Respiratory Therapy – Formulas Practice Test

Partial pressure follows Dalton’s law: a gas’s partial pressure in a mixture equals its fraction of the mixture times the total pressure. In inspired air, oxygen’s fraction is FiO2 and the total pressure is barometric pressure PB, so PiO2 = FiO2 × PB. This multiplication captures how the available oxygen pressure scales with both its presence in the air and the ambient pressure. For example, at sea level with FiO2 ≈ 0.21 and PB ≈ 760 mmHg, PiO2 ≈ 0.21 × 760 ≈ 160 mmHg. In more precise terms, you would subtract the water vapor pressure before applying the factor, but for atmospheric PiO2 the product FiO2 × PB is the correct relationship.