The hemoglobin molecule is made up of four polypeptide chains: two alpha chains of 141 amino acid residues each and two beta chains of 146 amino acid residues each. The alpha and beta chains have different sequences of amino acids, but fold up to form similar three-dimensional structures. The four chains are held together by noncovalent interactions. There are four binding sites for oxygen on the hemoglobin molecule, because each chain contains one heme group which consists of an organic part and an iron atom. The iron atom in heme binds to the four nitrogens in the center of the protoporphyrin ring. Iron atom is main component that binds with oxygen. Hence each molecule of haemoglobin is capable of carrying four oxygen molecules. Corporation between the four subunits in a haemoglobin molecule becomes mandatory i.e all four subunits bind with oxygen collectively. When oxygen gets bound to one site among the four subunits, it also increases the chances of other remaining sites being bound to oxygen. Deoxyhemoglobin is a taut moleucule, contrained by its eight salt links between the four subunits. Some of these salt links are broken so that the iron atom can move into the plane of the heme group to prompt oxygenation. Diffusion of oxygen into the body cells occur when haemoglobin releases its content in the capillaries. The capillary walls are relaxed by nitric oxide to allow expansion thus facilitating the transport of oxygen to the cells.