How is plasma osmolarity regulated?
The kidneys use a countercurrent flow system to maintain plasma osmolarity.
In the steady state, the osmolarity of the blood is about 300 mOsm/L.
Blood flowing through the descending loop of Henle loses water by osmosis. The osmolarity increases to 1200 mOsmol/L as solutes become more concentrated.
The ascending limb is permeable to salt but not to water. NaCl exits into the medulla, where the osmolarity is 1200 mOsmol/L. The osmolality decreases to 100 mOsm/L.
Thus, the two limbs cooperate with each other. The filtrate becomes saltier as it moves down the descending limb. The ascending limb exploits this concentration of NaCl to help maintain a high osmolarity in the medulla.
Capillaries carry blood in opposite (countercurrent) directions to the loop of Henle. As they convey blood toward the inner medulla, water leaves the blood and NaCl diffuses into the blood.
In the ascending capillaries, water re-enters the blood and salt diffuses out. Thus, the countercurrent flow helps maintain plasma osmolarity.