Haemoglobin  is the iron-containing oxygen transport metalloprotein in the red blood cells (erythrocytes) of almost all vertebrates (the exception being the fish family Channichthyidae as well as the tissues of some invertebrates. Hemoglobin in blood carries oxygen from the lungs or gills to the rest of the body (i.e. the tissues). There it releases the oxygen to permit aerobic respiration to provide energy to power the functions of the organism in the process called metabolism. A healthy individual has 12 to 20 grams of hemoglobin in every 100 ml of blood.


In mammals, the protein makes up about 96% of the red blood cells' dry content (by weight), and around 35% of the total content (including water).Hemoglobin has an oxygen-binding capacity of 1.34 mL O2 per gram, which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood. The mammalian hemoglobin molecule can bind (carry) up to four oxygen molecules.