Molecules naturally disperse from areas of higher concentration to lower concentration.
As oxygen-rich (and carbon dioxide-poor) blood travels by a cell the oxygen diffuses through the cell membrane to the area of lower concentration inside the cell. It can do this easily because the oxygen molecule (O2) is very small and has no charge or polarity. The oxygen is used up rapidly by mitochondria. This rapid consumption causes oxygen to constantly move into the cell from the blood.
The mitochondria creates carbon dioxide (CO2) as a waste product of cellular respiration (the process that makes energy for your body). Because the CO2 is of a higher concentration in the cell than in the blood passing by, this gas continually diffuses out of the cell. It too is small and uncharged so it can pass through cell membranes easily.
These movements require no energy (in the form of ATP) on behalf of the cell.
