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