Most of the cells in the blood are red blood cells. These are highly specialized cells that are designed specifically to transport oxygen from the lungs to the cells of the body.

Note that the red blood cells are concave and have no nucleus. This improves their ability to carry oxygen.

Oxygenation of Blood

Blood enters the heart from the body. From there it moves to the pulmonary (lung) circulation where it picks up oxygen. The oxygenated blood moves back into the heart and is then pumped into the body. The oxygen is exchanged for waste products like carbon dioxide at the cellular level. The deoxygenated blood then returns to the heart (Animation of heart and lung circulation).

Hemoglobin

Red blood cells are filled with a special red colored molecule called hemoglobin. It picks up oxygen in areas where oxygen is abundant and releases oxygen in tissues where the oxygen concentration is lowest.