Reabsorption

Reabsorption is a physiological process in which substances that have been filtered from the blood into the filtrate are transported back into the bloodstream. In the human body, reabsorption is essential for reclaiming water, electrolytes, nutrients, and other useful substances that would otherwise be lost in urine. While the term applies to various organ systems, it is most extensively studied in renal physiology, where the kidney reabsorbs the majority of filtered fluid.

Within the kidney, reabsorption occurs along the nephron tubules. The proximal tubule reclaims about the majority of filtrate volume, including most of the water, sodium, bicarbonate, glucose, and amino acids, largely by active and co-transport mechanisms and by osmosis. The loop of Henle establishes a countercurrent osmotic gradient that enables concentration of urine. The distal convoluted tubule and collecting ducts perform fine-tuning of reabsorption under hormonal control.

Transport mechanisms include transcellular routes, using membrane transport proteins such as sodium-glucose cotransporters (SGLTs) and various exchangers, coupled to the basolateral Na+/K+ ATPase; and paracellular pathways through tight junctions. Water reabsorption follows osmotic gradients, largely in response to solute reabsorption. Hormonal regulation by aldosterone, antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP) modulates sodium and water reabsorption in the distal nephron and collecting ducts.

Clinical relevance: defects in reabsorption can lead to dehydration, electrolyte disturbances, and impaired kidney function. Diuretics act by inhibiting reabsorption in specific nephron segments (for example, loop diuretics block the NKCC2 transporter in the thick ascending limb; thiazide diuretics inhibit NaCl reabsorption in the distal tubule). In diabetes treatment, SGLT2 inhibitors reduce glucose reabsorption in the proximal tubule to promote glycosuria.