Final answer:
The alveolar partial pressure is calculated using the mole fraction of the gas and the total atmospheric pressure, taking into account the respiratory quotient (RQ) and the presence of carbon dioxide for gases like oxygen.
Step-by-step explanation:
The alveolar partial pressure (PA) is determined by the concentration of the gas within the alveoli and the total pressure of the gas mixture inhaled. According to Dalton's law, the partial pressure of a gas is proportional to its mole fraction in a mixture of gases and can be calculated using the formula PA = XA * Ptot, where XA is the mole fraction and Ptot is the total pressure. For oxygen in the air we breathe, which is approximately 21% by volume, its partial pressure can be determined by the total atmospheric pressure, typically around 760 mmHg. This means we can calculate the partial pressure of oxygen (alveolar Po₂) in the alveoli by simply knowing its mole fraction and the total pressure.
However, within the lungs, the situation is a bit more complex due to the exchange of gases and the fact that alveoli never fully deflate, mixing incoming air with residual air. Additionally, other physiological factors such as the respiratory quotient (RQ) and the presence of carbon dioxide also affect alveolar Po₂ values. The RQ helps in calculating the alveolar Po₂ by taking into account the partial pressure of CO₂ in the alveoli and the ratio of CO₂ produced to O₂ consumed. Furthermore, intrapleural and intra-alveolar pressures influence the distribution and movement of gases within the lungs, as explained by Boyle's law, which relates gas pressure to its volume.