Tubular Reabsorption and Secretion

Tubular reabsorption occurs along the length of the renal tubules and collecting ducts. This process returns water and solutes from the filtrate to the blood.

It is essential for maintaining blood volume and regulating the electrolyte balance of the body.

During reabsorption, substances pass from the tubular fluid into the blood capillaries.

To do so, they can follow either the transcellular or paracellular routes.

In the transcellular route, substances move through the apical membrane, cytosol, basolateral membrane of the tubule cell, and finally, the endothelium of the peritubular capillaries.

The paracellular route allows movement between the tubule cells through the leaky tight junctions.

Active tubular reabsorption, powered by ATP, moves solutes against their concentration gradients. Passive reabsorption uses osmosis for water movement and diffusion for solutes along their electrochemical gradients.

Tubular secretion is reabsorption in reverse. It clears the plasma of unwanted substances, like urea, ammonium ions, creatinine, and certain drugs.

Additionally, tubular secretion helps control blood pH by regulating the secretion of hydrogen ions in the filtrate.