In the loop of Henle, water passes out of the descending limb and salts are pumped out of the ascending limb. Salts pumped out of the ascending limb cause the osmotic concentration of the medulla to increase. Water flows passively out of the descending limb (by osmosis) in response to the increased osmotic concentration of the medulla. Vasa recta are straight capillaries that penetrate deep into medulla in juxtamedullary nephrons. They ensure water is efficiently reabsorbed in the loops of Henle.
The distal convoluted tubule and collecting ducts are impermeable to water in the absence of ADH (anti-diuretic hormone). ADH is produced by the hypothalamus and released into the posterior pituitary gland. It increases the permeability of the distal convoluted tubule and collecting duct so that water is reabsorbed. Reabsorption of water in the distal tubules makes the urine more concentrated and conserves body water.
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