Partial pressure and the alveolar air equation made simple
Gases are in constant motion, colliding with each other and any surfaces they bump into. And the number of collisions determines the pressure of the gas—the more collisions, the higher the pressure.
In a mixture of gases, the total pressure of the gas mixture equals the sum of the partial pressures of the individual gases. This is Dalton’s Law.
And, the partial pressure of each gas is proportional to the number of molecules of the gas in the mixture. So the partial pressure of any gas in the mixture can be calculated by multiplying the fractional concentration (or proportion) of the gas by the total pressure of the gas mixture.
The same is true for the air around us. The kinetic energy of the mixture of gases that constitutes the atmosphere creates atmospheric or barometric pressure, Pbarometric.
An important determinant of barometric pressure is the height above sea level at which the measurement is made. At sea level, barometric pressure is 760 mmHg; it declines with the ascent to higher altitudes.
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As it is inhaled, dry atmospheric air is warmed to body temperature and fully humidified. The resulting humidification adds a component, water vapor, to the gas mixture which has a partial pressure of 47 mmHg. However, by convention, fractional concentrations of inspired gases are calculated after water vapor pressure has been subtracted (i.e., as dry gases).
Therefore, the partial pressure of each inspired gas, including oxygen, is calculated as the fractional concentration of the gas in the mixture times the difference between atmospheric pressure (e.g., 760 mmHg at sea level) and water vapor pressure (47 mmHg).
But when calculating the partial pressure of a gas in the alveoli, we also need to consider the concentration of carbon dioxide evolved into the alveoli from the mixed venous blood returning to the lungs.
So the partial pressure of oxygen in the alveoli (PAO2) is determined by the partial pressure of oxygen in inhaled gas, minus water vapor pressure, minus the concentration of carbon dioxide evolved into the alveoli from mixed venous blood returning to the lungs. This is expressed in the alveolar air equation.
The factor of 1.25 reflects the fact that metabolizing tissues generate slightly less carbon dioxide than oxygen consumed.
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