reabsorbed

Reabsorbed is the past participle of the verb absorb. In general usage, it describes a process in which something that has been taken up or absorbed is taken back into another medium, or absorbed again after initial entry. In scientific writing, reabsorbed often refers to substances moving from a lumen, filtrate, or interstitial space into tissue or the bloodstream.

In physiology, reabsorption denotes the selective transfer of substances from filtrate back into the blood during fluid processing, such as urine formation. The kidneys reabsorb most of the filtered water and solutes, returning them to circulation. The proximal tubule reabsorbs roughly two-thirds of filtered water and solutes, including glucose, amino acids, sodium, and bicarbonate. The descending limb of the loop of Henle reabsorbs water, while the ascending limb reabsorbs ions. The distal tubule and collecting ducts fine-tune reabsorption, with hormones such as aldosterone increasing sodium reabsorption and antidiuretic hormone promoting water reabsorption via aquaporins. Substances reabsorbed from the tubular fluid enter the peritubular capillaries to rejoin systemic circulation; inadequate or excessive reabsorption can lead to dehydration, electrolyte imbalance, or related disorders.

Reabsorption also occurs in other biological contexts. In the gastrointestinal system, water and certain nutrients are reabsorbed from the lumen into the bloodstream, while enterohepatic circulation recycles bile acids through reabsorption in the ileum. In pharmacology, the extent of renal reabsorption influences drug excretion, with lipid-soluble substances more readily reabsorbed and urine pH affecting ion trapping. In neuroscience, some discussions refer to neurotransmitter reabsorption by presynaptic terminals or glial cells, though “reuptake” is the more common term.