Which portion of the kidney reabsorbs the majority of glomerular ultrafiltrate?

• A. Descending limb
• B. Distal tubule
• C. Proximal convoluted tubule
• D. Ascending limb

Answer: (C)

Most of the filtrate is reabsorbed in the proximal convoluted tubule because this segment has the highest transport capacity, with a vast brush border and many transporters that reclaim Na+, glucose, amino acids, bicarbonate, and water. About two-thirds of the filtered load is reabsorbed here, and water follows solutes osmotically through aquaporin channels, so the filtrate leaving this tubule remains nearly isosmotic to plasma. The descending limb is highly permeable to water and concentrates the filtrate but reabsorbs few solutes, while the ascending limb pumps out solutes like Na+, K+, and Cl− but is impermeable to water, diluting the filtrate. The distal tubule and collecting ducts fine-tune reabsorption and excretion more selectively. Therefore, the proximal convoluted tubule reabsorbs the majority of glomerular ultrafiltrate.