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Urinary System

Kidneys

1. The paired kidneys are retroperitoneal in the superior lumbar region. Each kidney has a medial indentation (hilus), where the renal artery, renal vein (Figure 15.1). and ureter are seen. Each kidney is enclosed in a tough fibrous capsule. A fatty cushion holds the kidneys against the trunk wall.

2. A longitudinal section of a kidney reveals an outer cortex, deeper medulla, and medial pelvis. Extensions of the pelvis (calyces) surround the tips of medullary pyramids and collect urine draining from them.

3. The renal artery, which enters the kidney, breaks up into segmental, lobar, and then interlobar arteries that travel outward through the medulla. Interlobar arteries split into arcuate arteries, which branch to produce interiobular arteries, which serve the cortex.

4. Nephrons are structural and functional units of the kidneys (Figure 15.2). Each consists of a glomerulus and a renal tubule. Subdivisions of the renal tubule (from the glomerulus) are glomerular capsule, proximal convoluted tubule, loop of Henle, and distal convoluted tubule. A second (peritubular) capillary bed is also associated with each nephron.

5. Nephron functions include filtration, reabsorption, and secretion (Figure 15.3). Filtrate formation is the role of the high-pressure glomerulus. Filtrate is essentially plasma without blood proteins. In reabsorption, done by tubule cells, needed substances are removed from filtrate (amino acids, glucose, water, some ions) and returned to blood. The tubule cells also secrete additional substances into nitrate. Secretion is important to rid the body of drugs and excess ions (potassium, hyrdogen, and ammonia) and to maintain acid-base balance of blood.

6. Blood composition depends on diet, cellular metabolism, and urinary output. To maintain blood composition, the kidneys must:

   1. Allow nitrogen-containing wastes (urea, ammonia, creatinine, uric acid) to go out in the urine.

   2. Maintain water and electrolyte balance by absorbing more or less water and reclaiming ions in response to hormonal signals. ADH, which acts on the collecting ducts, increases water reabsorption and conserves body water. Aldosterone increases reabsorption of sodium and water and decreases potassium reabsorption.

   3. Maintain acid-base balance by actively secreting bicarbonate ions (and retaining H+) and by absorbing bicarbonate ions (and secreting H+. Chemical buffers tie up excess H+ or bases temporarily: respiratory centers modify blood pH by retaining C02 (decreases pH) or by eliminating more CO2 from the blood (increases blood pH). Only the kidney can remove metabolic acids and excess bases from the body.

7. Urine is clear, yellow, and usually slightly acid, but its pH value varies widely. Substances normally found in urine are nitrogenous wastes, water, various ions (always sodium and potassium). Substances normally absent from urine include glucose, blood proteins, blood, pus (WBC's), bile.

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Ureters, Urinary Bladder, and Urethra

1. The ureters are slender tubes running from each kidney to the bladder (Figure 15.4). They conduct urine by peristalsis from kidney to bladder.

2. The bladder is a muscular sac posterior to the pubic symphysis. It has two inlets (ureters) and one outlet (urethra). In males, the prostate gland surrounds its outlet. The function of the bladder is to store urine.

3. The urethra is a tube that leads urine from the bladder to the body exterior. In females, it is 3-4 cm long and conducts only urine. In males, it is 20 cm long and conducts both urine and sperm. The internal sphincter of smooth muscle is at the bladder-urethra junction. The external sphincter of skeletal muscle is located more inferiorly.

4. Micturition is emptying of the bladder. The micturition reflex causes the involuntary internal sphincter to open when stretch receptors in the bladder wall are stimulated. Since the external sphincter is voluntarily controlled, micturition can ordinarily be temporarily delayed. Incontinence is the inability to control micturition.

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Developmental Aspects of the Urinary System

1. The kidneys begin to develop in the first few weeks of embryonic life and are excreting urine by the third month,

2. Common congenital abnormalities include polycystic kidney and hypospadias.

3. Common urinary system problems in children and young to middle-aged adults are infections caused by fecal microorganisms, sexually transmitted disease-causing microorganisms, and streptococcus.

4. Renal failure is an uncommon, but extremely serious, problem in which kidneys are unable to concentrate urine, and dialysis must be done to maintain chemical homeostasis of blood.

5. With age, filtration rate decreases and tubule cells become less efficient at concentrating urine, leading to urgency, frequency, and incontinence. In males, urinary retention is another common problem.