Physiology of red and white blood cells

Blood is made up of plasma and formed elements, which are red blood cells, white blood cells and platelets. The red blood cells (erythrocytes) make up the vast majority of the cells present in the blood. Their principal function is the transport of oxygen from the lungs to the tissues and the transport of carbon dioxide from those tissues back to the lungs. This is due to the presence of haemoglobin, a protein that binds easily and reversibly with oxygen. The affinity of haemoglobin for oxygen changes under certain conditions allowing increased off-loading of oxygen at the respiring tissues as required. White blood cells (leucocytes) form the body's defence against invading pathogens. They can be subdivided into granulocytes and agranulocytes, which have different mechanisms of attack against those pathogens.