återabsorption

Återabsorption, or reabsorption, is the process by which a substance that has been absorbed or filtered is moved back into its original medium. The term is used in physiology, spectroscopy, and radiative-transfer contexts. Reabsorption reduces net loss of material or energy and can affect concentrations, spectra, or fluxes.

In physiology, kidneys reabsorb substances from filtrate as it passes through the nephron. The proximal tubule reclaims most water, sodium, glucose, and amino acids back into the bloodstream, mainly by active transport and osmosis. The distal nephron and collecting ducts fine-tune water and electrolyte balance, with hormones such as antidiuretic hormone increasing water reabsorption and aldosterone increasing sodium reabsorption. Reabsorption preserves nutrients and helps maintain fluid and electrolyte homeostasis.

In radiative transfer and spectroscopy, reabsorption describes photons that are reabsorbed after emission or scattering by matter. Self-absorption can weaken spectral lines, alter line profiles, and increase optical depth, influencing models of stellar and planetary atmospheres and laboratory plasmas.

Reabsorption contrasts with excretion or adsorption. It is a consideration in medical therapy, optical materials, and detector design, where reabsorption can affect efficiency or accuracy.