Where does most reabsorption occur in the nephron?

• A. Proximal convoluted tubule
• B. Distal convoluted tubule
• C. Loop of Henle
• D. Collecting duct

Answer: (A)

Most reabsorption happens in the proximal tubule because it has the greatest capacity for reclaiming filtered substances. The proximal tubule is lined with a tall brush border of microvilli, which massively increases surface area for transport. It also has many mitochondria to power active transport. Sodium reabsorption here drives most of the process: Na+ is actively pumped out of the tubule cells by the basolateral Na+/K+ ATPase, creating a favorable gradient for Na+ to be reabsorbed from the filtrate via transporters like the Na+/H+ exchanger and other co-transporters. Water follows this solute movement, so a large portion of filtered water is reabsorbed in the same regions, often while the filtrate remains isosmotic with blood.

Because of these features, the proximal tubule reclaims about two-thirds of the filtered Na+ and water, and it reabsorbs nearly all filtered glucose and amino acids, as well as a large share of bicarbonate. The loop of Henle continues reabsorption, especially concentrating urine and handling ions in its ascending limb, while the distal tubule and collecting duct handle smaller, hormonally regulated adjustments. This combination makes the proximal tubule the primary site of reabsorption.