Haemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and the tissues of some invertebrates.
In mammals, the protein makes up about 97% of the red blood cell's dry content, and around 35% of the total content (including water). Haemoglobin transports oxygen from the lungs or gills to the rest of the body where it releases the oxygen for cell use. It also has a variety of other roles of gas transport and effect-modulation which vary from species to species, and are quite diverse in some invertebrates.