Maintenance of gradient of sodium ions by the loop of Henle

• The loop of Henle is a hairpin-shaped tubule that extends into the medulla of the kidney. It is responsible for water being reabsorbed from the collecting duct, thereby concentrating the urine so that it has a lower water potential than the blood.
• The concentration of the urine produced is directly related to the length of the loop of Henle.
• The loop of Henle has two major regions : the descending limb and the ascending limb.
• The descending limb is narrower, with thin walls that are highly permeable to water.
• The ascending limb is wider, with thick walls that are impermeable to water.
• The water that passes out of the collecting duct via osmosis does so through channel proteins that are specific to water (aquaporins). Antidiuretic hormone (ADH) can alter the number of these channels and hence control water loss.
• By the time the filtrate (urine) leaves the collecting duct on the way to the bladder, it has lost most of its water, so has a lower water potential than the blood.