Although carbon dioxide (CO2) is not an acid per se, it becomes a proton (H+) donor when it is combined with water:

When PCO2 increases above its physiological levels (i.e. above 40 to 45 mmHg (5.33 to 6 kPa), corresponding to 1.2 to 1.35 mmol/l), acidemia usually develops. Thus respiratory acidosis can be defined as a process in which the tendency for the arterial pH to decrease is primarily due to an increase in PCO2. However, it is important to note that, although this process is usually associated with acidemia, there are circumstances in which the pH may be maintained within near-physiological limits owing to the concurrent increase in bicarbonate HCO 3-. In fact, the pH, which reflects the free H + activity in the blood, is a function of the ratio of HCO 3- to CO2 and not of their absolute values.

The CO2 load

The amount of CO2 produced metabolically in a normal male is approximately 200 to 250 ml/min, corresponding to an acid load of 13 000 to 16 000 mmol/day. However, because of increased metabolism, this load may be increased by 30 to 100 per cent depending on the basic disease (burns, infection, sepsis, etc.) ( Lon.g e.t... al 1979). An excessive caloric intake, particularly during parenteral nutrition, may also greatly increase the CO 2 production (Covelli elM- 1981.).

Another possible source of CO2 is provided by HCO3- when titrated by fixed acid, such as lactic acid (LAH):

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