The cortical collecting duct is where all of the distal tubules
of the nephrons in the kidneys feed. It is fed by many nephrons,
and is the last chance for the kidney to reabsorb any of the
filtrate before it enters the renal pelvis and is later excreted
Angiotensin II causes sodium and chloride reabsorption in the
collecting duct, as well as in the distal tubule. Its
permeability to water is controlled by antidiuretic hormone (or
vasopressin, as it is also known). Water reabsorption is aided
by the duct's positioning within the renal medulla, where the
interstitial side of the membrane has a very high osmolality
(hyperosmolality). This draws water back in when the membrane is
permeable to it, via osmotic pressure. This increased
permeability is due to the insertion of more aquaporin proteins
in the membrane upon binding of vasopressin. The collecting duct
also reabsorbs urea, and excretes around 30 percent of the
absorbed potassium. The potassium absorption in the collecting
duct is due to potassium-specific