A. Filtration,
B. Reabsorption,
C. and Secretion
1. Urine is a fluid that rids the body of water and solutes that are in excess of the amounts needed to maintain the extracellular fluid.
2. Urine forms through a sequence of three processes:
a. In filtration, blood pressure forces filtrate out of the glomerular capillaries into Bowman’s capsule, then into the proximal tubule.
1) Blood cells, proteins, and other large solutes cannot pass the capillary wall into the capsule.
2) Water, glucose, sodium, and urea are forced out.
b. Reabsorption takes place in the tubular parts of the nephron where water and solutes move across the tubular wall out of the nephron (by diffusion or active transport) and into the surrounding capillaries.
c. Secretion moves substances from the capillaries into the nephron walls.
1) Capillaries surrounding the nephrons secrete excess amounts of hydrogen ions and potassium ions into the nephron tubules.
2) This process also rids the body of drugs, uric acid, hemoglobin breakdown products, and other wastes.
3. Urination is a reflex response which empties the bladder.
a. The internal urethral sphincter (involuntary control) regulates flow from the bladder into the urethra.
b. The external urethral sphincter (voluntary control) opens to void urine from the body.
c. Kidney stones are deposits of uric acid that collect in the renal pelvis or lodge in the ureter; they can be removed by surgery or lithotripsy.
B. Factors That Influence Blood Filtration
1. The kidneys can process about 1.5 quarts of blood each minute because of two factors:
a. Blood enters the glomerulus under high pressure in arterioles that have wider diameters than most arterioles.
b. Glomerular capillaries are highly permeable to water and small solutes.
2. The rate at which the kidneys filter a given volume of blood depends on the flow of blood through them and the rate of reabsorption in the tubules; neural and hormonal controls operate.
DIAGRAM 3
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