired air. Also, as long as life continues, an irreducible minimum of water diffuses through the skin.
Regulation of Urine Volume
Urine volume is determined by the glomerular filtration rate and the rate of water reabsorption by the renal tubules. The glomerular filtration rate, except under abnormal conditions, remains fairly constant—hence it does not normally cause urine volume to fluctuate. The rate of tubular reabsorption of water, on the other hand, fluctuates considerably. The rate of tubular reabsorption, therefore, rather than the glomerular filtration rate, normally adjusts urine volume to fluid intake. The amount of antidiuretic hormone (ADH) and aldosterone secreted regulates the amount of water reabsorbed by the kidney tubules (see Figure 23-14). In other words, urine volume is regulated chiefly by hormones released by the posterior pituitary (ADH) and secreted by the adrenal cortex (aldosterone), and by atrial natriuretic hormone (ANH).
Although changes in the volume of fluid loss via the skin, the lungs, and the intestines also affect the fluid intake-output ratio, these volumes are not automatically adjusted to intake volume, as is the volume of urine.
Factors that Alter Fluid Loss Under Abnormal Conditions
The rate of respiration and the volume of sweat secreted may greatly alter fluid output under certain abnormal conditions. For example, a patient who hyperventilates for an extended time loses an excessive amount of water via the expired air. If, as frequently happens, the individual also takes in less water by mouth than normal, the fluid output then exceeds intake and a fluid imbalance develops, namely, dehydration (that is, a decrease in total body water). The severity of dehydration can be measured by weight loss as a percentage of the normal (hydrated) body weight. Symptoms range from simple thirst to muscle weakness and kidney failure (Figure 23-16).
Clinically, dehydration is often detected by loss of skin elasticity or turgor. If a fold of skin, when pinched, returns to its original shape slowly—a condition called “tenting”—dehydration is suspected. Other abnormal conditions such as vomiting, diarrhea, or intestinal drainage also cause fluid and electrolyte output to exceed intake and thus produce fluid and electrolyte imbalances.
Edema is a classic example of fluid imbalance. It is defined as the presence of abnormally large amounts of fluid in the intercellular tissue spaces of the body. Edema may occur in any organ or tissue of the body. However, the lungs, brain, and dependent body areas such as the legs and lower part of
FIGURE 23-16 The effects of dehydration.
the back are affected most often. One of the most common areas for swelling to occur is in the subcutaneous tissues of the ankle and foot. The term pitting edema is used to describe depressions in swollen subcutaneous tissue in this area that do not rapidly refill after an examiner has exerted finger pressure (Figure 23-17). The condition may be caused by disturbances in any of the factors that govern the interchange between blood plasma and the interstitial fluid compartments.
FIGURE 23-17 Pitting edema. Note the finger-shaped depressions that do not rapidly refill after an examiner has exerted pressure.
23. Identify the two substances that are most important in regulating the amount of water reabsorbed by the kidney tubules.
24. Name the two most important factors that alter fluid loss under abnormal conditions.
25. Define the term edema.
REGULATION OF WATER AND ELECTROLYTE LEVELS IN ICF
The plasma membrane plays a critical role in the regulation of intracellular fluid composition. The mechanism that regulates water movement through cell membranes is similar to the one that regulates water movement through capillary membranes. As Figure 23-12 shows, most of the body sodium is outside the cells, and sodium is by far the chief electrolyte in interstitial fluid. In contrast, the main electrolyte of intracellular fluid is potassium. Therefore, a change in the sodium or the potassium concentration of either of these fluids causes the exchange of fluid between them to become unbalanced.
Any change in the solute concentration of extracellular fluid will have a direct effect on water movement across the cell membrane in one direction or another. If for any reason dehydration occurs, the concentration of solutes in the extracellular fluid will increase, and osmosis will cause water to move from the intracellular space into the extracellular space. In severe dehydration, the increasing concentration of intracellular fluid caused by water loss to the extracellular space results in abnormal metabolism or even cell death. Increased movement of water into the cell is caused by a decreased concentration of solutes in the extracellular fluids. Thus, fluid balance depends on electrolyte balance. Conversely, electrolyte balance depends on fluid balance. An imbalance in one produces an imbalance in the other.
