Hem"o*glo"bin (?), n. [Hemo- + globe.] (Physiol.) The normal coloring matter of the red blood corpuscles of vertebrate animals. It is composed of hematin and globulin, and is also called hæmatoglobulin. In arterial blood, it is always combined with oxygen, and is then called oxyhemoglobin. It crystallizes under different forms from different animals, and when crystallized, is called hæmatocrystallin. See Blood crystal, under Blood.

• The iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globin), and haem (a porphyrin ring with an atom of iron at its centre).