Renal Physiology: Reabsorption and Secretion

Renal Physiology: Reabsorption and Secretion

Overview

  • This lecture focuses on reabsorption and secretion processes in the nephron tubule, following filtration in the glomerulus.

  • Reabsorption is crucial, reclaiming most of the 180 liters of filtrate back into the bloodstream, leaving only 1-2 liters as urine.

Reabsorption Locations

  • Proximal Tubule: Reclaims most nutrients (approximately 65% of water).

  • Nephron Loop (Loop of Henle):

    • Descending Limb: Primarily responsible for water reabsorption.

    • Ascending Limb: Primarily responsible for salt reabsorption.

  • Distal Tubule & Collecting Duct: Facultative reabsorption of sodium and water based on the body's needs.

Reabsorption and Secretion

  • Reabsorption and secretion occur in the tubules, while filtration occurs in the glomerulus.

Sodium Reabsorption Mechanism in Proximal Tubule

  • Sodium-Potassium Pump (Primary Active Transport):

    • Located on the basolateral side of the tubule cells.

    • Pumps sodium out of the cell, creating a low intracellular sodium concentration.

    • This establishes an electrochemical gradient favoring sodium entry from the filtrate.

    • Requires ATP.

    • Na^+/K^+

  • Secondary Active Transport (Symporter):

    • Located on the apical side (facing the filtrate).

    • Utilizes the sodium gradient created by the sodium-potassium pump.

    • Sodium moves down its concentration gradient, pulling other molecules (e.g., glucose) along with it.

    • Symporter moves two molecules in the same direction.

Countercurrent Multiplication

  • Establishes a medullary concentration gradient, crucial for concentrating urine.

  • Process:

    1. Salt (NaCl) is actively transported out of the ascending limb into the interstitial fluid.

    2. This increases the osmolarity of the interstitial fluid, drawing water out of the descending limb via osmosis.

    3. The loss of water increases the osmolarity of the filtrate in the descending limb.

    4. The increased filtrate osmolarity in the descending limb, now entering the ascending limb, facilitates further salt (NaCl) extraction from the ascending limb.

  • The cycle repeats, progressively increasing the concentration gradient in the medulla.

  • One process causes the other, which causes the other, which causes the other. Again, that's why it's called countercurrent multiplication.

  • Concentration gradient inside promotes water and salt to leave, which promotes concentration inside and so forth/

Impact of Loop Diuretics

  • Loop diuretics impede the establishment of this concentration gradient.

  • By increasing glucose concentration, water will have a harder time leaving.

Collecting Duct and ADH

  • The collecting duct's permeability to water is regulated by antidiuretic hormone (ADH).

  • ADH Mechanism:

    • In the presence of ADH, aquaporins (water channels) are inserted into the apical membrane of collecting duct cells.

    • This increases water reabsorption from the filtrate back into the bloodstream.

    • Results in concentrated urine (reduced volume, high osmolarity).

    • Without ADH, the collecting duct is less permeable to water, leading to dilute urine.

  • Conditions Leading to Increased ADH Release:

    • Low blood volume.

    • Low blood pressure.

    • Dehydration.

    • Bleeding.

Atrial Natriuretic Peptide (ANP)

  • ANP is released by the heart in response to high blood volume or high blood pressure.

  • Effects of ANP:

    • Inhibits sodium reabsorption in the distal tubule and collecting duct.

    • Increases glomerular filtration rate (GFR).

    • Overall effect: promotes sodium and water excretion, decreasing blood volume and blood pressure.