Vattenreabsorption

Vattenreabsorption (water reabsorption) is the physiological process by which water is reabsorbed from the filtrate in the kidney nephrons back into the bloodstream. It is essential for maintaining body water balance, urine concentration, and serum osmolarity.

Most water reabsorption occurs in the proximal tubule, where about two-thirds to three-quarters of filtered water follows solutes via paracellular pathways and aquaporin-1 channels. The descending limb of the loop of Henle is highly permeable to water, allowing it to exit the tubule as filtrate is concentrated; the ascending limb, in contrast, is impermeable to water.

In the distal nephron and collecting ducts, water reabsorption is variable and tightly regulated by antidiuretic hormone (ADH, vasopressin). When ADH is present, aquaporin-2 channels are inserted into the apical membrane of principal cells, increasing water permeability and enabling substantial reabsorption to concentrate urine. Without ADH, these segments reabsorb little water, producing dilute urine.

Regulation of water reabsorption involves osmotic and volume cues sensed by osmoreceptors in the hypothalamus and by baroreceptors. ADH release is stimulated by increased plasma osmolality and decreased blood volume. Aldosterone indirectly influences water reabsorption by promoting sodium reabsorption, which affects osmotic water movement, while atrial natriuretic peptide promotes water and sodium excretion.

Clinical relevance includes disorders of water handling such as diabetes insipidus (deficient or ineffective ADH leading to excessive water loss) and SIADH (excess ADH causing hyponatremia). Normal total water reabsorption is about 99% of filtered water; final urine osmolality adapts to hydration and ADH status, ranging from very dilute to highly concentrated urine.