REGULATION OF SODIUM AND POTASSIUM LEVELS IN BODY FLUIDS
A normal sodium concentration in interstitial fluid and potassium concentration in intracellular fluid depend on many factors, especially on the amount of ADH and aldosterone secreted. As shown in Figure 23-18, ADH regulates extracellular fluid electrolyte concentration and (colloid) osmotic pressure by regulating the amount of water reabsorbed into blood by renal tubules. Aldosterone, on the other hand, regulates extracellular fluid volume by regulating the amount of sodium reabsorbed into blood by renal tubules (see Figure 23-14).
If for any reason body sodium must be conserved, the normal kidney is capable of excreting essentially sodium-free urine. For this reason, the kidney is the chief regulator of sodium levels in body fluids. Sodium lost from sweat can
FIGURE 23-18 Antidiuretic hormone (ADH) mechanism for ECF homeostasis. The ADH mechanism helps maintain homeostasis of extracellular fluid (ECF) colloid osmotic pressure by regulating its volume and thereby its electrolyte concentration, that is, mainly ECF Na+ concentration. ICF, Intracellular fluid.
become significant when we experience elevated environmental temperatures or fever. However, the thirst that results may lead to replacement of water but not the lost sodium. In fact, because of the increased fluid intake, the remaining sodium pool may be diluted even more. Sodium loss in sweat is therefore not considered to be a normal means of regulation.
In addition to the well-regulated movement of sodium into and out of the body and between the three primary fluid compartments, there is a continuous movement or circulation of this important electrolyte between a number of internal secretions. More than 8 liters of various internal secretions such as saliva, gastric and intestinal secretions, bile, and pancreatic fluid are produced every day (Figure 23-19). You can see why the precise regulatory and conservation mechanisms for sodium are required for survival.
Chloride is the most important extracellular anion and is almost always linked to sodium. Generally ingested together, they provide in large part for the isotonicity of extracellular fluid. Chloride ions are usually excreted in the urine as a potassium salt, and therefore chloride deficiency—hypochloremia—is often found in cases of potassium loss. The body may lose a third to a half of its intracellular potassium reserves before the loss is reflected in lowered plasma potassium levels.
FIGURE 23-19 Sodium-containing internal secretions. Average volumes are shown. Depending on circumstances, the actual total volume of these secretions may reach 8,000 or more milliliters in a 24-hour period.
Potassium deficit, or hypokalemia, occurs whenever there is cell breakdown, as in starvation, burns, trauma, or dehydration. As individual cells disintegrate, potassium enters the extracellular fluid and is rapidly excreted because it is not reabsorbed efficiently by the kidney.
Acid-base balance is one of the most important of the body’s homeostatic mechanisms. The term refers to regulation of hydrogen ion concentration in the body fluids, and the study of acid-base physiology is, in a very real sense, the study of the hydrogen ion (H+). Many of the body’s most biologically important molecules contain chemical groups that can either “donate” or “accept” a hydrogen ion (H+) and thus behave as a weak acid or base (for a review of acid, base, and buffer, see Chapter 2). As a molecule’s pH changes, so does its shape—and thus also its biological activity. The functional ability of ion channels, membrane receptors, and a variety of enzymes and other important body proteins that influence protein conformation and structure, such as hemoglobin and chaperonins, closely depends on the maintenance of precise regulation of hydrogen ion concentration.
Thus, even slight deviations from the normal pH range in body cells and fluids result in pronounced, systemic, and potentially fatal changes in metabolic activity. For example, activity of the Na+-K+ pump, arguably the most important active transport mechanism in the cell membrane, falls by 50% when pH decreases by approximately 1 pH unit. An even more dramatic effect is seen in the activity of a key enzyme involved in the breakdown of glucose in the absence of O2 during anaerobic catabolism (glycolysis). The biological activity of this key enzyme falls by approximately 90% when the pH decreases by only 0.1 unit! Maintaining acid-base balance within narrow and precise ranges is necessary for survival.
