The colouring compound which produces the red colour of blood. Haemoglobin is a chromoprotein, made up of a protein called globin and the iron-containing pigment, haemin. When separated from the red blood corpuscles – each of which contains about 600 million haemoglobin molecules – it is crystalline in form.
Haemoglobin exists in two forms: simple haemoglobin, found in venous blood; and oxy-haemoglobin, which is a loose compound with oxygen, found in arterial blood after the blood has come into contact with the air in the lungs. This oxyhaemoglobin is again broken down as the blood passes through the tissues, which take up the oxygen for their own use. This is the main function of haemoglobin: to act as a carrier of oxygen from the lungs to all the tissues of the body. When the haemoglobin leaves the lungs, it is 97 per cent saturated with oxygen; when it comes back to the lungs in the venous blood, it is 70 per cent saturated. The oxygen content of 100 millilitres of blood leaving the lungs is 19·5 millilitres, and that of venous blood returning to the lungs, 14·5 millilitres. Thus, each 100 millilitres of blood delivers 5 millilitres of oxygen to the tissues of the body. Human male blood contains 13–18 grams of haemoglobin per 100 millilitres; in women, there are 12–16 grams per 100 millilitres. A man weighing 70 kilograms (154 pounds) has around 770 grams of haemoglobin circulating in his red blood corpuscles.