# Alveolar Carbon Dioxide

Overview
• Qualitatively, the partial pressure of carbon dioxide within the alveoli is determined by two opposing processes. The first involves entry of carbon dioxide into the alveolus and occurs due to diffusion of carbon dioxide gas from the pulmonary capillaries into the alveolar space (This process is discussed in Carbon Dioxide Pulmonary Gas Exchange). The second involves the rate at which carbon dioxide is expelled from the alveolus during refreshing of alveolar air with external air and is determined by the rate of alveolar ventilation. The balance between these two opposing processes ultimately determines the steady-state carbon dioxide gas tension within the alveolus and can be quantitatively described using the equation below.
Calculation
• Overview
• Calculation of the partial pressure of alveolar carbon dioxide can be done simply by rearranging the alveolar ventilation equation (see page). It may be initially unclear why the rate of CO2 diffusion from the pulmonary capillaries into the alveolar space is not a variable in the following equation given our discussion above. This is because the equation assumes a constant rate of uniform pulmonary blood flow in which case the rate of CO2 diffusion into the pulmonary capillaries is largely determined by the rate of CO2 production by the body which is included in the equation below. Thus, when metabolic carbon dioxide generation increases, the diffusion of CO2 from the pulmonary capillaries into the alveoli will also increase, assuming a constant rate of pulmonary blood flow.
• Alveolar Carbon Dioxide Equation
• PACO2 = V'CO2/V'A
• PACO2 = Alveolar Partial Pressure of CO2
• V'CO2 = Metabolic Rate of CO2 production
• V'A = Alveolar Ventilation (ml/min)
• Qualitative Relationships
• Given the equation above, it is clear that the alveolar partial pressure of carbon dioxide is proportional to the body's metabolic rate of carbon dioxide generation and inversely proportional to the rate of alveolar ventilation. Thus, if in a context of exercise the body's metabolic rate of CO2 production were to double (V'CO2), then the alveolar partial pressure of CO2 (PACO2) would also double if alveolar ventilation remained the same (V'A). Conversely, if the rate of alveolar ventilation (V'A) doubled then the alveolar partial pressure of carbon dioxide (PACO2) would become half, given a constant rate of metabolic CO2 production (V'CO2). Qualitative Features of Alveolar Carbon Dioxide On the left we see that for the same level of metabolic CO2 production (Vâ€CO2), doubling the alveolar ventilation rate (Vâ€A) will halve the alveolar (and thus arterial) partial pressure of carbon dioxide (PaCO2). In contrast, if the metabolic rate of carbon dioxide is doubled the curve shifts to the left. Consequently, to maintain the same level of alveolar carbon dioxide, the alveolar ventilation rate must be doubled.

Physiological Significance
• As discussed in Carbon Dioxide Pulmonary Gas Exchange, the partial pressure of carbon dioxide in the alveoli always equalizes with that in the pulmonary capillaries by the time the end of the capillaries is reached in nearly all physiological and pathological contexts. As a result, the partial pressure of alveolar CO2 is of significant physiological importance as it largely determines the arterial carbon dioxide partial pressure (i.e. PACO2 determines PaCO2).
• Because of the near equivalence of arterial and alveolar partial pressure of carbon dioxide, the equation above is typically unnecessary in calculating the PACO2 since the PaCO2 can be easily and directly measured by obtaining arterial blood gases. In fact, direct measurement of arterial carbon dioxide tension is typically used to calculate the alveolar ventilation rate, rather than the converse. However, we provided the extensive discussion above to demonstrate how PACO2 is dependent on the body's metabolic production of CO2 and the alveolar ventilation rate. These considerations will become important in our discussion of alveolar oxygen in the next section.