The loop of Henle • The descending limb of the loop of Henle is permeable to water • The ascending limb is impermeable. • Na+ and Cl- ions are actively transported out of the ascending limb and passively move into the descending limb. • This creates a concentration gradient across the medulla. • There is a lower ion concentration gradient at the top of the ascending limb and a much higher ion concentration gradient at the bottom.
The Role of ADH • The water potential of the tissue fluid in the medulla is always more negative than that of the filtrate in the collecting duct. • Whether the water actually leaves the collecting duct (by osmosis) is determined by the hormone ADH (anti-diuretic hormone) • Osmoreceptors in the hypothalamus detect the low levels of water,so the hypothalamus sends an impulse to the pituitary gland which releases ADH into the bloodstream. • ADH makes the wall of the collecting duct more permeable to water. • Therefore, when ADH is present more water is reabsorbed and less is excreted.