MECHANISMS THAT CONTROL pH OF BODY FLUIDS
Recall from Chapter 2 that water and all water solutions contain hydrogen ions (H+) and hydroxide ions (OH−). pH is a symbol used to represent the negative logarithm (exponent of 10) of the number of hydrogen ions (H+) present in 1 liter of a solution. It is expressed as a number between 0 and 14. In Figure 23-20, the pH value is shown on the right side of the scale and the corresponding logarithmic value is on the left.
Take a moment to review the pH unit. pH indicates the degree of acidity or alkalinity of a solution. As the concentration of hydrogen ions increases, the pH goes down and the solution becomes more acid; a decrease in hydrogen ion concentration makes the solution more alkaline and the pH goes up. A pH of 7 indicates neutrality (equal amounts of H+ and OH−), a pH of less than 7 indicates acidity (more H+ than OH−), and a pH greater than 7 indicates alkalinity (more OH− than H+).
With a pH of about 1.5, gastric juice is the most acid substance in the body. With a pH of 7.0, intracellular fluid is essentially neutral.
Arterial and venous blood is slightly alkaline because both have a pH slightly higher than 7.0. The slight increase in acidity of venous blood (pH 7.36) compared with arterial blood (pH 7.40) results primarily from carbon dioxide entering venous blood as a waste product of cellular metabolism. Although any pH value above 7.0 is considered chemically basic, in clinical medicine the term acidosis is used to describe an arterial blood pH of less than 7.35 and alkalosis is used to describe an arterial blood pH greater than 7.45.
Buffers in the blood are chemicals that automatically neutralize small amounts of acid or base that enter the bloodstream. Although buffers act quickly, they are soon used up—so the body needs additional mechanisms to maintain stable blood pH.
The urinary system can alter its secretion of acids and bases into the urine. This results in a net gain or loss of these substances by the blood—thus correcting the blood’s pH.
The lungs remove the equivalent of more than 30 liters of carbonic acid each day from the venous blood by elimination of carbon dioxide, and yet 1 liter of venous blood contains only about 1/100,000,000 gram more hydrogen ions than does 1 liter of arterial blood. What incredible constancy! The pH homeostatic mechanism does indeed control effectively.
FIGURE 23-20 The pH range.
Cycle of LIFE
The kidney plays a critical role in homeostasis by regulating the levels of many substances in the blood. Primary renal functions include filtration, reabsorption, and secretion. All are interrelated by homeostatic control systems involving central nervous system activity and hormonal secretions. More than 1 million nephron units in each kidney serve as the structural framework permitting normal function to occur.
Normally, life cycle changes in kidney structure and function occur only within rather narrow limits. Significant structural changes, such as dramatic decreases in the number of nephron units, almost always indicate serious disease or result from trauma such as crush injuries. Functionally, the kidney is able to operate normally throughout life under a wide array of conditions. If, however, the kidneys cannot cope with extreme conditions, such as water deprivation or disease, death will occur from the buildup of toxins in the blood.
Initially, kidney function in a newborn is less efficient than in an older child or adult. As a result, the urine is less concentrated because the regulatory mechanisms required to retain water are not fully operative. Urinary incontinence, or an inability to control urination, is normal in very young children. Reflex emptying occurs when the bladder fills, but normal sphincter activity keeps urine in the bladder until filling occurs. In contrast, many older adults have problems with incontinence because of loss of sphincter tone, or control.
Renal clearance is the ability of the kidneys to clear, or cleanse, the blood of a certain substance in a given unit of time, generally 1 minute. This value for certain substances tends to decrease with advanced age, thus indicating deterioration of kidney function. Changes in the porosity of the filtration membrane also occur in the elderly. Loss of functional nephron units is yet another consequence of aging. It contributes to the gradual decline in renal function in this age-group.
