Deoxygenated blood gets oxygenated in the lungs to become oxygenated again. This process is called gas exchange, and it takes place in the alveoli and the capillaries that surround them. Deoxygenated blood arrives at the alveoli via the pulmonary artery. The alveoli receive the carbon dioxide that was carried by the red blood cells through the process of diffusion. This CO2 disappears from the body when the air in the lungs is exhaled. During inhalation, oxygen enters the lungs again. The O2 passes the alveoli wall through diffusion, enters the bloodstream and binds to the red blood cells. The blood becomes oxygenated again and flows to the left heart through the pulmonary vein.