Carbon Dioxide and Acid-Base Balance
From the previous discussion it is appreciated that CO2 acts like an acid and therefore its excretion serves a very essential function in maintaining acid-base balance.
The image illustrates proton buffering in the blood.
After blood buffers have acted, the next most immediate means of regaining acid-base stability is through the respiratory system and the manipulation of CO2 levels. Subsequent to the lungs, the kidneys effect change but there is at least a six-hour lag before they can achieve their aims.
Metabolic acidosis
When there is an excess of protons in the blood, they are first mopped up by the proteins, including haemoglobin and the phosphate buffers, the former contributing about 90% of the response within usual physiological limits. However, when these are exhausted the respiratory system is able to excrete H+ through the exhalation of CO2, thus, hyperventilation raises the serum pH by excreting the proxy of protons, namely carbon dioxide.
Beyond the lungs ability the renal system influences H+ excretion by filtering protons which combine with filtered HCO3– as well as HPO42-. Proton reaction with HPO42- permits H+ excretion without sacrificing the HCO3– which is available to be reabsorbed as necessary and so helps to raise the body pH.
Respiratory acidosis
Additionally the amino acid glutamine is held in the cells between the tubule and the bloodstream, and can be catabolised to ammonium (NH4+) and HCO3–. The body then excretes the acidic NH4+ into the tubule which is lost with the urine, while the HCO3– migrates to the bloodstream. This mechanism also helps to normalise pH, and the level of NH4+ excreted in the urine can be used to detect respiratory acidosis.
Mixed acidosis
In circumstances where the respiratory effort is also impaired e.g. in exhaustion, a mixed metabolic and respiratory acidosis ensue. This can be suggested in the presence of a negative base excess with normal or elevated PaCO2. Even the presence of normocapnia with a marked metabolic acidosis show that compensatory mechanisms are failing.
Learning Bite
- The development of normocarbia or hypercarbia in severe metabolic acidosis indicates respiratory muscle fatigue and imminent respiratory failure, and should prompt the consideration of assisted ventilation e.g. non invasive ventilation (NIV).