Definition
The pressure exerted by one specific gas within a mixture of gases. In any gas mixture, each gas contributes its own share of the total pressure, and that share is its partial pressure. The sum of all the partial pressures equals the total pressure of the mixture.
Plain English
Air is made up of several gases mixed together. Each one pushes with its own amount of pressure. That individual share is called its partial pressure. Add all the shares together and you get the total air pressure.
Context Anchor
Seen in high-altitude flying, oxygen use, cabin pressure, and discussions of how the body gets enough oxygen.
Derivation
From Latin 'partialis' meaning 'of a part.' The pressure belonging to one part of the mixture, not the whole.
Why Pilots Care
Partial pressure of oxygen determines whether supplemental oxygen is required even when total atmospheric pressure appears adequate.
Analogy
Think of several people pushing on a door at the same time. The total push is all of them together; each person's share of the push is like that gas's partial pressure.
Grounding Statement
At sea level, oxygen makes up about 21% of the air, and the air pushes down at about 14.7 pounds per square inch. So oxygen's partial pressure is roughly 21% of that — about 3 psi. Climb to 18,000 feet and total pressure is cut in half, so oxygen's partial pressure is also halved, even though it's still 21% of the air around you.
Intuition Check
Partial does not mean incomplete or less accurate here. It means one gas's share of the total pressure.
Example Sentence 1
As the aircraft climbed through 12,000 feet, the partial pressure of oxygen dropped low enough that the pilot started using supplemental oxygen.
Example Sentence 2
Pilots calculate the partial pressure of water vapor in the lungs before determining available oxygen for breathing at altitude.