Respiratory acidosis
[Ref: KB's online acid-base text]
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