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
POSTED BY:
SHAMILY
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