BIOL111-R8.3-UrinarySystem-TubularResorptionAndSecretion

Tubular Reabsorption and Secretion

• Introduction to Urine Formation

  • Urine formation processes involve several interactions within the nephron structure.

  • Key components: Glomerulus, Efferent and Afferent arterioles, PCT (Proximal Convoluted Tubule), DCT (Distal Convoluted Tubule), Collecting ducts, etc.

Urine Formation Processes

• Glomerular Filtration

  • Occurs in the glomerulus where blood substances move into the capsular space.

• Tubular Reabsorption

  • Involves the movement of substances from the tubular fluid back into the blood.

  • Powered by diffusion, osmosis, and active transport processes.

  • Vital solutes and most water are reabsorbed; excess solutes and waste remain in tubular fluid.

• Tubular Secretion

  • The movement of substances from the blood into the tubular fluid, usually through active transport.

Transport Processes

• Types of Transport

  • Transcellular Transport: Across epithelial cells through the basolateral and luminal membranes.

  • Paracellular Transport: Between epithelial cells.

• Renal Mechanisms

  • Transport maximum (Tm): Maximum rate substances can be reabsorbed or secreted based on the number of transport proteins.

  • Renal threshold: Max concentration of a substance in blood without appearing in urine.

Mechanisms of Substance Movement

• Nutrients

  • Completely reabsorbed with specific transport proteins; e.g., glucose reabsorbed via secondary active transport and facilitated diffusion.

• Proteins

  • Small/medium proteins transported from PCT back to blood via endocytosis and facilitated diffusion.

• Sodium

  • Reabsorbed along the entire tubule; primarily in PCT through facilitated diffusion and active transport.

  • Hormonal influence:

    • Aldosterone: Increases Na+ reabsorption.

    • ANP: Inhibits Na+ reabsorption.

• Water

  • Reabsorbed by osmosis; controlled by hormones:

    • Aldosterone: Increases Na+ and water reabsorption.

    • ADH: Increases number of aquaporins for more water reabsorption.

• Potassium

  • Both reabsorbed and secreted, dependent on Na+ movement.

  • Secreted in response to aldosterone levels.

• Calcium and Phosphate

  • Regulated by PTH; inhibits PO43- reabsorption at PCT, stimulates Ca2+ reabsorption at DCT to increase blood Ca2+ levels.

• Bicarbonate and Hydrogen Ions

  • HCO3- moves freely; urine and blood pH regulated in collecting tubules.

• Urea

  • A nitrogenous waste product: both reabsorbed and secreted; eliminated with drugs and other metabolic wastes.

Concentration Gradient

• Establishment of a concentration gradient in the interstitial fluid surrounding renal tubules using solutes (e.g., Na+, Cl-).

• Countercurrent Multiplier: Establishes concentration gradient mainly involving juxtamedullary nephrons.

  • Involves,

    • Descending Limb: Permeable to water and impervious to salts.

    • Ascending Limb: Impermeable to water and permeable to salts.

• Countercurrent Exchange: Maintains concentration gradients through vasa recta.

• Urea Recycling: Contributes to interstitial fluid concentration, accounting for 50% of solute gradient.