The failure of tubule cells to reabsorb some solutes is an important way of clearing plasma of unwanted substances. Another way is tubular secretion-essentially, reabsorption reverse. Substances such as H+, K+,NH4+, creatinin and certain organic acids either move into the filtrate from the peritubular capillaries through the tubule cells or are synthesized in the tubule cells and secreted. As a result, the urine eventually excreted contains both filtered and secreted substances. With one major exception (K+), the proximal convoluted tubule is the main site of secretion, but the cortical parts of the collecting ducts are also active.
Tubular secretion is important for:
- Disposing of substances, such as certain drugs and metabolites, that are tightly bound to plasma protein. Because plasma proteins are generally not filtered, the substances they bind are not filtered and so must secreted.
- Eliminating undesirable substances or end products that have been reabsorbed by passive processes. Urea and uric acid, two nitrogenous wastes, are both handled in this way. Urea handling in the nephron is complicated, but the net effect is that 40-50% of the urea in the filtrate is excreted.
- Ridding the body excess K+. Because virtually all K+ present in the filtrate is reabsorbed in the proximal convoluted tubule and ascending loop of Henle, nearly all K+ in urine is from aldosteron driven active tubular secretion into the late distal convoluted tubule and collecting ducts.
- controlling blood pH. When blood pH drops toward acidic end of its homestatic range, the renal tubule cells actively secrete more H+ into the filtrate and retain and generate more HCO3- (a base). As a result, the blood pH rises and the urine drains off the excess H+. Conversely, When blood pH approaches the alkaline end of its range, Cl- is reabsorbed instead of HCO3-, which is allowed to leave the body in urine.