Scheme of renal tubule and its vascular supply. (Loop of Henle visible center-left.)
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In the kidney, the loop of Henle is the portion of the nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. It is named after its discoverer, F. G. J. Henle. The loop has a hairpin bend in the renal medulla. Its primary function uses a countercurrent multiplier mechanism in the medulla to reabsorb water and ions from the urine.
The descending limb has low permeability to ions and urea, while being highly permeable to water. The medullary interstitium is highly concentrated (because of the activity of the ascending limb), leading to a strong osmotic gradient from the descending limb to the medulla. Water is readily reabsorbed from the descending limb by osmosis, increasing the concentration of the urine. Osmolality can reach up to 1200 mOsmol/kg by the end of the descending limb.
Thin ascending limb
As the filtrate passes back up the thin ascending limb, the concentration of the surrounding medulla decreases. The thin ascending limb is not permeable to water, but it is permeable to ions. Sodium ions (Na+) diffuse along this concentration gradient, from the thin ascending limb into the medulla.
Medullary thick ascending limb
The medullary thick ascending limb remains impermeable to water. Sodium, potassium (K+) and chloride (Cl-) ions are reabsorbed by active transport. K+ diffuses along its concentration gradient, back into the lumen of the ascending limb. This 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+).
Loop diuretics block the K+/Na+/2Cl- co-transporter.
Urea which remains in the loop creates a solute potential that prevents water completely osmosing out into the interstitial space. This means that while almost all the ions are reabsorbed, there will still be some water in the urine, and hence, the concentration of the filtrate in the loop is decreased here. (If only ions were present, and a certain amount of ions were reabsorbed, one would expect the same amount of the water to be reabsorbed too, and hence the concentration would remain the same, but this is not true.)
Cortical thick ascending limb
The difference between the medullary and cortical thick ascending limbs is mainly anatomical. Functionally, they are very similar.
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 recta; recta is from the Latin for "straight") also have a countercurrent exchange 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.
