Loop of Henle

In the kidney, the loop of Henle (or Henle's loop or ansa nephroni) is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discover F. G. J. Henle, the loop of Henle's main function is to create a concentration gradient in the medulla of the kidney.

By means of a countercurrent multiplier system, which utilizes sodium pumps, the loop of Henle creates an area of high sodium concentration deep in the medulla, near the collecting duct. Water present in the filtrate in the collecting duct flows through aquaporin channels out of the collecting duct, moving passively down its concentration gradient. This process reabsorbs water and creates a concentrated urine for excretion.

Structure
It can be divided into five parts:
 * Thick descending limb of loop of Henle -- The descending limb has low permeability to ions and urea, while being highly permeable to water.
 * Thin descending limb of loop of Henle -- The descending limb has low permeability to ions and urea, while being highly permeable to water.
 * Thin ascending limb of loop of Henle -- The thin ascending limb is not permeable to water, but it is permeable to ions.
 * Medullary thick ascending limb of loop of Henle -- Sodium (Na+), potassium (K+) and chloride (Cl-) ions are reabsorbed from the urine by active transport. K+ is passively transported along its concentration gradient through a K+ channel in the apical aspect of the cells, back into the lumen of the ascending limb. This K+ "leak" generates a positive electrochemical potential difference in the lumen. The electrical gradient drives more reabsorption of Na+, as well as other cations such as magnesium (Mg2+) and importantly calcium (Ca2+).
 * Cortical thick ascending limb -- The cortical thick ascending limb drains urine into the distal convoluted tubule.

The loop has a sharp bend in the renal medulla.

Blood supply
The loop of Henle is supplied by blood in a series of straight capillaries descending from the cortical efferent arterioles. These capillaries (called the vasa recta; recta is from the Latin for "straight") also have a countercurrent multiplier mechanism that prevents washout of solutes from the medulla, thereby maintaining the medullary concentration. As water is osmotically driven from the descending limb into the interstitium, it readily enters the vasa recta. The low bloodflow through the vasa recta allows time for osmotic equilibration, and can be altered by changing the resistance of the vessels' efferent arterioles.

Also, the vasa recta still has the large proteins and ions which were not filtered through the glomerulus, which provides an oncotic pressure for ions to enter the vasa recta from the interstitium.

The main function of the Loop of Henle is to set up a concentration gradient.