Physiological Action of Carbon Dioxide

Respiration in man is directly related to the proportion of carbon dioxide in the air of the lung alveoli. Breathing regulates the fluctuations due to varying carbon dioxide production within the body, or varying carbon dioxide pressures in the inspired air; and there is maintained during rest a fixed partial pressure of about 5.6 per cent, or 40 mm. within the lungs. A rise of about 0.22 per cent, or 1.6 mm. in this pressure increases the rate of breathing by 100 per cent., and a corresponding diminution of alveolar carbon dioxide causes apnoea (cessation of natural breathing).

"When the alveolar carbon dioxide reaches about 8 per cent, dizziness and loss of memory, etc., begin. With higher percentages there is loss of consciousness, but no immediate danger till the percentage exceeds 20 or 30. Recovery is rapid and complete."

The action of carbon dioxide within the blood depends upon the formation of carbonic acid, and the consequent hydrion concentration reached by this acid. Apart from the fluctuations caused by varying carbon dioxide pressure, this concentration is regulated with exquisite exactitude, chiefly by the kidneys.

As regards the relation between oxygen and carbon dioxide in the blood, oxygenation of the blood in the lungs helps to drive out carbon dioxide and increases by about 50 per cent, the amount of carbon dioxide given off at each round of the circulation; whilst the de- oxygenation of the blood in the tissues helps the absorption of carbon dioxide and diminishes by about 40 per cent, the rise of carbon dioxide pressure and of hydrion concentration in the venous blood. Thence it follows that the giving off of carbon dioxide by the Jungs is to a large extent consequent on the taking up of oxygen.