Oxygen enters the lungs and into the alveolus when air is inhaled. From there, the oxygen diffuses into the blood capillaries (if you want, you could mention the changing of the air pressure that causes diffusion to take place) that are surrounding the alveolus. Thousands of red blood cells are found in the blood capillaries. The oxygen then comebines with a pigment found in the red blood cells, called haemoglobin. When combined, it forms oxyhaemoglobin. It then travels to the heart, to be pumped to all parts of the body.