Although the amount of total water in the body does not vary much from day to day, the proportions of water to fat and dry solids in the body change considerably over our life spans. We start with over two thirds of our body mass as water and progress to about half of our body mass as water in adulthood. Because muscles and other metabolically active cells are high in water content, active adults have more water content than nonactive adults. Often we become less active during late adulthood. This also contributes to less water in our bodies as we age. Advanced age can also bring on some of the kidney problems mentioned earlier.
MECHANISMS OF DISEASE
Many people are afflicted by disorders and diseases of the urinary system, sometimes chronically, throughout their lives. These range from the discomfort and pain of common bladder infections to much more serious chronic and acute diseases.
Renal hypertension, for example is often caused by atherosclerotic plaque, which interferes with blood flow to the kidneys. There are also a number of renal obstructive disorders that interfere with normal urine flow at different locations within the urinary tract. Renal calculi, or kidney stones, are a common and very painful condition (sometimes likened by mothers to the closest pain to that of childbirth men will ever experience!). Neurogenic disorders and overactive bladders can also be chronic and embarrassing conditions. In addition to these, urinary tract infections (UTIs) such as urethritis, cystitis, and nephritis are caused by microorganisms that infect the urethra, bladder, ureter, and/or kidneys. Finally, there are a number of glomerular disorders that can cause progressive kidney damage, and of course, there are a number of cancers that afflict the kidneys and bladder. Kidney failure can result from a number of these disorders and diseases.
Find out more about these diseases and disorders of the urinary system online at Mechanisms of Disease: Urinary System and Fluid Balance.
The BIG Picture
As our study of the urinary system and fluid balance has shown us, homeostasis of water and electrolytes in body fluids depends largely on proper functioning of the kidneys. Each nephron within the kidney processes blood plasma in a way that adjusts its content to maintain a dynamic constancy of the internal environment of the body. Without renal processing, blood plasma characteristics would soon move out of their set point range. On the other hand, without the blood pressure generated by cardiovascular mechanisms, the kidney could not filter blood plasma and therefore could not process blood plasma. Thus the urinary system and the cardiovascular system are interdependent.
Regulation of urinary function and fluid balance, we have seen, is often centered outside the kidney—mainly in the form of endocrine hormone action. Urinary function is also regulated to some extent by nerve reflexes. Thus both the endocrine system and the nervous system must operate properly to ensure efficient kidney function. The urinary system also interacts with many other body systems and tissues. For example, the kidneys clear the blood plasma of nitrogenous wastes and excess metabolic acids produced by the chemical activity of nearly every cell in the body. The kidneys also can clear some toxins and other compounds that enter the blood via the digestive tract, skin, or respiratory tract.
Each of the more than 100 trillion cells of our bodies must be bathed in a precisely controlled and homeostatically regulated fluid medium. In fact, fluid volumes, buffers, electrolyte levels, nutrients, circulating wastes, and protein concentrations must exist within very narrow ranges of their set point values in order for us to function normally. Recall from previous chapters that many ions must exist in precise balances within various fluid compartments of our bodies for normal operation of many vital functions. For example, proper calcium ion balance is required in bone formation or reabsorption, contraction of all three muscle types, synaptic transmission, some types of endocrine signal transduction, and other vital functions.
Also recall that acids and bases are ions, and thus pH homeostasis is related to ion homeostasis. For this reason, homeostasis of fluid and electrolyte levels, and the maintenance of proper pH throughout our bodies, is vital to our existence.
