The loop of Henle is a section of the nephron that leads from the proximal convoluted tubule to the distal convoluted tubule in the kidney. It is named after its discoverer, F. G. J. Henle. The loop has a hairpin bend in the medulla. Its primary function uses a countercurrent mechanism in the medulla to reabsorb water and ions from the urine.

It can be divided into four parts:

• Descending limb
• Thin ascending limb
• Medullary thick ascending limb
• Cortical thick ascending limb

Descending Limb

The descending limb has low permeability to ions and urea, while being highly permeable to water. The medullary interstitium is highly concentrated, 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 urine 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.

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 capillaries descending from the renal cortex. These capillaries (called vasa recta) also have a countercurrent mechanism that prevents washout of solutes from the medulla, thereby maintaining the medullary concentration. As water osmoses out in the descending limb, it is readily absorbed back into the vasa recta.

See Also

• Nephron

External links

Loop of Henle physiology