Now, if the air becomes less dense, then the total pressure in the atmosphere decreases. And if we multiply zero point 21 by a smaller total pressure, that gives us a smaller partial pressure of oxygen. So if we increase the altitude, the partial pressure of oxygen in the atmosphere decreases. Now, what exactly does that mean physiologically? Well, what that means is the lungs will not be able to pump as much oxygen into the blood. And if there's less oxygen circulating inside our blood, less oxygen will be delivered to the cells and tissues of our body.