Urinary System Help (page 2)
The urinary system plays a critical role in regulating the composition of body fluids (water balance, electrolyte balance, and acid-base balance). It also rids the body of metabolic wastes and foreign matter (chemicals, drugs). The kidneys also have a minor endocrine function.
Components of the Urinary System
Kidneys. The kidneys are located on either side of the vertebral column in the abdominal cavity, between the twelfth thoracic and third lumbar vertebrae. They form urine. The macroscopic structure of the kidneys is diagramed and labeled in Figure 21-1.
Ureters. The ureters transfer urine from the renal pelvises of the kidneys to the urinary bladder. The ureters are retroperitoneal.
Urinary bladder. The urinary bladder lies posterior to the symphysis pubis and anterior to the rectum. It stores urine. The urinary bladder consists of four tunics: the (innermost) mucosa, which is folded into rugae that allow distension of the bladder; the submucosa, which provides a rich vascular supply; the muscularis, a smooth muscle layer, (the detrusor muscle); and the serosa, a continuation of the peritoneum. The floor of the urinary bladder is a triangular area called the trigone. It has an opening at each of its three angles for the two ureters laterally and the urethra at the apex. Sympathetic fibers innervate the trigone, urethral openings, and blood vessels; parasympathetic fibers innervate the smooth muscle wall.
Urethra. The urethra conveys urine from the urinary bladder to the outside of the body. The internal urethral sphincter, composed of smooth muscle, and the external urethral sphincter, of skeletal muscle, constrict the lumen of the urethra causing the urinary bladder to fill. The urethra of a female is about 4 cm long, and that of a male about 20 cm. long. In a male, the spongy urethra also carries semen during ejaculation.
The Nephron and Its Function
Figure 21-2 depicts the functional (urine-forming) unit of the kidney, the nephron.
There are over 1 million nephrons per kidney. The components of the nephron are described below.
Juxtaglomerular apparatus: Cells of the macula densa along with specialized juxtaglomerular cells of the afferent arteriole compose a sensory apparatus for monitoring blood pressure. A drop in blood pressure or an increased NaCl concentration in the distal tubule stimulates renin to be released from the juxtaglomerular cells. This activates the renin-angiotensin system (see Figure 13-2).
Glomerular filtration: Fluid and solutes in the blood plasma of the glomerulus pass into the glomerular capsule. Filtrate has the same composition as blood plasma excluding proteins. Glomerular filtration rate (GFR) is the volume of filtrated formed by all nephrons each minute.
Tubular reabsorption: Approximately 99 percent of the filtrate is transported passively or actively out of the tubule into the interstitial fluid, then into the peritubular capillaries; 1 percent is excreted as urine. Most of the solutes are reabsorbed: 100 percent of the glucose, 99.5 percent of the sodium, and 50 percent of the urea.
Tubular secretion: Noxious substance such as hydrogen, potassium, poisons, drugs, and metabolic toxins are actively transferred from the peritubular capillaries into the interstitial fluid, then into the tubular lumen.
The kidneys produce either a concentrated or dilute urine depending on the operation of a countercurrent exchange mechanism in the nephron loop, and the amount of circulating antidiuretic hormone (ADH) secreted from the posterior pituitary.
- A concentration gradient exits in the renal medulla due to active transport of Cl- out of the tubular fluid in the ascending limb of the nephron loop, and the movement of Na+ ions out of the tubule. The ascending limb is impermeable to water and as Na+ and Cl- move out, the fluid in the ascending limb becomes more dilute.
- Na+ and Cl- diffuse into the descending limb. The descending limb is permeable to water, and as water diffuses out into the interstitium as a result of the osmotic gradient, the tubular fluid in the descending limb becomes more concentrated.
- Ions are actively transported into the intersititium from the collecting duct; urea passively diffuses out of the collecting duct into the interstitium.
- Thin-wall vessels, the vasa recta, parallel the course of the nephron loops. Na+, Cl-, and water diffuse into the descending vasa recta and Na+ and Cl- diffuse out of the ascending vasa recta. These vessels function as countercurrent exchangers.
- The amount of water reabsorbed form the distal convoluted tubules and collecting ducts is dependent on levels of ADH present. When low levels of ADH are secreted from the posterior, these tubules are impermeable to water, and a dilute urine is excreted. When ADH levels are high, these tubules are highly permeable to water, which is forced by the osmotic gradient out into the interstitium and a more concentrated urine is excreted.
In acidosis, increased amounts of H+ are secreted into the kidney tubule and bicarbonate ions in the tubule are reabsorbed. In alkalosis, decreased amounts of H+ are secreted and less bicarbonate is reabsorbed. Two buffer systems in the tubular fluid carry excess H+ into the urine:
- Phosphate buffer system: HPO32– + H+ → H2PO4–
- Ammonia buffer system: NH3 + H+ → NH4+
Micturition is the physiological process of urination. Distention of the urinary bladder sends signals via sensory neurons to the spinal cord and up to the brain. Parasympathetic impulses stimulate the detrusor muscle to contract and the internal urethral sphincter to relax. If the decision is to urinate, the external urethral sphincter is relaxed and micturition results.
Practice problems for these concepts can be found at:
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