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Loop of Henle
'
Scheme of renal tubule and its vascular supply. (Loop of
Henle visible center-left.)
Latin
ansa nephroni
Gray's
subject #253 1223
Precursor
MeSH
Metanephric blastema
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
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1 Anatomy
2 Blood supply
3 Physiology
4 Additional images
5 Notes
6 Further reading
7 External links
Anatomy
It can be divided into five parts:
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Thin descending limb of loop of Henle
The thin descending limb has 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.
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Thin ascending limb of loop of Henle
The thin ascending limb is not permeable to water, but it is permeable to ions.
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Thick ascending limb of loop of Henle
Sodium (Na+), potassium (K+) and chloride (Cl-) ions are reabsorbed from the urine by
secondary active 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+).
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Cortical thick ascending limb—The cortical thick ascending limb drains urine into the
distal convoluted tubule.[1]
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, .
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.[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, the loop
becomes impermeable to water 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 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.[1]
Overall the loop of Henle resorbs about 25% of filtered ions and 20% 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]