What is Loop of Henle?

• A. A filtering unit of the kidney that begins urine formation
• B. Part of the glomerular tubule responsible for sodium & water balance in kidneys
• C. A duct that stores urine
• D. A hormone-secreting gland

Answer: (B)

Loop of Henle is a segment of the nephron that handles reabsorption of water and ions to regulate urine concentration and sodium balance. The descending limb is highly permeable to water, so as filtrate moves downward, water leaves and is reabsorbed, concentrating the filtrate. The ascending limb actively reabsorbs sodium, potassium, and chloride but is relatively impermeable to water, which dilutes the filtrate and helps create the medullary osmotic gradient. This arrangement, part of the countercurrent multiplier system, is essential for concentrating urine and maintaining electrolyte and fluid balance. It is not the site where filtration begins (that’s the glomerulus), not a urine storage duct (urine is stored in the bladder/collecting system), and not a hormone-secreting gland.