partial pressure

Imagine a crowded elevator where each person pushes a little on the walls. The total push is the sum of everyone's individual pushes; if you knew how much one particular person contributed, that share would be their 'partial' push. Air is a crowd of gases — mostly nitrogen, with oxygen, carbon dioxide and water vapour mixed in — and the partial pressure of one gas is the share of the total pressure that that single gas is responsible for.

Partial pressure matters because it, not the raw amount, is what drives a gas to diffuse. A gas always moves from where its partial pressure is high to where it is low. At sea level, oxygen makes up about 21 percent of dry air, so in the inhaled air its partial pressure is high; by the time air reaches the air sacs it is diluted by water vapour and the carbon dioxide leaving the blood, which is why alveolar oxygen pressure is lower than in the outside air. The notation uses a capital P, as in the oxygen pressure in arterial blood.

Two consequences follow. At altitude the air is thinner, so even though it is still 21 percent oxygen, the partial pressure of oxygen falls and less can diffuse into the blood. And because a dissolved gas exerts a partial pressure in liquid just as it does in air, we can speak of the oxygen and carbon dioxide pressures in blood — the very numbers reported on a blood gas test.