Reabsorptioon

Reabsorptioon, or reabsorption, is the physiological process by which substances filtered from the blood into the tubular fluid of the kidney are transported back into the bloodstream. It is essential for conserving water, electrolytes, and nutrients while forming urine.

Location and scope

Most reabsorption occurs in the nephron. About two-thirds of the filtrate is reabsorbed in the proximal tubule, including water, sodium, glucose, amino acids, and bicarbonate. The loop of Henle reabsorbs water in the descending limb and salts in the ascending limb, creating medullary osmotic gradients and continuing the reabsorption of certain ions. The distal tubule and collecting duct reabsorb additional sodium and water, with the final amount adjusted by hormonal control to meet the body's needs.

Mechanisms

Reabsorption involves a mix of active transport and passive diffusion. Sodium reabsorption is a key driver, often via Na+/K+ ATPase pumps on tubular cells that maintain the electrochemical gradient, enabling cotransport of glucose and amino acids in the proximal tubule. Water moves by osmosis through aquaporin channels, mainly in the proximal tubule and collecting ducts. Ion channels and transporters in the distal parts enable selective reabsorption or, in some cases, secretion.

Regulation

Hormones finely tune reabsorption. Antidiuretic hormone (vasopressin) increases water reabsorption in the collecting ducts. Aldosterone enhances sodium (and thus water) reabsorption in the distal tubule and collecting duct. Parathyroid hormone increases calcium reabsorption in the distal tubule. The process also depends on filtration rate, tubular flow, and the osmotic gradient within the renal medulla.

Clinical relevance

Impaired reabsorption can lead to dehydration, electrolyte disturbances, glycosuria, or altered acid-base balance. Reabsorption efficiency is a key determinant of urine concentration and overall fluid homeostasis.