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A. The urinary system not only produces urine, it also balances the composition of our blood plasma
B. The urinary system regulates the content of blood plasma so that the homeostasis, or “dynamic constancy,” of the entire internal fluid environment can be maintained within normal limits
ANATOMY OF THE URINARY SYSTEM
A. Gross structure—principal organs of the urinary system are the kidneys
a. Kidneys resemble kidney beans; roughly oval, about the size of a mini-football (Figure 23-1)
b. Right kidney is often slightly smaller and lower than the left
c. Kidneys lie in a retroperitoneal position
d. Lie on either side of the vertebral column between T12 and L3
e. Renal fat pad—encases and cushions each kidney and helps to hold it in position
f. Hilum—concave notch on medial surface where vessels and tubes enter kidney
g. Internal structures
(1) Renal cortex—outer region
(2) Renal medulla—inner region
(3) Renal pyramids—make up much of the medullary tissue
(4) Renal columns—cortical tissue between the pyramids
(5) Calyx—cuplike structure that marks the beginning of the “plumbing system”
(6) Renal pelvis—region where the calyces join together; large collection “basin”
2. Blood vessels of the kidneys—highly vascular organs (Figure 23-2, B)
a. Renal artery—brings blood into each kidney
b. Segmental arteries—division of renal arteries; divide to become lobar arteries
c. Interlobar arteries—branches of the lobar arteries that extend out toward the cortex
d. Arcuate arteries—formed from the interlobar arteries
e. Interlobular arteries—branches of the arcuate arteries; penetrate the cortex
f. Afferent arterioles—carry blood directly to the nephrons
3. Ureter—tube running from each kidney to the urinary bladder (Figure 23-1)
a. Lined with transitional epithelium; permits significant stretching
b. Composed of three layers of tissue (Figure 23-4):
(1) Mucous lining
(2) Muscular middle layer
(3) Fibrous outer layer
4. Urinary bladder—muscular, collapsible bag located directly behind the pubic symphysis and in front of the rectum (Figures 23-1, C, and 23-3)
a. Lies below the parietal peritoneum
b. Wall of the bladder is made mostly of smooth muscle tissue
c. Three openings in the floor of the bladder:
(1) Two from the incoming ureters
(2) One from the outgoing urethra
d. Bladder performs two major functions:
(1) It serves as a reservoir for urine before it leaves the body
(2) Aided by the urethra, it expels urine from the body
5. Urethra—small tube lined with mucous membrane; leads from the floor of the bladder to the exterior of the body
a. In females it extends downward and forward from the bladder for a distance of about 3 cm
b. Male urethra extends along a winding path for about 20 cm; passes through the center of the prostate gland just after leaving the bladder; extends downward, then forward, then upward to enter the base of the penis and ends as a urinary meatus at the tip of the penis
(1) Also serves as a pathway for semen
6. Micturition (urination)
a. As bladder volume increases, micturition contractions (of detrusor muscle) increase and the internal urethral sphincter relaxes
b. Relaxation of the internal sphincters along with the micturition contractions of the bladder wall can force urine out of the bladder and through the urethra
B. Microscopic structure
1. Nephron—microscopic functional unit; nephrons make up bulk of each kidney; each nephron is made up of two regions and connects to a shared collecting duct (Figure 23-5)
a. Renal corpuscle—first part of the nephron; made up of the Bowman capsule and the glomerulus (Figure 23-6, A)
b. Bowman capsule—cup-shaped mouth of a nephron
(1) Formed by two layers of epithelial cells with a capsular space (Bowman space)
(2) Pedicels in the visceral layer are packed closely together to form filtration slits; a slit diaphragm prevents filtration slits from enlarging under pressure
c. Glomerulus—network of fine capillaries surrounded by Bowman capsule (Figure 23-6, A and B)
(1) Glomerular capillaries have thin, membranous walls composed of a single layer of endothelial cells; fenestrations (pores) are present in the glomerular endothelium
d. Renal tubule—winding, hollow tube extending from the renal corpuscle to the end of the nephron; divided into different regions (Figure 23-5)
(1) Proximal convoluted tubule
(2) Henle loop
(3) Distal convoluted tubule
e. Proximal convoluted tubule (proximal tubule)—first part of the renal tubule nearest the Bowman capsule; follows a winding, convoluted course
