Molecules tend to move from places of high concentration to places of low concentration, just by moving randomly. For example, there is more oxygen in a lung than there is oxygen in the blood so oxygen molecules will tend to move into the blood. Similarly, there is more carbon dioxide molecules in the blood than in the lung so carbon dioxide molecules will tend to move into the lung. It happens in cell biology, where small molecules simply diffuse through the cell membrane, but larger molecules only get through by using energy: see active transport.