Summary
Proximal convoluted tubules (PCT)
Major site of absorption of glucose, amino acids, other organic substance
Major site of bicarbonate reabsorption
Proximal straight tubules (PST)
(After PCT)
Major site of secretion of organic acid and bases (including drugs)
Loop of Henle (LOH)
- Reabsorption of water (10%)
- Reabsorption of Na+ (25%)
--> More solute is reabsorbed than water
--> Dilution of filtrate
Thus,
- Iso-osmotic filtrate from proximal tubule becomes hypo-osmotic
Distal convoluted tubule (DCT)
- Impermeable to water
- Active reabsorption of Na+ and Cl-
* via Na-Cl symporter
- ENaC also present
- Calcium channle present
* Major site of action in regulation of Ca2+ by PTH
Thus,
- Hypo-osmotic filtrate from LOH becomes even more hypo-osmotic
Collecting duct
Reabsorption of sodium and water by principle cells
Cortical collecting duct
Principle cells
- About 70% of cells
- Sodium reabsorption via apical Na+ channel (i.e. ENaC)
- Water reabsorption via apical aquaporin 2
- K+ secretion
* via apical K+ channel
* Driven by electrical and chemical gradients
- Cl- absorption via paracellular route
Intercalated cell - Type A
- Active secretion of H+
* via apical H-ATPase and apical H-K-ATPase
Intercalated cell - Type B
- Active secretion of base (HCO3-)
* Apical Cl-HCO3 antiporter
* ???? Isohydric cycle
NB:
- Type B is relatively rare in non-vegatarians
- Essentially same as type A, but with apical and basolateral transporters swapped around