Tuesday, November 17, 2009

Loop of Henle

The loop of Henle is the specialized portion of the nephron which leads from the proximal convoluted tubule to the distal convoluted tubule. The main function of the Henle's loop is to filter solutes. The ascending limb of the loop of Henle transports solutes, such as Sodium Chloride (NaCl), out of the tubule lumen with little or no water, generating an hyperosmotic medullary interstitium and delivering an hyposmotic tubule fluid to the distal tubule. 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. This portion of the nephron was named after its discovery by the German physician Friedrich Gustav Jakob Henle.

The descending limb of the loop of Henle has low permeability to ions and urea, while being highly permeable to water. The thin ascending limb is not permeable to water, but it is permeable to ions. The medullary thick ascending limb remains impermeable to water with sodium, potassium (K+) and chloride (Cl-) ions being reabsorbed by active transport; K+ is passively transported along its concentration gradient through a K+ leak 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+.

The loop of Henle is supplied by blood in a series of straight capillaries descending from the cortical efferent arterioles. These capillaries also have a countercurrent exchange 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.