Respiratory acidosis

PaCO2

PaCO2 is proportional to VCO2/VA

Thus,

PaCO2 increase when

• CO2 production (VCO2) increase
• Alveolar ventilation (VA) decrease
• Presence of excess CO2 in inspired gas

Clinically, most cases are due to alveolar hypoventilation

Renal compensation in respiratory acidosis

Renal response (compensating by retention of bicarbonate) to a chronic respiratory acidosis is slower
* Takes 3 or 4 days to reach its maximum

Increased PaCO2
--> Increased intracellular pCO2 inside proximal tubular cells
--> Increased H+ secretion from PCT into lumen

End result

• Increased HCO3 production which crosses the basolateral membrane and enters the circulation
  --> Plasma [HCO3] increases further (in addition to effects of increased [CO2])
• Increased Na+ reabsorption in exchange for H+ and less in exchange for Cl-
  --> Decreased Cl- resorption
  --> Plasma [Cl-] falls
• Increased 'NH3' production to 'buffer' the H+ in the tubular lumen
  --> Urinary excretion of NH4Cl increases