The Human Body

Structure and Function of Urinary System

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Kidney
The kidneys filter the blood and rid the body of wastes. Approximately one million nephrons (right) compose each bean-shaped kidney (left). The filtration unit of the nephron, called the glomerulus, regulates the concentration within the body of important substances such as potassium, calcium, and hydrogen, and removes substances not produced by the body such as drugs and food additives. Cancers that originate in the filtration tissues of the kidney, called renal cell cancer, account for 85 percent of all cancers of the kidney. A small percentage of cancers originate in the renal pelvis, a cavity in the center of each kidney.

The kidneys lie embedded in fat tissue on either side of the backbone at about waist level. Each fist-sized kidney is reddish-brown, weighs 140 to 160 g (5 to 6 oz), and is similar in shape to the kidney beans.

On the inner border of each kidney is a depression called the hilum, where the renal artery, the renal vein, and the ureter connect with the kidney (the adjective renal is from the Latin term renalis, meaning of or near the kidneys). The renal artery delivers over 1700 liters (450 gal) of blood to the kidneys each day, which these organs filter and return to the heart via the renal vein. Each kidney contains about 1 million microscopic coiled channels, called nephrons, which perform this critical blood-filtering function and produce urine in the process.

The bulblike upper portion of the kidney’s nephrons filters water; urea, the nitrogen-containing breakdown product of protein; salts; glucose; amino acids, the building blocks of proteins; yellow bile compounds from the liver; and other trace substances from the blood. As this material moves through a long, looped tubule, many of these filtered materials are reabsorbed into the blood to be reused by the body to maintain normal body functions. Less than 1 percent of the water and other materials remain behind to be excreted as waste products in the urine.

These waste materials then pass from the nephrons into a funnel-shaped area called the renal pelvis. From the renal pelvis, waste trickles out of the kidney into the ureter. The ureter empties into a hollow, muscular sac called the urinary bladder. A valvelike flap of tissue at the point of entry into the bladder prevents urine from flowing backward into the ureter. The urinary bladder is able to expand and contract according to how much urine it contains. As it fills with urine, the walls of the bladder stretch and become thinner, with the bladder itself lengthening to 12.5 cm (5 in) or more and holding up to about 0.5 liter (1 pt) of urine. A ringlike sphincter muscle surrounds the bladder’s outlet and prevents spontaneous emptying.

As the bladder becomes full, stretch-sensitive receptors in its walls are stimulated, and the person becomes aware of the fullness. When the person is ready to urinate, the sphincter relaxes and urine flows from the bladder to the outside through the urethra. In females, the urethra is about 3.8 cm (1.5 in) long and is strictly a urinary passage. In males, the urethra is about 20 cm (8 in) long; it passes through the penis and also serves to convey semen during sexual intercourse.

In addition to their vital role in ridding the body of wastes through the production of urine, kidneys play important regulatory roles. They maintain water balance, ensuring that the amount of water in body tissues remains at a constant level. The kidneys also control calcium levels in the blood to maintain healthy bones. They aid in regulating the acid-base balance of the blood and body fluids so that all body processes can proceed smoothly. By controlling salt levels, the kidneys help regulate blood pressure. Finally, they stimulate the body to make red blood cells, the primary component of healthy blood. Properly functioning kidneys are so vital to health that if they cease to function, death follows within days.