The collecting duct of the nephron is the last segment of the functioning nephron and is connected to minor calyces and the ensuing renal pelvis of the kidney where urine continues before it is stored in the bladder. The collecting duct is mainly responsible for the excretion and reabsorption of water and ions. It is composed of two important cell types: intercalated cells that are responsible for maintaining acid-base homeostasis, and principal cells that help maintain the body's water and salt balance. When renin is released from the kidneys, it causes the activation of angiotensin I in the blood circulation which is cleaved to become angiotensin II. Angiotensin II stimulates the release of aldosterone from the adrenal cortex and release of vasopressin from the posterior pituitary gland. When in the circulation, vasopressin eventually binds to receptors on epithelial cells in the collecting ducts. This causes vesicles that contain aquaporins to fuse with the plasma membrane. Aquaporins are proteins that act as water channels once they have bound to the plasma membrane. As a result, the permeability of the collecting duct changes to allow for water reabsorption back into the blood circulation. In addition, sodium and potassium are also reabsorbed back into the systemic circulation at the collecting duct via potassium and sodium channels. However, aldosterone is a major regulator of the reabsorption of these ions as well, as it changes the permeability of the collective duct to these ions. As a result, a high concentration of sodium and potassium in the blood vessels occurs. Some urea and other ions may be reabsorbed as well. The reabsorption of ions and water increases blood fluid volume and blood pressure.

Kidney Function - Collecting Duct References