Not quite. When you hold your breath at or near sea level, the partial pressure of oxygen inside your lungs is 760 mm Hg(Hg = mercury) minus the partial pressure of water dissoved in the air (call it 60), times the proportion of air that is oxygen (.21). This leaves a partial pressure of oxygen in inhaled air of ~ 147 mm Hg which would fall only gradually over the minute. Breathing is actually driven not by low oxygen in the blood, but by build-up of CO2.
In the lungs, though, oxygen is diffusing in, and CO2 is diffusing out, so you wind up with a partial pressure of oxygen of around 105-115. This is at sea level.
Here in El Paso, it’s more like 95-105.
At 30,000 feet, it would be drastically less, (made-up number disclaimer) maybe like 20-25 mm Hg. This means that air at 30,000 feet which is still 21% oxygen, only oxygenates your blood as effectively as sea level air which is 3% oxygen. And in the event of sudden depressurization, it would not be humanly possible to hold sea level pressure air in your lungs at 30,000 without causing massive tissue rupture. Good thing you’d exhale involuntarily…
Sue from El Paso