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.