Note:  Do not rely on this information. It is very old.

Haemoglobin

Haemoglobin, the constituent of the red corpuscles of the blood, to which their chief importance as oxygen carriers in the human economy is due. It occurs to a variable extent in venous blood, but in arterial blood it only exists loosely combined with oxygen as oxy-haemoglobin. Its chemical composition has not as yet been definitely ascertained. It may be obtained in crystals which ordinarily appear of a dark red or purplish colour, but the hue varies with the direction of the light. With a number of gases, other than oxygen, haemoglobin can also enter into combination - as e.g. with carbon monoxide, nitric oxide - forming compounds resembling oxyhemoglobin.