(Eqn 4-9)
PCO2 AND THE RESPIRATORY
QUOTIENT
PCO2 is determined by the ratio of carbon dioxide production (VCO2) over alveolar ventilation (see the previous section). Carbon dioxide production is determined by several factors, including the type of foods ingested, level of exercise, body temperature, and hormonal activity (e.g., that of the thyroid hormone). Basal VCO2, approximately 200 ml/min for the average sized adult, can increase several fold during exercise (see Chapter 12).
The ratio of oxygen consumption (VO2) to VCO2 is called the respiratory quotient (RQ).
(Eqn 4-9)
In a person at rest who consumes a normal diet (mixture of fats, carbohydrates, and proteins), VO2 is approximately 250 ml/min, for an RQ of approximately 0.80. When the diet consists of only carbohydrates, RQ increases to l.0 - the quantity of carbon dioxide produced equals the quantity of oxygen consumed. If only fats are eaten, the RQ is 0.71.
There is an RQ for both metabolism in the tissues and gas exchange in the lungs. In the steady state, metabolic RQ (production of carbon dioxide and consumption of oxygen by the tissues) is exactly matched by the pulmonary RQ (output of carbon dioxide and uptake of oxygen by the lungs). In fact,
(Eqn 4-10)
defines a steady state for the respiratory system. Normal ventilation is governed by the body's metabolic demands. For example, during exercise the body consumes far more oxygen and produces more carbon dioxide than at rest; as a result, minute (and alveolar) ventilation increase to meet the increase in metabolic demand.
From Equation 8 it can be seen that if VCO2 rises proportionately more than VA, PaCO2 will increase. Does this ever happen in a healthy person? For example, can one exercise to such a point that VCO2 exceeds the capacity of the respiratory system to excrete carbon dioxide? Apparently not. In fact the more a healthy person exercises, the more likely his PaCO2, will fall because of compensatory hyperventilation in response to developing metabolic acidosis (see Chapter 12).
If VCO2 increases and if VA cannot rise proportionately, PaCO2 will increase. In such cases the cause of hypercapnia is inadequate VA for the amount of carbon dioxide presented to the lungs for excretion.