Which structure in the nephron is primarily involved in filtration?

Bowman's capsule is the structure in the nephron that is primarily involved in the filtration process. It surrounds the glomerulus, a network of capillaries where blood is first filtered. During this filtration, blood pressure forces water, ions, and small molecules from the blood into Bowman's capsule, forming a filtrate that will eventually become urine. Importantly, larger molecules like proteins and blood cells are typically too big to pass through the filtration barrier, ensuring that the filtrate contains mostly small solutes.

The other structures mentioned play different roles in the nephron's function. The proximal convoluted tubule is primarily involved in reabsorption, taking back valuable substances from the filtrate back into the bloodstream. The distal convoluted tubule also plays a role in reabsorption and secretion, adjusting the composition of the fluid as it moves towards the collecting duct. The collecting duct is mainly responsible for the final concentration and reabsorption of water, influenced by hormones like antidiuretic hormone (ADH). Thus, while all these structures are important in nephron function, Bowman's capsule is the key site for the initial step of filtration.