b. Glucose. Glucose reabsorption occurs in the proximal tubule and usually
goes to completion at this point. Any glucose that is not reabsorbed in the proximal
tubule is usually excreted.
c. Sodium. The reabsorption of sodium is by active transport and is associated
with the reabsorption of Cl- and HCO3-. Sodium is reabsorbed in exchange for H+ and
K+. To preserve electrostatic equilibrium, it is necessary that for each Na+ reabsorbed,
there must be the reabsorption of an anion (Cl- or HCO3-) or the secretion of a cation
(H+ or K+).
d. Chloride. Chloride is reabsorbed in the proximal tubule. Chloride
reabsorption is always incomplete, but it increases as the filtered load increases. About
80 percent of the sodium that is reabsorbed is in association with chloride.
e. Hydrogen. Hydrogen ions are secreted into the renal tubule. Hydrogen is
secreted into the distal portion of the nephron in exchange as sodium is reabsorbed
from the lumen. Hydrogen also serves to acidify the urine. H+ secretion is an important
pH regulatory mechanism.
f. Potassium. Reabsorption of potassium is probably by active transport. As
sodium is reabsorbed from the lumen, interstitial potassium is secreted. Reabsorption
of potassium occurs in the proximal tubule and secretion of potassium is into the distal
g. Water. Most of the water filtered from the plasma is reabsorbed in the
proximal convoluted tubule. The flow of urine and the concentration of solute particles
in the urine may vary widely. The concentration of solute in the urine depends on the
concentration of antidiuretic hormone (ADH) in the plasma and on the rate of urine flow.
The amount of ADH that is released depends on the concentration of solutes in the
plasma. If the concentration is high, the hypophysis (posterior pituitary) releases ADH
into the blood. When this hormone reaches the kidney, water reabsorption begins. If
the concentration of solutes in the plasma is low, no ADH is released and water
In the proximal convoluted tubule, the reabsorption of glucose and amino acids
begins. Because of the active transport of sodium, chlorides and bicarbonate are
reabsorbed. Approximately 90 percent of the sodium is reabsorbed in the proximal
tubule. Water is reabsorbed in the proximal tubule as needed and determined by the
concentration of solutes in the plasma. In the distal tubule, sodium is reabsorbed in
exchange for hydrogen and potassium. The distal tubule is also permeable to water in
the presence of ADH. When ADH is present, the collecting tubules also are permeable
to water; but if ADH is not present, these tubules act only as conduits.