Loop of Henle
Encyclopedia
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.
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.
; 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.
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.
The main function of the Loop of Henle is to set up a concentration gradient.
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.
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.
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.
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.
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
, the loop of Henle (or Henle's loop or ansa nephroni) is the portion of a nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....
that leads from the proximal convoluted tubule to the distal convoluted tubule
Distal convoluted tubule
The distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system.- Physiology :It is partly responsible for the regulation of potassium, sodium, calcium, and pH...
. Named after its discoverer F. G. J. Henle
Friedrich Gustav Jakob Henle
Friedrich Gustav Jakob Henle was a German physician, pathologist and anatomist. He is credited with the discovery of the loop of Henle in the kidney. His essay "On Miasma and Contagia" was an early argument for the germ theory of disease...
, the loop of Henle's main function is to create a concentration gradient in the medulla of the kidney.
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
Aquaporin
Aquaporins are proteins embedded in the cell membrane that regulate the flow of water.Aquaporins are integral membrane proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells....
channels out of the collecting duct, moving passively down its concentration gradient. This process reabsorbs water and creates a concentrated urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
for excretion.
Anatomy
It can be divided into five parts:- Thick descending limb of loop of HenleDescending limb of loop of HenleThe descending limb of loop of Henle is the portion of the renal tubule constituting the first part of the loop of Henle.-Physiology:The permeability is as follows:...
- Thin descending limb of loop of HenleDescending limb of loop of HenleThe descending limb of loop of Henle is the portion of the renal tubule constituting the first part of the loop of Henle.-Physiology:The permeability is as follows:...
- The descending limbs both thick and thin have low permeability to ions and ureaUreaUrea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
, while being highly permeable to water. The loop has a sharp bend in the renal medullaRenal medullaThe renal medulla is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the arcuate arterioles. The arcuate arterioles each in turn branch into...
going from descending to ascending thin limb.- Thin ascending limb of loop of HenleThin ascending limb of loop of HenleThe thin ascending limb of loop of Henle is a sub-portion of the loop of Henle in the juxtamedullary nephron of the kidney. The thin ascending limb is impermeable to water, and is also permeable to ions in particular Na and Cl...
- Thin ascending limb of loop of Henle
- The thin ascending limb is not permeable to water, but it is permeable to ions.
- Thick ascending limb of loop of HenleThick ascending limb of loop of HenleThe thick ascending limb of loop of Henle also known as distal straight tubule, is a segment of the nephron in the kidney. It can be divided into two parts: that in the renal medulla, and that in the renal cortex.-Medullary thick ascending limb:...
- Thick ascending limb of loop of Henle
- Sodium (Na+), potassiumPotassiumPotassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
(K+) and chlorideChlorideThe chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
(Cl-) ions are reabsorbed from the urine by passive transportPassive transportPassive transport means moving biochemicals and other atomic or molecular substances across membranes. Unlike active transport, this process does not involve chemical energy, because, unlike in an active transport, the transport across membrane is always coupled with the growth of entropy of the...
by the Na-K-2Cl symporter (NKCC2). The electrical and concentration gradient drives more reabsorption of Na+, as well as other cations such as magnesiumMagnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
(Mg2+) and importantly calciumCalciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
(Ca2+).- Cortical thick ascending limb -- The cortical thick ascending limb drains urine into the distal convoluted tubuleDistal convoluted tubuleThe distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system.- Physiology :It is partly responsible for the regulation of potassium, sodium, calcium, and pH...
.
- Cortical thick ascending limb -- The cortical thick ascending limb drains urine into the distal convoluted tubule
Blood supply
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 rectaVasa recta
In the blood supply of the kidney, the vasa recta renis form a series of straight capillaries in the medulla...
; 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.
Also, the vasa recta still has the large proteins and ions which were not filtered through the glomerulus, which provides an oncotic pressure
Oncotic pressure
Oncotic pressure, or colloid osmotic pressure, is a form of osmotic pressure exerted by proteins in blood plasma that usually tends to pull water into the circulatory system.Throughout the body, dissolved compounds have an osmotic pressure...
for ions to enter the vasa recta from the interstitium.
The main function of the Loop of Henle is to set up a concentration gradient.
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.
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.
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.
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
Carbonic anhydrase
The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...
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.