Physiology of the Genitourinary System II: Tubular Reabsorption and Secretion

The kidneys produce approximately 180 liters of filtrate daily through glomerular filtration. About 99% of this filtrate is reabsorbed back into the bloodstream, with the proximal convoluted tubule reclaiming roughly 80% of the filtrate. This highly efficient reabsorption process recovers essential substances like glucose, amino acids, electrolytes, and water, leaving only waste products and excess ions to form urine.

The proximal convoluted tubule reabsorbs nearly all glucose, amino acids, and small proteins, along with sodium and water through active and passive transport mechanisms. The sodium-potassium adenosine triphosphatase pump plays a crucial role in driving sodium reabsorption, which facilitates the movement of other solutes. This segment reclaims approximately 80% of the total filtrate, making it the most efficient reabsorption site in the nephron.

In the descending limb, water passively moves out due to high solute concentration in the surrounding medulla, concentrating the filtrate. The ascending limb actively transports sodium and chloride ions into the interstitium while remaining impermeable to water, diluting the filtrate. This progressive amplification creates a steep osmotic gradient between the cortex and medulla, essential for water reabsorption in the collecting ducts regulated by antidiuretic hormone.

Antidiuretic hormone enhances the permeability of the distal convoluted tubule and collecting ducts to water, promoting water reabsorption during dehydration. This hormone-regulated mechanism allows the kidneys to conserve water by increasing its reabsorption back into the bloodstream. ADH works in conjunction with the osmotic gradient established by the loop of Henle to fine-tune urine concentration and maintain body fluid balance.

Aldosterone promotes sodium reabsorption while enhancing potassium secretion in the distal convoluted tubule and collecting ducts. This hormone-driven process helps maintain electrolyte balance by reclaiming sodium back into the bloodstream and removing excess potassium into the urine. Aldosterone works alongside antidiuretic hormone to fine-tune both electrolyte and water balance in the final stages of urine formation.

Tubular secretion is the active transport of substances from peritubular capillaries into the tubular lumen, removing excess ions, waste products, and other compounds not adequately filtered by the glomerulus. Secreted substances include potassium, hydrogen ions, ammonia, uric acid, and drugs. This process is essential for maintaining acid-base balance, electrolyte balance, and excreting waste products that contribute to urine formation.

The distal convoluted tubule and collecting ducts make final adjustments to water, sodium, and potassium levels through hormone-regulated reabsorption and secretion. Antidiuretic hormone increases water permeability during dehydration, while aldosterone enhances sodium reabsorption and potassium secretion. These processes produce final urine containing waste products, excess ions, and water in concentrations appropriate for body homeostasis.