Which structure serves as the filtration unit at the top of each nephron?

Bowman's capsule is a crucial component of the nephron, the functional unit of the kidney, serving as the initial site for the filtration of blood. It is a cup-like structure that surrounds the glomerulus, a network of capillaries, and plays a key role in the process of urine formation.

When blood flows into the glomerulus, the high pressure within this capillary network forces water, ions, and small molecules from the blood into Bowman's capsule, effectively filtering out waste products and excess substances. This filtered fluid, known as filtrate, then moves into the renal tubule, where further processing occurs to reabsorb necessary substances back into the bloodstream and remove wastes.

By contrast, structures like the collecting duct and renal pelvis have different functions in the renal system. The collecting duct is primarily involved in the reabsorption of water and salts and the concentration of urine. The renal pelvis serves as a funnel for urine that drains from the collecting ducts into the ureter. The glomerulus, while essential for filtering blood, is not a structure that collects the filtrate; that role is specifically fulfilled by Bowman's capsule. This distinction highlights why Bowman's capsule is identified directly as the filtration unit in this context.