f. Henle loop—segment of renal tubule just beyond the proximal tubule; two regions (Figure 23-5):
(1) Descending limb
(2) Ascending limb
g. Distal convoluted tubule—convoluted portion of the tubule beyond (distal to) the Henle loop; conducts filtrate out of the nephron and into a collecting duct
(1) Juxtaglomerular apparatus—found at the point where the afferent arteriole brushes past the distal convoluted tubule; important in maintaining homeostasis of blood flow (Figure 23-6, B)
h. Collecting duct—formed by the joining of renal tubules of several nephrons
(1) Release urine through their openings into one of the minor calyces
i. Blood supply of nephron (Figure 23-7)
(1) Afferent arteriole enters glomerular capillary network
(2) Efferent arteriole leaves glomerulus and extends to the peritubular blood supply
j. Types of nephrons
(1) Cortical nephrons—located almost entirely in the renal cortex
(2) Juxtamedullary nephrons—adjoin the medulla
PHYSIOLOGY OF THE URINARY SYSTEM
A. Overview of kidney function—chief functions of the kidneys is to process blood plasma and excrete urine; maintain homeostasis
1. Kidneys are the most important organs in the body for maintaining fluid-electrolyte and acid-base balance
2. Nitrogenous wastes from protein metabolism leave the blood by way of the kidneys
3. Kidneys influence the rate of secretion of the hormones antidiuretic hormone (ADH) and aldosterone
4. Kidneys synthesize the active form of vitamin D, the hormone erythropoietin, and certain prostaglandins
5. Basic functional unit of the kidney is the nephron; forms urine by means of three processes (Table 23-1):
a. Filtration—movement of water and protein-free solutes from blood plasma in the glomerulus into the capsular space of the Bowman capsule
b. Tubular absorption—movement of molecules out of the various segments of the tubule and into the peritubular blood
c. Tubular secretion—movement of molecules out of peritubular blood into the tubule for excretion
B. Filtration—first step in blood processing; occurs in renal corpuscles
1. Blood flows through the glomerular capillaries; water and small solutes filter out of the blood into Bowman capsules
C. Mechanism of filtration
1. Filtration from glomeruli into Bowman capsules occurs because a pressure gradient exists between them
2. Intensity of glomerular hydrostatic pressure is influenced by systemic blood pressure and the resistance to blood flow through the glomerular capillaries
D. Tubular reabsorption—second step in urine formation; requires passive and active transport mechanisms from all parts of the renal tubules
1. Major portion of water and electrolytes and (normally) all nutrients are reabsorbed from the proximal convoluted tubules
E. Reabsorption in the proximal convoluted tubule
1. Sodium is actively transported out of the tubule fluid and into blood
2. Glucose and amino acids “hitch a ride” with sodium and passively move out of the tubule fluid by means of the sodium cotransport mechanism
3. About half the urea present in the tubule fluid passively moves out of the tubule, with half the urea thus left to move on to the Henle loop
4. The total content of the filtrate has been reduced greatly by the time it is ready to leave the proximal convoluted tubule
F. Reabsorption in the Henle loop
1. Two countercurrent mechanisms
a. Henle loop is a countercurrent structure because the contents of the ascending limb travel in a direction opposite to the flow of urine in the descending limb
b. Vasa recta also have a countercurrent structure because arterial blood flows down into the medulla and venous blood flows up toward the cortex
2. The Henle loop reabsorbs water from the tubule fluid (and picks up urea from the interstitial fluid) in its descending limb; it reabsorbs sodium and chloride from the tubule fluid in the ascending limb
a. By reabsorbing salt from its ascending limb, it makes the tubule fluid dilute (hypotonic)
b. Reabsorption of salt in the ascending limb also creates and maintains a high osmotic pressure, or high solute concentration, of the medulla’s interstitial fluid
G. Reabsorption in the distal tubules and collecting ducts
1. Distal convoluted tubule is similar to the proximal convoluted tubule; it also reabsorbs some sodium by active transport, but in smaller amounts
2. ADH is released by the posterior pituitary and targets the cells of the distal tubules and collecting ducts to make them more permeable to water
