The bloodstream delivers oxygen to cells and removes carbon dioxide waste through a process called internal respiration. In the lungs, oxygen is absorbed into the erythrocytes in the blood stream. These cells, commonly called red blood cells, contain gas-transporting molecules called hemoglobin. Oxygen binds to these molecules. The oxygenated blood then moves through the vasculature. At the narrow capillaries within body tissues, red blood cells release oxygen, which then diffuses through the capillary wall into tissues. Meanwhile, the waste product carbon dioxide diffuses into the bloodstream, where it is carried inside red blood cells and in plasma. The deoxygenated blood travels through the vasculature and back to the lungs, where carbon dioxide is expelled from the body.