Blood accounts for about 8% of total body weight and is made up of three types of specialised cells, one of which is the red blood cells also known as erythrocytes. The main function of erythrocytes is to carry oxygen around the body and deliver it to tissues.
In appearance erythrocytes look a bit like a doughnut with a flat middle instead of a hole. They have a large surface area and are relatively thin biconcave discs. This maximises their capacity for rapid diffusion of oxygen across the membrane ensuring their efficiency to transporting oxygen around the body. In addition to this the membrane of an erythrocyte cell is flexible, this enables them to squeeze through narrow capillaries and deliver the oxygen directly to tissues.
Erythrocytes contain haemoglobin which is made up of two parts: (1) the globin, made up of four highly folded polypeptide chains, and (2) four iron elements the heme group, each of which is bound to one of the polypeptide chains and can combine with one oxygen molecule. This means that each erythrocyte can carry four molecules of oxygen.
Blood is red as a result of haemoglobin which is a pigment. Because of its iron content when it is combined with oxygen it has a reddish appearance.
In addition to carrying oxygen haemoglobin can combine with other molecules such as carbon dioxide and contributes to transporting carbon dioxide away from tissues back to the lungs.