Urinary

© The McGraw-H Companies, 2001

© The McGraw-H Companies, 2001

chapter objectives

After you have studied this chapter, you should be able to

1. Name the organs of the urinary system and list their general functions.

2. Describe the locations of the kidneys and the structure of a kidney.

3. List the functions of the kidneys.

4. Trace the pathway of blood through the major vessels within a kidney.

5. Describe a nephron and explain the functions of its major parts.

6. Explain how glomerular filtrate is produced and describe its composition.

7. Explain how various factors affect the rate of glomerular filtration and how this rate is regulated.

8. Discuss the role of tubular reabsorption in urine formation.

9. Explain why the osmotic concentration of the glomerular filtrate changes as it passes through a renal tubule.

10. Describe a countercurrent mechanism and explain how it helps concentrate urine.

11. Define tubular secretion and explain its role in urine formation.

12. Describe the structure of the ureters, urinary bladder, and urethra.

13. Discuss the process of micturition and explain how it is controlled.

14. Describe how the components of the urinary system change with age.

Understanding ^Vo rds af-, to: afferent arteriole—

arteriole that leads to a nephron. calyc-, small cup: major calyces—cuplike subdivisions of the renal pelvis.

cort-, covering: renal cortex—

shell of tissue surrounding the inner region of a kidney. cyst-, bladder: cystitis— inflammation of the bladder. detrus-, to force away: detrusor muscle—muscle within the bladder wall that causes urine to be expelled. glom-, little ball: glomerulus—

cluster of capillaries within a renal corpuscle. juxta-, near to: /uxtamedullary nephron—nephron located near the renal medulla. mict-, to pass urine:

micturition—process of expelling urine from the bladder. nephr-, pertaining to the kidney: nephron—functional unit of a kidney. papill-, nipple: renal papillae— small elevations that project into a renal calyx. prox-, nearest: proximal tubule— coiled portion of the renal tubule leading from the glomerular capsule. ren-, kidney: renal cortex—outer region of a kidney. trigon-, triangular shape:

trigone—triangular area on the internal floor of the bladder.

Cook that burger! Hemolytic uremic syndrome is a complication of infection by a strain of E. coli bacteria that produces shigatoxin. Destruction of the proximal portions of kidney tubules allows proteins and blood cells to enter urine.

elicia had looked forward to summer camp all

Fyear, especially the overnight hikes. A three-day expedition in July was wonderful, but five days after returning to camp, Felicia developed severe abdominal cramps. So did seventeen other campers and two counselors, some of whom had bloody diarrhea, too. Several of the stricken campers were hospitalized, Felicia among them. Although the others improved in a few days and were released, Felicia suffered from a complication, called hemolytic uremic syndrome (HUS). Her urine had turned bloody, and she also had blood abnormalities-severe anemia and lack of platelets.

Camp personnel reported the outbreak to public health officials, who quickly recognized the signs of food poisoning and traced the illness to hamburgers cooked outdoors on the trip. The burgers were served rare, the red meat not hot enough to kill a strain of Escherichia coli bacteria that releases a poison called shigatoxin.

Most people who eat meat tainted with E. coli toxin become ill, but the damage usually is restricted to the digestive tract, producing cramps and diarrhea for several days. In about 6% of affected people, mostly children, HUS develops because the bloodstream transports the toxin to the kidneys. Here, the toxin destroys cells of the microscopic tubules that normally filter proteins and blood cells from forming urine. With the tubule linings compromised, proteins and blood cells, as well as damaged kidney cells, appear in the urine.

HUS is a leading cause of acute renal (kidney) failure, killing 3-5% of affected children. Felicia was in the lucky majority. Blood clotted around the sites of her damaged kidney cells, and over a few weeks, new cells formed. Three weeks after the bloody urine began, her urine was once again clear, and she was healthy.

Cook that burger! Hemolytic uremic syndrome is a complication of infection by a strain of E. coli bacteria that produces shigatoxin. Destruction of the proximal portions of kidney tubules allows proteins and blood cells to enter urine.

The urinary system consists of a pair of glandular kidneys, which remove substances from the blood, form urine, and help regulate certain metabolic processes; a pair of tubular ureters, which transport urine from the kidneys; a saclike urinary bladder, which collects urine from the ureters and serves as a urine reservoir; and a tubular urethra, which conveys urine to the outside of the body. Figures 20.1 and 20.2 show these organs.

