Haemoglobin

an iron-containing red pigment, which is found within the red blood cells (called ERYTHROCYTES) of vertebrates and which is responsible for the transport of oxygen around the body. In actively metabolizing tissue such as the muscles, haemoglobin exchanges oxygen for carbon dioxide (CO₂), which is then carried in the blood back to the heart and pumped to the lungs, where the haemoglobin loses the CO₂ and regains oxygen.

Taken from Dictionary of Science