Factors Operating:

Fig. 152.—Carbon dioxide absorption curves. (Modified from Peters, Barr, and Rule, and Van Slyke).

Figure 152 is a graphic representation of essential facts in acid-base equilibrium. Ordinates represent total CO2 content, which comprises that in simple solution and that as bicarbonate of whole blood in volumes per cent, and abscissae the mm. CO2 tension in the blood as drawn. The line OT gives the proportion of total CO2 present in simple solution. pH values are shown by the lines OL, OM, etc. The extreme normals for carbon dioxide absorption curves are OP and OR. The CO2 tension of alveolar air may be the same or vary as much as 20 mm. below, while that of venous blood will be about 6 (0.8-10.0) mm. higher than that of arterial blood. The “CO2 capacity” (or “CO2 combining power”) of plasma may be as much as 15 vol. % more than the total CO2 of whole blood.

The actual state of acid-base balance, then, can only be determined by the use of any two of a number of interdependent variables, such as total CO2, CO2 tension, pH, H2CO3 concentration, other buffers than bicarbonate, plasma chloride, ratio of oxyhaemoglobin to haemoglobin, etc. Findings that fall within ABCD and at about 40 mm. tension indicate a normal equilibrium for the resting individual at ordinary altitudes. Or, such a normal would be a total CO2 of about 49 (43-56) vol. % for whole blood, and 50-65 vol. % for plasma. The normal for the individual falls within narrower limits.

If either H2CO3 or bicarbonate varies from normal values, there is apparently an effort on the part of the body to compensate by adjusting the other so as at least to maintain a normal pH. This is accomplished by respiration, or by diverting alkali from or recalling it to the blood stream. Naturally, treatment of any such abnormal condition will do well to imitate Nature’s efforts. Haggard and Henderson have demonstrated that blood alkali may be decreased in two ways—by acids (the acidotic process) or by acapnia (the acapnial process). By the forced breathing of the acapnial process the lungs are over-ventilated and an excessive amount of carbon dioxide is washed out of the blood, thus bringing on a temporary alkalosis. In case of prolonged forced breathing, nature prevents an extreme alkalosis by causing the alkali to leave the blood, it being stored in the tissues or excreted in the urine. Blood relatively poor in carbonic acid or relatively rich in alkali acts to depress respiration, and the slowing of respiration produces an acidosis by the resultant retention of H2CO3, this causing alkali to be recalled from the tissues. Thus acidosis, in calling more alkali into the blood from the tissues, represents what may be regarded as a restorative effort. Hence, administration of bicarbonate is indicated in acidotic processes, and of CO2 in acapnial; the use of the wrong one is dangerous.

The numbered regions of the chart are associated with various clinical conditions, e.g., tetany from 1, 2, and 3; the acidosis of diabetes mellitus, nephritis, or infantile marasmus with 6 or 9; pneumonia, morphine narcosis, and breathing of air containing 3-5% CO2 with 7 or 8; emphysema with 4; some cardiac cases with 9; overdose of bicarbonate with 1 or 4; fever with 2; as the result of high altitudes, 2 or 3, or, when acclimated, 6; shock (handling of intestines), deep ether anesthesia, and carbon monoxide asphyxia with lowered bicarbonate. The disturbances of acid-base equilibrium in the last two are the result of acapnial processes.