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Collecting Duct

Reabsorption of water by osmosis

(Note: Although the collecting duct is not anatomically part of the nephron, it is functionally connected.)

(Note: Although the collecting duct is not anatomically part of the nephron, it is functionally connected.)

Abnormal constituents of urine may not indicate illness. Glucose in the urine may result from a sugary meal or may occur toward the end of pregnancy; protein may appear in the urine following vigorous physical exercise; ketones appear in the urine during a prolonged fast or when a person follows a very low calorie or low carbohydrate diet.

The volume of urine produced usually varies between 0.6 and 2.5 liters per day. Such factors as fluid intake, environmental temperature, relative humidity of the surrounding air, and a person's emotional condition, respiratory rate, and body temperature influence the exact urine volume. An output of 50-60 milliliters of urine per hour is considered normal, and an output of less than 30 milliliters per hour may indicate kidney failure. Clinical Application 20.4 discusses renal clearance, which is a measure of kidney efficiency.

Parents of infants may be startled when a physician hospitalizes their child for an illness that in an adult might be considered mild—a day or two of vomiting and diarrhea. Because the kidneys of infants and young children are unable to concentrate urine and conserve water as effectively as those of adults, they can lose water rapidly, which may lead to dehydration. A 20-pound infant can lose a pound in just a day of an acute viral illness, and this is a sufficiently significant proportion of body weight to warrant hospitalization, where intravenous fluids are given to restore water and electrolyte balance (see chapter 21, p. 862).

List the normal constituents of urine.

What is the normal hourly output of urine? The minimal hourly output?

Elimination of Urine

After forming along the nephrons, urine passes from the collecting ducts through openings in the renal papillae and enters the major and minor calyces of the kidney. From there it passes through the renal pelvis, into a ureter, and into the urinary bladder. The urethra delivers urine to the outside.

Ureters

Each ureter is a tubular organ about 25 centimeters long, which begins as the funnel-shaped renal pelvis. It extends downward posterior to the parietal peritoneum and parallel to the vertebral column. Within the pelvic cavity, it courses forward and medially to join the urinary bladder from underneath.

The wall of a ureter is composed of three layers. The inner layer, or mucous coat, includes several thicknesses of transitional epithelial cells and is continuous with the linings of the renal tubules and the urinary bladder. The middle layer, or muscular coat, largely consists of smooth muscle fibers arranged in circular and longitudinal bundles. The outer layer, or fibrous coat, is composed of connective tissue (fig. 20.27).

Because the linings of the ureters and the urinary bladder are continuous, bacteria may ascend from the bladder into the ureters, causing infection. An inflammation of the bladder, called cystitis, is more common in women than in men because the female urethral pathway is shorter. Inflammation of the ureter is called ureteritis.

Muscular peristaltic waves, originating in the renal pelvis, help move the urine along the length of the ureter. The presence of urine in the renal pelvis initiates

Renal Clearance

The rate at which a particular chemical is removed from the plasma indicates kidney efficiency. This rate of removal is called renal clearance.

Tests of renal clearance can detect glomerular damage or judge the progress of renal disease. One such test, the inulin clearance test, uses in-ulin (not to be confused with insulin), a complex polysaccharide found in certain plant roots. In the test, a known amount of inulin is infused into the blood at a constant rate. The in-ulin passes freely through the glomerular membranes, so its concentration in the glomerular filtrate equals that in the plasma. In the renal tubule, inulin is not reabsorbed to any significant degree, nor is it secreted. Consequently, the rate at which it appears in the urine can be used to calculate the rate of glomerular filtration.

Similarly, the kidneys remove creatinine from the blood. Creatinine is produced at a constant rate during muscle metabolism. Like inulin, creati-nine is filtered, but neither reabsorbed nor secreted by the kidneys. Thus, the creatinine clearance test, which compares a patient's blood and urine creati-nine concentrations, can also be used to calculate the GFR. A significant advantage is that the bloodstream normally has a constant level of creatinine. Therefore, a single measurement of plasma creatinine levels provides a rough index of kidney function. For example, significantly elevated plasma creatinine levels suggest that GFR is greatly reduced. Because nearly all of the creatinine the kidneys filter normally appears in the urine, a change in the rate of creatinine excretion may reflect renal failure.

Another plasma clearance test uses para-aminohippuric acid (PAH), a substance that filters freely through the glomerular membranes. However, unlike inulin, any PAH remaining in the peritubular capillary plasma after filtration is secreted into the proximal convoluted tubules. Therefore, essentially all PAH passing through the kidneys appears in the urine. For this reason, the rate of PAH clearance can be used to calculate the rate of plasma flow through the kidneys. Then, if the hematocrit is known (see chapter 14, p. 548), the rate of total blood flow through the kidneys can also be calculated. ■

Mucous coat

Muscular coat

Fibrous coat

Figure 20.27

Cross section of a ureter (75x).

Mucous coat

Muscular coat

Fibrous coat

Figure 20.27

Cross section of a ureter (75x).

these waves, whose frequency keeps pace with the rate of urine formation. If this rate is high, a peristaltic wave may occur every few seconds; if the rate is low, a wave may occur every few minutes.

When a peristaltic wave reaches the urinary bladder, it spurts urine into the bladder. A flaplike fold of mucous membrane covers the opening where the urine enters. This fold acts as a valve, allowing urine to enter the bladder from the ureter but preventing it from backing up from the bladder into the ureter.

If a ureter becomes obstructed, such as by a small kidney stone (renal calculus) in its lumen, strong peristaltic waves are initiated in the proximal portion of the tube, which may help move the stone into the bladder. The presence of a stone usually also stimulates a sympathetic reflex (ureterorenal reflex) that constricts the renal arterioles and reduces urine production in the affected kidney.

