Haemoglobin crystals. Polarized light micrograph of freeze-dried (lyophilized) crystals of haemoglobin. This iron-containing metalloprotein transports oxygen in red blood cells from the lungs to the body's tissues, and returns carbon dioxide from the body tissues back to the lungs. Each red blood cell contains around 280 million haemoglobin molecules. Related disorders are sickle-cell anaemia, thalassaemia, anaemia and hypoxia (lack of oxygen supply to tissues). Elsewhere in the body, haemoglobin functions as an antioxidant and a regulator of iron metabolism. This is bovine haemoglobin, from the blood of a cow. Field of view size: 0.740 x 1.110 millimetres.