Now, we know that at sea level, the total atmospheric pressure is 760 mercury. And because the amount of oxygen in the atmosphere is 21%, to find the partial pressure of oxygen, we simply multiply zero point 21 by the total pressure of 760. And that gives us about 159 mercury is the partial pressure of oxygen at sea level. Now, as we begin to increase an altitude, for example, as we climb a mountain, what happens is the air becomes less dense. So the distance between the gas molecules increases. And what that means is we'll find less gas molecules in the same volume of space at a higher altitude.