How does osmolarity of the blood maintain homeostasis?
The kidney uses a countercurrent flow system to maintain homeostasis.
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 mOsm/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 mOsm/mol. 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 homeostasis.