Loop of Henle


Loop of Henle
Loop of Henle
Gray1128.png
Scheme of renal tubule and its vascular supply. (Loop of Henle visible center-left.)
Latin ansa nephroni
Gray's subject #253 1223
Precursor Metanephric blastema
MeSH 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 discoverer F. G. J. Henle, the loop of Henle's main function is to create a concentration gradient in the medulla of the kidney.[1]

By means of a countercurrent multiplier system, which utilizes electrolyte pumps, the loop of Henle creates an area of high urine 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.[1]

Contents

Anatomy

It can be divided into five parts:

The descending limbs both thick and thin have low permeability to ions and urea, while being highly permeable to water. The loop has a sharp bend in the renal medulla going from descending to ascending thin limb.
The thin ascending limb is not permeable to water, but it is permeable to ions.
Sodium (Na+), potassium (K+) and chloride (Cl-) ions are reabsorbed from the urine by passive transport by the Na-K-2Cl symporter (NKCC2). The electrical and concentration gradient drives more reabsorption of Na+, as well as other cations such as magnesium (Mg2+) and importantly calcium (Ca2+).

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.[citation needed]

Physiology

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 looses 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.[1]

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, water become impermeable and the cells of the loop actively reabsorb solutes, therefore water is not reabsorbed and ions are readily reabsorbed. As ions leave 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.[1]

Flow of the fluild 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 loose osmolarity as well. Increases in flow will disrupt the kidney's ability to form concentrated urine.[1]

Overall the loop of Henle resorbs about 25% of filtered ions and 15% of the filtered water in a normal kidney. These ions are mostly Na, Cl, K, Ca 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 utilizing 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 for the antiporter come from the enyzme carbonic anhydrase which takes water and carbon dioxide and forms bicarbonate and hydrogen. The hydrogen is exchanged for the Na in the tubular fluid of the loop of Henle.[1]

Additional images

Notes

  1. ^ a b c d e f g Dunn R. B.; Kudrath W., Passo S.S., Wilson L.B. (2011). "8" (in English). Kaplan USMLE Step 1 Physiology Lecture Notes. p. 209-223. 

Further reading

External links


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Look at other dictionaries:

  • loop of Henle — [hen′lē] n. pl. loops of Henle the long U shaped section of a nephron in the kidney, where urine is formed …   English World dictionary

  • loop of Henle — /hen lee/, Anat. the part of a nephron between the proximal and distal convoluted tubules that extends, in a loop, from the cortex into the medulla of the kidney. Also called Henle s loop. [1880 85; named after F. G. J. Henle (1809 85), German… …   Universalium

  • loop of Henle — loop of Hen·le hen lē n the U shaped part of a vertebrate nephron that lies between and is continuous with the proximal and distal convoluted tubules, that leaves the cortex of the kidney descending into the medullary tissue and then bending back …   Medical dictionary

  • loop of Henle — loop′ of Hen′le [[t]ˈhɛn li[/t]] n. anat. the part of a kidney tubule that loops from the cortex into the medulla of the kidney • Etymology: 1880–85; after F. G. J. Henle (1809–85), German pathologist …   From formal English to slang

  • loop of henle — ˈhenlē, lə Usage: usually capitalized H Etymology: after F. G. J. Henle died 1885 German pathologist : the part of a vertebrate nephron that lies between the proximal and distal convoluted tubules, leaves the cortex of the kidney descending into… …   Useful english dictionary

  • loop of Henle — U shaped loop in the kidney discovered by Friedrich Gustav Jacob Henle …   English contemporary dictionary

  • loop of Henle — Etymology: F. G. J. Henle died 1885 German pathologist Date: 1890 a U shaped part of the nephron of birds and mammals that lies between and is continuous with the proximal and distal convoluted tubules and that functions in water resorption …   New Collegiate Dictionary

  • loop of Henle — [ hɛnli] noun the part of a kidney tubule from which water and salts are resorbed into the blood. Origin C19: named after the German anatomist F. G. J. Henle …   English new terms dictionary

  • Descending limb of loop of Henle — Scheme of renal tubule and its vascular supply. (Labeled at center left.) …   Wikipedia

  • Thick ascending limb of loop of Henle — Scheme of renal tubule and its vascular supply. (Labeled at center left.) …   Wikipedia