H. Tubular secretion—movement of substances out of the blood and into tubular fluid
1. Descending limb of the Henle loop removes urea by means of diffusion
2. Distal tubules and collecting ducts secrete potassium, hydrogen, and ammonium ions
3. Aldosterone—hormone that targets the cells of the distal tubule and collecting duct cells; causes increased activity of the sodium-potassium pump
I. Regulation of urine volume
1. ADH has a central role in the regulation of urine volume; reduces water loss by the body
2. Aldosterone increases distal tubule and collecting duct absorption of sodium, thus promoting the reabsorption of water from the tubule
3. Atrial natriuretic hormone (ANH)—influences water reabsorption in the kidney
a. ANH indirectly acts as an antagonist of aldosterone; promotes the secretion of sodium into the kidney tubules rather than sodium reabsorption
4. Urine volume also relates to the total amount of solutes other than sodium excreted in urine
J. Urine composition (Table 23-2)
1. 95% water
2. Nitrogenous wastes
7. Abnormal constituents
FLUID AND ELECTROLYTE BALANCE
A. Electrolytes—have chemical bonds that allow dissociation into ions, which carry an electrical charge; of critical importance in fluid balance
B. Fluid balance and electrolyte balance are interdependent
TOTAL BODY WATER
A. Normal values for total body water will vary between 45% and 75% of total body weight
B. Differences occur because of age, fat content of the body, and gender
BODY FLUID COMPARTMENTS
A. Two major fluid compartments (Figure 23-11)
B. Extracellular fluid (ECF)—makes up the internal environment of the body
1. Consists mainly of the plasma found in the blood vessels and the interstitial fluid that surrounds the cells; also lymph, cerebrospinal fluid, joint fluids, and humors of the eye
2. ECF serves the dual vital functions of providing a relatively constant environment for cells and transporting substances to and from them
C. Intracellular fluid (ICF)—water inside the cells
1. Functions to facilitate intracellular chemical reactions that maintain life
2. By volume, ICF is the largest body fluid compartment
ELECTROLYTES IN BODY FLUIDS
A. Extracellular vs. intracellular fluids
1. Two extracellular fluids (plasma and interstitial fluid) are almost identical; intracellular fluid shows striking differences
2. Extracellular fluids
a. Difference between blood and interstitial fluid—blood contains a slightly larger total of ions than interstitial fluid does
b. Functionally important difference between blood and interstitial fluid is the number of protein anions
3. Extracellular fluids and intracellular fluid are more unlike than alike chemically
4. Chemical structure of the three fluids helps control water and electrolyte movement between them
HOW WATER ENTERS AND LEAVES THE BODY
A. Water enters the body by way of the digestive tract (Figure 23-13)
B. Water is added to the body’s total fluid volume by metabolic activity of its billions of cells
C. Water normally leaves the body in four basic ways:
1. As urine through the kidney
2. As water in expired air through the lungs
3. As sweat through its skin
4. In feces from the intestine
GENERAL PRINCIPLES OF FLUID BALANCE
A. Mechanisms for varying output so that it equals intake are the most important means of maintaining fluid balance; e.g. renin-angiotensin-aldosterone system (RAAS) (Figure 23-14)
B. Mechanisms for controlling water movement between the fluid compartments of the body are the most rapid-acting fluid balance processes
HOMEOSTASIS OF TOTAL FLUID VOLUME
A. Under normal conditions, homeostasis of the total volume of water in the body is maintained or restored primarily by adjusting urine volume and secondarily by fluid intake
B. Regulation of fluid intake—decrease in fluid intake causes osmoreceptors in “thirst center” to increase ADH secretion
C. Regulation of urine volume—determined by the glomerular filtration rate and the rate of water reabsorption by the renal tubules (Figure 23-14)
D. Factors that alter fluid loss under normal conditions—rate of respiration and the volume of sweat secreted may greatly alter fluid output
1. Vomiting, diarrhea, or intestinal drainage also produce fluid and electrolyte imbalances
REGULATION OF WATER AND ELECTROLYTE LEVELS IN ICF
A. Plasma membrane plays a critical role in the regulation of intracellular fluid composition
B. Mechanism that regulates water movement through cell membranes is similar to the one that regulates water movement through capillary membranes
