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Reabsorption is a biological process in which substances are transported from a filtrate or lumen back into the bloodstream or surrounding tissues. In humans, the term is often used for the reclamation of water, ions, and nutrients from the fluid that has been filtered by the kidneys, helping to conserve essential substances and regulate fluid balance.

In the kidney, reabsorption occurs along the tubular system after glomerular filtration. About 65–70% of filtered water and a large share of sodium, bicarbonate, glucose, and amino acids are reabsorbed in the proximal tubule. The loop of Henle reabsorbs additional solutes and water in a segment-dependent manner: the descending limb is permeable to water, while the ascending limb actively reabsorbs ions but is impermeable to water. The distal convoluted tubule and collecting ducts fine-tune reabsorption, with hormonal control determining how much sodium and water are reabsorbed. Aldosterone stimulates sodium reabsorption (raising blood volume and pressure), and antidiuretic hormone (ADH) increases water reabsorption in the collecting ducts.

Mechanisms of reabsorption include transcellular transport, driven by carrier proteins and ion pumps, and paracellular pathways, where substances pass between cells. Water follows solutes osmotically through aquaporin channels. Reabsorption is contrasted with secretion, a process by which substances move from blood into the tubular fluid, collectively shaping the final composition of urine.

Outside the kidneys, reabsorption occurs in other tissues, including the gastrointestinal tract, where nutrients and fluids can be reabsorbed, and in various epithelial systems. Clinically, impaired reabsorption can contribute to dehydration, electrolyte disturbances, or metabolic imbalances, while excessive reabsorption can affect blood pressure and volume.

Overall, reabsorption is a key mechanism for conserving vital resources and maintaining homeostasis in the body's internal environment.