14. Transport processes inn the proximal tubule Flashcards
Reabsorption takes place in two phases:
1) Actively to the interstitium,
2) Passively to the peritubular capillaries
Sodium (Na+)
Main motor: Na+/K+ - ATPase pump
- Forwards sodium form the cell to the interstitium and potassium from the interstitium to the cell.
Hydrogen (H+)
The Na+ entry results in H+ secretion.
- The process has a high capacity, and ensures the removal of the majority of the metabolic hydrogen from the organism.
Bicarbonate ion (HCO3-)
The cell is impermeable to HCO - 3
CO diffusing into the cell rapidly transforms into H+ and 2 HCO - with the help of IC carbonic anhydrase (CA) 3 indirect HCO - transport 3
Chloride ion (Cl-)
the prerequisite for the transcellular chloride transport is the lower pH in the lumen than in the cell.
- Then the chloride-acidic anion antiporter protein of the luminal membrane transports chloride to the cell, and acidic anion from the cell to the lumen.
- In the lumen the acidic anion binds H+ and as free acid easily diffuses back to the cell. Here it dissociated and another chloride transport can be carried out.
Water (H20
The water as a result of increased peritubular oncotic pressure paracellularly migrates from the lumen to the interstitium.
Glucose, amino acids
The glucose and amino acids are 100% withdrawn from the proximal tubule together with Na+, via secondary active symport maintained by the Na+/K+-ATPase pump.
Urea
Approximately half of the urea are passively resorbed through the cells and the paracellular pathways, the rest stays in the interstitium and contributes to the formation of the special osmotic layering in the kidney
Proteins
gets back to the tubular cells by
pinocytosis, then from here as amino acids to the interstitium. The fluid originating from the distal tubule is therefore free of proteins.
