Tubular reabsorption
Also known as: renal reabsorption
Tubular reabsorption is the process by which the kidney's nephrons return filtered water and solutes from the tubular fluid back into the blood. It reclaims most of what the glomerulus filters, including nearly all glucose, amino acids, and sodium.
Tubular reabsorption is one of the three basic processes of urine formation, alongside glomerular filtration and tubular secretion. The glomerulus filters roughly 180 liters of plasma per day, yet urine output is only 1–2 liters — because the renal tubules reabsorb the overwhelming majority of the filtrate, moving water and solutes out of the tubular lumen and back into the peritubular capillaries.
Most reabsorption happens in the proximal convoluted tubule (PCT), which reclaims about two-thirds of filtered sodium and water and essentially 100% of filtered glucose and amino acids under normal conditions. Much of this transport is driven by the basolateral Na⁺/K⁺-ATPase, with glucose and amino acids carried across the apical membrane by sodium-coupled cotransporters. When plasma glucose exceeds the transporters' capacity — as in uncontrolled diabetes — glucose spills into the urine.
Downstream segments fine-tune the filtrate. The thick ascending limb of the loop of Henle reabsorbs sodium, potassium, and chloride but is impermeable to water; the distal convoluted tubule reabsorbs sodium and calcium; and the collecting duct adjusts final water and sodium recovery under hormonal control — antidiuretic hormone (ADH) inserts aquaporins to reabsorb water, while aldosterone promotes sodium reabsorption and potassium secretion.
Tubular reabsorption is high-yield renal physiology on the USMLE Step 1 exam. Expect questions on where each solute is reabsorbed, transporter mechanisms and their diuretic targets, glucose handling and the transport maximum, and how ADH and aldosterone shape the final urine.
Key takeaways
- Tubular reabsorption returns filtered water and solutes from the nephron's tubular fluid to the peritubular capillaries.
- The proximal convoluted tubule reabsorbs about two-thirds of filtered sodium and water and normally all glucose and amino acids.
- Glucose appears in urine when plasma levels exceed the sodium-glucose transporters' capacity, as in diabetes.
- ADH and aldosterone regulate water and sodium reabsorption in the distal nephron and collecting duct.
- USMLE Step 1 tests segment-by-segment reabsorption, transporters, and hormonal regulation.