Kidneys

The basic structural unit of the urinary system is the kidney. Each kidney is about 4 inches (10 cm) long and 2 inches (5 cm) wide and weighs about 150 g. Each kidney contains about one to two million nephrons, the tiny structures that filter the blood (Fig. 17-2). The nephron is the location of formation of urine and is the functional unit of the urinary system. The process by which the urine is filtered in the nephron is complex (Table 17-1). The kidney has three layers (Fig. 17-3):

Inside the renal pelvis is the hilum, which serves as a passageway for lymph vessels, nerves, the renal artery, and renal vein.

Ureters

The ureters, small tubes composed of smooth muscle tissue, move the urine from the kidney to the bladder with peristaltic motion. Each ureter is about 10 to 12 inches (25 to 30 cm) long. At the junction with the bladder, a valvelike narrow region of the ureter prevents urine from flowing back to the kidneys.

Bladder

The bladder, a smooth muscular sac that expands as it fills with urine, can hold up to 1 L. When the bladder fills, nerves in the muscular wall are stimulated and cause the urge to urinate. Urination is also called voiding or micturition.

Urethra

The urethra moves urine from the bladder to be excreted from the body. It is about 1.0 to 1.5 inches (5 cm) long in the female and runs through the vulva. In the male it is 8 inches (20 cm) long and runs through the penis. At the end of the urethra is an opening called the urinary meatus, through which urine passes to the outside of the body.

Urine Formation

Each minute about 600 mL of blood plasma enters the nephrons of each kidney through the renal arteries. In the glomerulus of the nephron, particles are filtered from the blood to be eliminated from the body. Water, glucose, vitamins, amino acids, and chloride salts are reabsorbed into the blood in the renal tubules. Ammonia, potassium, hydrogen ions, and some drugs are secreted into the urine by tubular cells using the process of active transport. The blood leaves the kidneys through the renal veins.

Urine normally consists of 95% water. The remaining 5% includes waste products from the breakdown of protein, hormones, electrolytes, pigments, toxins, and any abnormal components (Table 17-2). Urine is normally sterile in the kidneys and bladder.

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