D Describe the structure of a ureter.

^9 How is urine moved from the renal pelvis to the urinary bladder?

^9 What prevents urine from backing up from the urinary bladder into the ureters?

Q How does an obstruction in a ureter affect urine production?

Kidney stones, which are usually composed of calcium oxalate, calcium phosphate, uric acid, or magnesium phosphate, sometimes form in the renal pelvis. If such a stone passes into a ureter, it may produce severe pain, beginning in the region of the kidney and radiating into the abdomen, pelvis, and lower limbs. Nausea and vomiting may also occur.

About 60% of kidney stones pass spontaneously; the others must be removed. In the past, such removal required surgery or instruments that could be passed through the tubes of the urinary tract to capture or crush the stones. Today, shock waves applied from outside the body are used to fragment kidney stones. This procedure, called extracorporeal shock-wave lithotripsy (ESWL), focuses high-energy shock waves through water (either in a tub or in a water-filled sack placed against the patient). The shock waves break the stones into fragments small enough to be eliminated with the urine.

Urinary Bladder

The urinary bladder is a hollow, distensible, muscular organ. It is located within the pelvic cavity, posterior to the symphysis pubis, and inferior to the parietal peritoneum (fig. 20.28 and reference plate 52). In a male, the bladder lies posteriorly against the rectum, and in a female, it contacts the anterior walls of the uterus and vagina.

The pressure of surrounding organs alters the spherical shape of the bladder. When the bladder is empty, its inner wall forms many folds, but as it fills with urine, the wall becomes smoother. At the same time, the superior surface of the bladder expands upward into a dome.

When it is greatly distended, the bladder pushes above the pubic crest and into the region between the abdominal wall and the parietal peritoneum. The dome can reach the level of the umbilicus and press against the coils of the small intestine.

The internal floor of the bladder includes a triangular area called the trigone, which has an opening at each of its three angles (fig. 20.29). Posteriorly, at the base of the trigone, the openings are those of the ureters. Anteriorly, at the apex of the trigone, is a short, funnel-shaped extension called the neck of the bladder, which contains the opening into the urethra. The trigone generally remains in a fixed position, even though the rest of the bladder distends and contracts.

The wall of the urinary bladder consists of four layers. The inner layer, or mucous coat, includes several thicknesses of transitional epithelial cells, similar to those lining the ureters and the upper portion of the urethra. The thickness of this tissue changes as the bladder expands and contracts. Thus, during distension, the tis

Figure

The urinary bladder is located within the pelvic cavity and behind the symphysis pubis. In a male, it lies against the rectum.

Figure

The urinary bladder is located within the pelvic cavity and behind the symphysis pubis. In a male, it lies against the rectum.

sue appears to be only two or three cells thick, but during contraction, it appears to be five or six cells thick (see fig. 5.9).

Former Vice President Hubert Humphrey died in 1978 of bladder cancer. He had been diagnosed in 1976, and an exam in 1973 had found a "borderline malignancy." In 1967, he had experienced his first symptom — blood in the urine. In 1994, researchers identified a mutation affecting only one DNA base, which caused the cancer. Today, a genetic test can detect several chromosomal changes indicative of bladder cancer, in cells shed in the urine.

Detecting a genetic change in cancer cells can be clinically useful in several ways. It can permit early diagnosis when treatment is more likely to be successful. Absence of the causative gene variant in cells bordering an excised tumor indicates that sufficient tissue has been removed to stop the cancer. Finally, a genetic test can detect a recurrence of cancer before symptoms arise.

The second layer of the bladder wall is the submucous coat. It consists of connective tissue and contains many elastic fibers.

The third layer of the bladder wall, the muscular coat, is primarily composed of coarse bundles of smooth muscle fibers. These bundles are interlaced in all directions and at all depths, and together they comprise the detrusor muscle (de-truz'or mus'l). The portion of the

Ureter

Ureter

Mucous coat

Ureteral openings

Trigone

Serous coat

Mucous coat

Ureteral openings

Trigone

Internal urethral sphincter Prostate gland Urethra

■ Region of external urethral sphincter

Figure 20.29

A male urinary bladder. (a) Coronal section. (b) Posterior view.

Urinary bladder

Serous coat

Urinary bladder

Internal urethral sphincter Prostate gland Urethra

Prostate gland

Urethra

Prostate gland

Urethra

detrusor muscle that surrounds the neck of the bladder forms an internal urethral sphincter. Sustained contraction of this sphincter muscle prevents the bladder from emptying until the pressure within it increases to a certain level. The detrusor muscle has parasympathetic nerve fibers that function in the reflex that passes urine.

The outer layer of the wall, the serous coat, consists of the parietal peritoneum. It is found only on the upper surface of the bladder. Elsewhere, the outer coat is composed of fibrous connective tissue (fig. 20.30).

O Describe the trigone of the urinary bladder. ^9 Describe the structure of the bladder wall. ^9 What kind of nerve fibers supply the detrusor muscle?

Urethra

The urethra is a tube that conveys urine from the urinary bladder to the outside of the body. Its wall is lined with mucous membrane and contains a thick layer of longitudinal smooth muscle fibers. It also contains numerous mucous glands, called urethral glands, which secrete mucus into the urethral canal (fig. 20.31).

In a female, the urethra is about 4 centimeters long. It passes forward from the bladder, courses below the symphysis pubis, and empties between the labia minora. Its opening, the external urethral orifice (urinary mea-tus), is located anterior to the vaginal opening and about 2.5 centimeters posterior to the clitoris (fig. 20.32a).

Muscular coat

Mucous coat

Submucous coat

Muscular coat

Mucous coat

\ Lumeri^—

/

__ j

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