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Loop of Henle

The loop of Henle is a U-shaped structure which leads to the distal convoluted tubule in human kidney. It is named after its discoverer, F. G. J. Henle. Its primary function is to provide the concentration gradient in the medulla necessary for the collecting duct to concentrate urine. This structure can be broken up into two parts, the descending limb, and the ascending limb.

• Descending Limb - Decreases the concentration of water in the tubule (increases osmolarity) due to the following characteristics:
  1. Low permeability of the membrane to NaCl and urea
  2. High permeability of the membrane to water
  3. Thin cells with NO active transport

• Ascending Limb - Increases the osmolarity of the medulla due to the following characteristics:
  1. Low permeability of the membrane to water & urea
  2. Active transport of Na⁺ and Cl^- into the medulla

Note that as the water osmoses out in the descending limb it is greatly absorbed back into the blood, whereas the salts that diffuse into the medulla in the ascending limb remain in the outer and inner medulla creating a concentration gradient. It is this concentration gradient (made much higher in the medulla than within the duct)that allows the collecting duct to lose water by osmosis and concentrate the urine.

See Also

• Nephron