Thick ascending limb of Henle's loop.

Thick ascending limbs of Henle's loop have at least three major roles: (1) They reabsorb sodium chloride which dilutes the urine. (2) The reabsorption of sodium chloride also produces concentration gradients that drive the countercurrent multiplier system in the medulla and medullary rays and thus concentrates the urine. (3) They reabsorb large amounts of potassium, calcium, and magnesium in an energy-efficient manner. The mechanisms involved in these functions are reviewed in this paper, emphasizing the results of direct studies on isolated tubule segments.