Understanding Proximal Convoluted Tubules

Sep 5, 2024

Lecture on Proximal Convoluted Tubules

Introduction

  • Continuation from previous lecture on glomerular filtration.
  • Brief recap: filtration of water, electrolytes (Na, K, Cl, Ca, Mg), nutrients (glucose, amino acids, vitamins), lipids, and small proteins.

Osmolality

  • Defined as particles per kilogram of solvent.
  • Blood osmolality: 300 milliosmoles per liter.

Tubular Processes

  • Tubular Secretion: Active process (requires ATP), moves substances from blood to kidney tubules.
  • Tubular Reabsorption: Can be active or passive, moves substances from kidney tubules back into the blood.

Mechanisms of Reabsorption

  • Sodium-Potassium ATPase: Pumps 3 Na+ out and 2 K+ into cells, requires ATP, maintains low sodium concentration inside cells.
  • Secondary Active Transport:
    • Sodium and glucose transport together via co-transporters.
    • Sodium moving down its gradient helps pull glucose against its gradient.
    • Same mechanism applies for amino acids and lactate.
  • Reabsorption Rates:
    • 100% of glucose, amino acids, lactate are reabsorbed under normal conditions.
    • 65% of sodium and water are reabsorbed (obligatory water reabsorption).

Bicarbonate Reabsorption

  • Carbonic Anhydrase Reaction:
    • CO2 and H2O form carbonic acid (H2CO3), which breaks into H+ and HCO3-.
    • Protons secreted, bicarb reformed in the blood.
    • 90% bicarbonate is reabsorbed.

Additional Ion Reabsorption

  • Chloride, Potassium, Calcium, Magnesium:
    • Paracellular transport (moves between cells) for ions like Ca++, Mg++, K+, Cl-.
  • Urea and Lipid Soluble Solutes:
    • Diffuse across the cell membrane.

Small Protein Reabsorption

  • Receptor-Mediated Endocytosis:
    • Insulin and hemoglobin can be endocytosed, broken down into amino acids, then reabsorbed.

Hormonal Influence

  • Parathyroid Hormone (PTH) and Phosphate:
    • PTH inhibits phosphate reabsorption leading to phosphate excretion.

Tubular Secretion

  • Substances Secreted:
    • Ammonia, drugs (penicillin, methotrexate), organic acids (uric acid, bile salts), organic bases (morphine).
  • Active Process: Requires ATP.
  • Metabolic Acidosis Compensation:
    • Deamination of glutamine produces ammonium ions and bicarbonate, bicarb enters blood to buffer pH.

Conclusion

  • Reviewed essential reabsorption and secretion processes in the proximal convoluted tubule.
  • Next topic: The Loop of Henle.