Haemoglobin (Hb) is a protein present in RBCs of organisms, carrying oxygen to the tissues and organs and CO2 from organs and tissues back to the lungs.
It is made of globin protein with heme as a prosthetic group.
Hb is reddue to the presence of the heme group, thus making the blood red-coloured.
One haemoglobin molecule can bind to 4 oxygen molecules.