Lucien Rizzo

Receive:

• Plasma ultrafiltrate from Bowman’s capsule containing ions, glucose, nitrogenous waste.

Process:

• The PCT is a reabsorption powerhouse recouping 2/3 of all Na+ and H2O. It also reabsorbs HCO3-, glucose, K+, amino acids, phosphate, lactate, and Cl-.
  • Active transport: Na+ is actively pumped into the peritubular capillary by Na+/K+ATPase on the basolateral tubular membrane. This maintains a low intracellular Na+ concentration allowing Na+ co-transporters at the luminal membrane to absorb glucose and amino acids down a concentration gradient. This is more efficient than by passive diffusion alone.
  • Passive transport: K+, Urea
  • Cotransport: Na+ and glucose reabsorbed together by cotransport. 
  • Countertransport: Na+ absorbed while H+ is excreted
  • Glucose Reabsorption: Glucose is reabsorbed with Na+ via sodium-glucose linked transporters (SGLT) on the luminal membrane. Na+ is then pumped into the peritubular capillary by Na+/K+ATPase while glucose is further absorbed by simple diffusion via glucose transporters (GLUT). Blood is always flowing and maintains the glucose gradient.

Produce:

• Regulates pH by balancing H+ and HCO3-.
  • Carbonic anhydrase is intracellular and on the luminal brush border. [H+ + HCO3- <-> H2CO3 <-> H2O + CO2]
  • Ultimately, there is net reabsorption of filtered HCO3-.
  • H+ in the lumen is partially bound by NH3 or HPO4 2- and excreted, therefore there is NOT net reabsorption.

Export:

• Isotonic filtrate (~300 mOsm/L)