^ Kidneys

A kidney is a reddish brown, bean-shaped organ with a smooth surface. It is about 12 centimeters long, 6 centimeters wide, and 3 centimeters thick in an adult, and it is enclosed in a tough, fibrous capsule (tunic fibrosa).

Location of the Kidneys

The kidneys lie on either side of the vertebral column in a depression high on the posterior wall of the abdominal cavity. The upper and lower borders of the kidneys are generally at the levels of the twelfth thoracic and third lumbar vertebrae, respectively, although the positions of the kidneys may vary slightly with changes in posture and with breathing movements. The left kidney is usually about 1.5 to 2 centimeters higher than the right one.

The kidneys are positioned retroperitoneally (re"tro-per"i-to-ne'al-le), which means they are behind the parietal peritoneum and against the deep muscles of the back. Connective tissue (renal fascia) and masses of adipose tissue (renal fat) surrounding the kidneys hold them in place (fig. 20.3 and reference plates 58, 59).

Kidney Structure

The lateral surface of each kidney is convex, but its medial side is deeply concave. The resulting medial depression leads into a hollow chamber called the renal sinus. Through the entrance to this sinus, termed the hilum, pass blood vessels, nerves, lymphatic vessels, and the ureter (see fig. 20.1).

The superior end of the ureter expands to form a funnel-shaped sac called the renal pelvis, which is located inside the renal sinus. The pelvis is subdivided into two or three tubes, called major calyces (sing., calyx), and they, in turn, are subdivided into eight to

Renal vein -

Hilum

Inferior-

vena cava

Urinary-

bladder

- Renal artery -Kidney

Abdominal aorta

Ureter

-Urethra

Figure 20.1

The urinary system includes the kidneys, ureters, urinary bladder, and urethra. Notice the relationship of these structures to the major blood vessels.

Figure 20.2

Structures of the urinary system are visible in this falsely colored radiograph.

Figure 20.2

Structures of the urinary system are visible in this falsely colored radiograph.

fourteen minor calyces (fig. 20.4). A series of small projections called renal papillae project into each minor calyx.

The kidney includes two distinct regions: an inner medulla and an outer cortex. The renal medulla (re'nal me-dul'ah) is composed of conical masses of tissue called renal pyramids, the bases of which are directed toward the convex surface of the kidney, and the apexes of which form the renal papillae. The tissue of the medulla appears striated because it consists of microscopic tubules that lead from the cortex to the renal papillae.

The renal cortex (re'nal kor'teks), which appears somewhat granular, forms a shell around the medulla. Its tissue dips into the medulla between the renal pyramids, forming renal columns. The cortex itself is surrounded by the renal capsule, a fibrous membrane surrounding each kidney that helps maintain its shape and provides some protection (figs. 20.4 and 20.5).

Two common inherited kidney abnormalities are polycystic kidney disease, which affects adults, and Wilms tumor, which affects young children. In polycystic kidney disease, cysts present in the kidneys since childhood or adolescence begin to produce symptoms in the thirties, including abdominal pain, bloody urine, and elevated blood pressure. The condition can lead to renal failure. In Wilms tumor, pockets of cells within a child's kidney remain as they were in the embryo — they are unspecialized and divide rapidly, forming a cancerous tumor. Loss of a tumor suppressor gene causes Wilms tumor.

Functions of the Kidneys

The main function of the kidneys is to regulate the volume, composition, and pH of body fluids. In the process, the kidneys remove metabolic wastes from the blood and excrete them to the outside. These wastes include nitrogenous and sulfur-containing products of protein metabolism. The kidneys also help control the rate of red blood cell formation by secreting the hormone erythropoietin (see chapter 14, p. 552), regulate blood pressure by secreting the enzyme renin (see chapter 13, p. 528), and regulate absorption of calcium ions by activating vitamin D (see chapter 13, p. 523).

Medical technology can take over the role of a kidney. In hemodialysis, a person's blood is rerouted across an artificial membrane that "cleanses" it, removing substances that would normally be excreted in the urine. A patient usually must use this artificial kidney three times a week, for several hours each time. Clinical Application 20.1 further discusses hemodialysis.

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