Hi, Aisy. Gases in our bodies will diffuse into other spaces (lungs, blood, cells) depending on their partial pressure. This partial pressure is the force exerted by a single gas (Dalton's law). In order for these gases to diffuse, they must travel (or diffuse) to an area of lesser pressure. For example, oxygen in our blood will circulate to our body cells where the partial pressure of oxygen is lower. Because of this partial pressure difference, oxygen will diffuse out of the blood, into the interstitial fluid surrounding the cell, and ultimately into the cells. This same principle is true for oxygen diffusing from the alveoli in our lungs to the capillaries surrounding the alveoli. A higher partial pressure of oxygen in the alveoli, when compared to the capillary partial pressure, allows oxygen to diffuse across the alveolar membrane and into the blood capillary.
