In the kidney, filtration begins at which structure?

• A. Renal pelvis
• B. Glomerulus
• C. Loop of Henle
• D. Collecting duct

Answer: (B)

Filtration begins at the glomerulus, a tuft of capillaries inside the renal corpuscle. Blood pressure forces water and small solutes across the filtration barrier—composed of fenestrated endothelium, the basement membrane, and podocyte slit diaphragms—into Bowman's space, creating the filtrate. Larger proteins and cells stay in the bloodstream. From there, the filtrate moves on for reabsorption and secretion in the rest of the nephron. The other structures are involved later in urine handling (renal pelvis collects urine; Loop of Henle reabsorbs water and salts; collecting duct concentrates urine and adjusts final composition).