QS Study

Respiratory acidosis

A rise in arterial PCO2 due to decreased ventilation causes respiratory acidosis. IIt is usually caused by a lung disease or condition that affects normal breathing or impairs the lungs’ ability to remove CO2. It develops when air exhaled out of the lungs does not adequately exchange the carbon dioxide formed in the body for the inhaled oxygen in the air.

The CO2 is retained in equilibrium with H2CO3

And H2CO3 is retained in equilibrium with HCO3.

As the CO2 is retained, the plasma rises HCO3 and a new equilibrium is reached at a lower pH.

Renal Compensation – of respiratory acidosis is by increased urinary excretion of hydrogen ions and resorption of HCO3.

HCO3 reabsorption depends upon –

Filtered load of HCO3 – Rate of H+ secretion

Filtered load of HCO3 = GFR X Plasma HCO3 concentration

HCO3 is reabsorbed in exchange for H+.

The rate of H+ secretion and the rate of HCO3 reabsorption in proportionate to arterial pH. The more CO2 is available to form H2CO3 in the cells, the more H+ can be secreted. Furthermore, when the PCO2 a high, the interior of most cells becomes more acidic.

In respiratory acidosis renal tubular H+ secretion is therefore increased, removing H+ from the body; and even though the plasma HCO3 is elevated, HCO3 reabsorption is increased, further raising the plasma HCO3, Cl excretion is increased, and plasma Cl falls as plasma HCO3 is increased.

In respiratory acidosis: [HCO3 / increased PCO2]

Compensation: [Increased HCO3 / increased PCO2]

Thus, increase in pH is compensated by HCO3 generation by the Kidney.

Again in respiratory alkalosis, the low PCO2 hinders renal H+ secretion, HCO3 reabsorption is depressed, and HCO3 is excreted, further reducing the already low plasma HCO3 and lowering the pH towards normal.

In respiratory alkalosis: [HCO3 / Decreased PCO2]

Compensation: [Decreased HCO2 / Decreased PCO2]

Thus, decrease in pH compensated by HCO3 excretion by the kidney.