Oxygen passes into the blood through a process called diffusion, which occurs in the lungs. When we breathe in, air containing oxygen enters the lungs and reaches the alveoli, which are tiny air sacs surrounded by a network of fine blood vessels called capillaries. The oxygen in inhaled air passes across the thin lining of the air sacs and into the blood vessels through the capillaries. The oxygen then moves into the red blood cells, which contain a protein called hemoglobin that binds to oxygen. The hemoglobin molecules inside red blood cells pick up and carry the oxygen, and these oxygen-rich cells travel in the blood vessels from the lungs to the left side of the heart. The blood is then pumped around the body through the arteries. Once the oxygenated blood reaches the body tissues, oxygen is released from the hemoglobin and diffuses into the cells where it is used to make energy.