GLOSSARY

The loop of Henle is a U-shaped part of the nephron that dips from the cortex into the medulla and returns to the cortex. Its main job is to create the medullary osmotic gradient, which is essential for concentrating urine; about 25 percent of filtered sodium is reabsorbed here.

Segments and Features

• Thin descending limb: permeable to water (via aquaporin-1) and impermeable to salt; as it descends, water leaves and the tubular fluid becomes more concentrated.
• Thin ascending limb: impermeable to water and permeable to salt (passive transport).
• Thick ascending limb: impermeable to water, with active salt reabsorption through the NKCC2 transporter (sodium, potassium and two chlorides); the tubular fluid becomes dilute, which is why this segment is called the “diluting segment”.

Countercurrent Mechanism

The opposite flow in the two limbs, together with salt reabsorption in the