reabsorbing

Reabsorbing, in a biological context usually called reabsorption, is the process by which substances are taken back into the bloodstream after an initial filtration or secretion. It is central to fluid and electrolyte balance in animals and is most prominently demonstrated in the kidneys and the digestive tract.

In the kidney, filtered fluid (the glomerular filtrate) travels through the nephron’s tubules. Along these segments, water, ions, and nutrients are transported from the tubule fluid back into the bloodstream. Water reabsorption occurs by osmosis, while sodium reabsorption is primarily active and builds the gradients that power the reabsorption of other substances. Glucose, amino acids, and bicarbonate are normally reabsorbed almost completely. The kidneys also reabsorb calcium, phosphate, and urea to varying degrees. Hormones regulate reabsorption: antidiuretic hormone (ADH) increases water reabsorption in the collecting ducts; aldosterone promotes sodium (and thus water) reabsorption; parathyroid hormone modulates calcium and phosphate reabsorption.

In the digestive system, reabsorption of water and electrolytes occurs along the small and large intestines, contributing to the concentration of waste and overall hydration.

Clinical relevance: disruptions in reabsorption can cause dehydration, electrolyte disturbances, or urinary loss of nutrients (as in diabetes mellitus when glucose is excreted).

Understanding reabsorption is essential for kidney physiology, fluid balance, and pharmacology, since many drugs modify or exploit tubular reabsorption.