NEPHRON LOOP

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.[citation needed] 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.[citation needed] Structure It can be divided into five parts[citation needed]: 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.[citation needed] Blood supply Counter current multiplier diagram 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.[citation needed] 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.[citation needed] The main function of the Loop of Henle is to
set up a concentration gradient.