b. Renal Clearance. A clearance value indicates the degree to which a
substance is removed from the blood by excretion in the voided urine. Glomerular
filtrate is practically identical to plasma, except that it contains very little protein. While
125 mL of glomerular filtrate is produced per minute, approximately 124 mL is
reabsorbed by the renal tubules. Since none of the creatinine in the glomerular filtrate
is reabsorbed, for example, we say that the plasma clearance of creatinine is 125 mL
per minute. Another example follows: Less than 10 percent of the potassium ions is
excreted (not reabsorbed); therefore, the clearance of potassium is said to be 12.5 mL
per minute. The formula is:
Plasma clearance = Milligrams excreted in urine per minute
Concentration (mg/mL) in plasma
c. Glomerular Filtration. In humans, clearance tests are used to measure the
rate of glomerular filtration. A substance is used that passes easily into the glomerular
filtrate but is neither secreted nor reabsorbed by renal tubular cells. The concentration
of such a material in the glomerular filtrate is approximately the same as the
concentration of the material in the plasma. Knowledge of the rate of excretion of this
material and the plasma concentration are the only factors necessary to compute the
rate of glomerular filtration.
224. TUBULAR TRANSPORT
a. Reabsorption. The principal activity of the kidney is to transport dissolved
materials and water across the tubular cells. Transport of materials from the renal
tubules to the interstitial fluid is called reabsorption. Most tubular reabsorption occurs in
the proximal convoluted tubule.
b. Secretion. The movement of dissolved materials and water from the
interstitial fluid into the tubular lumen is called secretion.
c. Passive Transport. Passive transport occurs by two methods. Bulk flow
occurs when an entire solution moves through a membrane taking all parts of the
solution with it. Diffusion occurs when only certain components of a solution move
across a membrane.
d. Active Transport. Active transport occurs when a dissolved material is
forced across a rather impermeable membrane by a pumping mechanism. Most
physiologically important solutes (glucose, amino acids, proteins, uric acid, and most
electrolytes) are reabsorbed in this manner.
225. TRANSPORT OF SPECIFIC SUBSTANCES
a. Urea. Urea clearance test values are ordinarily about 70 mL per minute.
Therefore, only about 50% of the urea in the glomerular filtrate is reabsorbed.