In a normal resting person, breathing air at sea level with RQ of 0.8, Pa CO2 of 40mm Hg, Pi O2 of 149 mm Hg and Fi O2 of 21 %. Calculate alveolar partial pressure of oxygen
**Core Concept**
The alveolar partial pressure of oxygen (PAO2) is a critical parameter in respiratory physiology, calculated using the alveolar gas equation. It reflects the oxygen available for diffusion into the blood from the alveoli, influenced by the partial pressure of oxygen in the inhaled air (PIO2), the partial pressure of carbon dioxide (PaCO2), and the respiratory quotient (RQ).
**Why the Correct Answer is Right**
The alveolar gas equation is given by: PAO2 = (FiO2 * (Pb - PH2O) - PaCO2 / RQ), where Pb is the barometric pressure at sea level (approximately 760 mmHg), PH2O is the vapor pressure of water (approximately 47 mmHg at 37°C), and FiO2 is the fraction of inspired oxygen (21% in room air). Plugging in the given values, we can calculate PAO2.
PAO2 = (0.21 * (760 - 47) - 40 / 0.8)
PAO2 = (0.21 * 713 - 50)
PAO2 = (149.73 - 50)
PAO2 = 99.73 mmHg
**Why Each Wrong Option is Incorrect**
**Option A:** This option would be incorrect if it did not account for the barometric pressure or the vapor pressure of water.
**Option B:** This option would be incorrect if it did not consider the respiratory quotient (RQ) or the partial pressure of carbon dioxide (PaCO2).
**Option C:** This option would be incorrect if it did not account for the fraction of inspired oxygen (FiO2) or the barometric pressure at sea level.
**Clinical Pearl / High-Yield Fact**
When calculating alveolar partial pressures, remember to consider the barometric pressure at sea level and the vapor pressure of water at body temperature (approximately 47 mmHg). This is crucial when assessing oxygenation or ventilation in patients.
**Correct Answer:**
**C**. 99.73 mmHg