What is loop of henle?

Question

Safora Sibilla · Answer

The human kidney is made up of about a million nephrons, the filtering units of this complex and highly vascular organ. Each nephron is composed of a highly coiled tubule, one end of which forms a cup-shaped structure.

Inside this cup and forming a network around its walls, is a tuft of capillaries called a glomerulus, with a special fenestrated basement membrane.

Each glomerulus filters out water and solutes from the blood passing through it into the surrounding space, which is the cavity between the two walls of the cup. This part is wholly within the renal cortex.

The next part of the tubule is highly coiled (the proximal convoluted tubule) and empties into a U-shaped loop which carries the filtered fluid deep down into the medulla and then back again into the cortex. This part of the nephron is called the loop of Henle.

Its main function is to reabsorb water and sodium chloride from the filtrate. This conserves water for the organism, producing highly concentrated urine.

The loop of Henle has a thin descending limb, a thick ascending limb and a thin ascending limb.

The fluid entering the descending limb contains sodium chloride and other salts, urea and other chemicals that have been filtered out from the blood.

The cells here are permeable to water and thus the salt and urea concentration rises within the fluid by the time it reaches the bend.

The ascending limb is permeable to sodium chloride, which passes out of the tubule into the medullary tissue surrounding it.

The absorption of water within the descending limb leads to an increasing osmotic gradient within the tubule by increasing the solute concentration.

Simultaneously, the active transport of sodium chloride into the interstitium through the water-impermeable cells of the ascending limb maintains the gradient by ensuring that the concentration declines as the fluid passes towards the distal convoluted tubule.

The loop of Henle is supplied by two vasa recta which are straight vessels closely accompanying the tubule’s hairpin-shaped course.

These carry blood in opposite directions, just like the passage of tubular fluid – the countercurrent mechanism. Thus they absorb water on the one hand and solute on the other.

This is called the countercurrent multiplier system, which is responsible for maintaining an osmotic medullary gradient in the outer medullary tissue. This gradient ensures the vasopressin-driven reabsorption of water from the luminal fluid in the collecting duct.

The thick ascending limb expresses a sodium-potassium-chloride cotransporter and helps reabsorb approximately a third of the filtered sodium and chloride from the fluid in the tubular lumen into the blood.

Other functions of the loop of Henle include:

Vanick Cohan · Answer

In the kidney, the loop of Henle (English: /ˈhɛnli/) (or Henle's loop, Henle loop,[1] nephron loop[2] or its Latin counterpart ansa nephroni) is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer, the German anatomist Friedrich Gustav Jakob Henle, the loop of Henle's main function is to create a concentration gradient in the medulla of the kidney.[3]

By means of a countercurrent multiplier system, which uses electrolyte pumps, the loop of Henle creates an area of high urea concentration deep in the medulla, near the papillary duct in the collecting duct system. Water present in the filtrate in the papillary duct flows through aquaporin channels out of the duct, moving passively down its concentration gradient. This process reabsorbs water and creates a concentrated urine for excretion.[3]

The loop of Henle can be divided into four parts:

The tissue type of the loop is simple squamous epithelium. The "thick" and "thin" terminology does not refer to the size of the lumen, but to the size of the epithelial cells.[4] The loop is also sometimes called the Nephron loop.

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 capillaries. 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]

As well, blood in the vasa recta still has large proteins and ions which were not filtered through the glomerulus. This 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.[citation needed]

The descending loop of Henle receives isotonic (300 mOsm/L) fluid from the proximal convoluted tubule (PCT). The fluid is isotonic because as ions are reabsorbed by the gradient time system, water is also reabsorbed maintaining the osmolarity of the fluid in the PCT. Substances reabsorbed in the PCT include urea, water, potassium, sodium, chloride, glucose, amino acids, lactate, phosphate, and bicarbonate. Since water is also reabsorbed the volume of fluid in the loop of Henle is less than the PCT, approximately one-third of the original volume.

The interstitium of the kidney increases in osmolarity outside as the loop of Henle descends from 600 mOsm/L in the outer medulla of the kidney to 1200 mOsm/L in the inner medulla. The descending portion of the loop of Henle is extremely permeable to water and is less permeable to ions, therefore water is easily reabsorbed here and solutes are not readily reabsorbed. The 300 mOsm/L fluid from the loop loses water to the higher concentration outside the loop and increases in tonicity until it reaches its maximum at the bottom of the loop. This area represents the highest concentration in the nephron, but the collecting duct can reach this same tonicity with maximum ADH effect.[3]

The ascending limb of the loop of Henle receives an even lower volume of fluid and has different characteristics compared to the descending limb. In the ascending portion, the loop becomes impermeable to water and the cells of the loop actively reabsorb solutes from the luminal fluid; therefore water is not reabsorbed and ions are readily reabsorbed. As ions leave the lumen via the Na-K-2Cl symporter and the Na-H antiporter, the concentration becomes more and more hypotonic until it reaches approximately 100-150 mOsm/L. The ascending limb is also called the diluting segment of the nephron because of its ability to dilute the fluid in the loop from 1200 mOsm/L to 100 mOsm/L.[3]

Flow of the fluid through the entire loop of Henle is considered slow. As flow increases, the ability of the loop to maintain its osmolar gradient is reduced. The vasa recta (capillary loops) also have a slow flow as well. Increases in vasa recta flow wash away metabolites and cause the medulla to lose osmolarity as well. Increases in flow will disrupt the kidney's ability to form concentrated urine.[3]

Overall the loop of Henle reabsorbs around 25% of filtered ions and 20% of the filtered water in a normal kidney. These ions are mostly Na+, Cl−, K+, Ca2+ and HCO3−. The powering force is the Na/K ATPase on the basolateral membrane which maintains the ion concentrations inside the cells. On the luminal membrane, Na enters the cells passively; using the Na-K-2Cl symporter. Then the Na/K ATPase will pump 3 Na out into the peritubular fluid and 2 K into the cell on the non-lumen side of the cell. This gives the lumen of the fluid in the loop a positive charge in comparison and creates a Na concentration gradient, which both push more Na into the cell via the Na-H antiporter. The hydrogen ion for the antiporter comes from the enzyme carbonic anhydrase, which takes water and carbon dioxide and forms bicarbonate and a hydrogen ion. The hydrogen ion is exchanged for the Na in the tubular fluid of the loop of Henle.[3]

Wells Hammer · Answer