C. Change in the sodium or the potassium concentration of either the interstitial or intracellular fluids causes the exchange of fluid between them to become unbalanced
D. Fluid balance depends on electrolyte balance
REGULATION OF SODIUM AND POTASSIUM LEVELS IN BODY FLUIDS
A. Normal sodium concentration in interstitial fluid and potassium concentration in intracellular fluid depend on many factors, especially on the amount of ADH and aldosterone secreted
1. ADH regulates extracellular fluid electrolyte concentration and (colloid) osmotic pressure by regulating the amount of water reabsorbed into blood by renal tubules (Figure 23-18)
2. Aldosterone regulates extracellular fluid volume by regulating the amount of sodium reabsorbed into blood by renal tubules (Figure 23-14)
B. When conservation of body sodium is required, the kidneys excrete an essentially sodium-free urine; kidneys are considered the chief regulator of sodium levels
C. Chloride—most important extracellular anion and is almost always linked to sodium
1. Chloride ions are usually excreted in the urine as a potassium salt
a. Hypochloremia—chloride deficiency
b. Hypokalemia—potassium deficiency; occurs whenever there is cell breakdown, as in starvation, burns, trauma, or dehydration; potassium enters the extracellular fluid and is rapidly excreted because it is not reabsorbed efficiently by the kidney
A. Acid-base balance is one of the most important of the body’s homeostatic mechanisms
B. Acid-base balance refers to the regulation of hydrogen ion concentration in the body fluids
C. Precise regulation of pH at the cellular level is necessary for survival
D. Slight pH changes have dramatic effects on cellular metabolism
MECHANISMS THAT CONTROL pH OF BODY FLUIDS
A. Review of pH concept
1. pH is a symbol used to represent the negative logarithm (exponent of 10) of the number of hydrogen ions (H+) present in 1 liter of a solution (Figure 23-20)
2. Acidity—increase in concentration of hydrogen ions
3. Alkalinity—decrease in concentration of hydrogen ions
4. pH of 7.0—intracellular fluid is essentially neutral
a. Acidosis—arterial blood pH less than 7.35
b. Alkalosis—arterial blood pH greater than 7.45
B. Mechanisms that regulate pH
1. Buffers—blood chemicals that quickly absorb excess acids or bases
2. Urinary system—stabilizes blood pH by secreting acids or bases into urine
3. Respiratory system—removes excess acid by releasing CO2 from the lungs
Write out the answers to these questions after reading the chapter and reviewing the Chapter Summary. If you simply think through the answer without writing it down, you won’t retain much of your new learning.
1. List the principal and accessory organs of the urinary system.
2. Name, locate, and give the main function(s) of each organ of the urinary system.
3. Describe the microscopic structure of the kidney.
4. Diagram the flow of blood through the kidney.
5. Define the terms filtration, tubular reabsorption, and tubular secretion.
6. What happens to sodium and chloride in the ascending limb of the Henle loop?
7. Identify three body systems in addition to the urinary system that also excrete unneeded substances.
8. Discuss the changes in total body water content from infancy to adulthood.
9. How does total body water content differ in men and women?
10. List the compartments of extracellular fluid.
11. What is the cardinal principle regarding fluid balance?
12. How is urine volume regulated?
CRITICAL THINKING QUESTIONS
After finishing the Review Questions, write out the answers to these items to help you apply your new knowledge. Go back to sections of the chapter that relate to items that you find difficult.
1. What would result if the nerves supplying the bladder and urethra were damaged?
2. Can you describe the mechanism of urine formation? How is each step related to the part of the nephron that performs it?
3. If the proximal convoluted tubules were unable to transport sodium ions into blood, why would you expect to find high concentrations of both sodium and chloride ions in urine?
4. Why do you think ADH prevents rapid dehydration of the body?
5. How is the function of ANH related to the increase in urine volume?
6. How would you summarize the role of aldosterone in thirst?
7. What information would you use to support the view that the homeostasis of fluid and electrolyte levels is one of the most crucial requirements for the maintenance of life itself?inar
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