Concept preview

What you will be able to do

1. 01Describe the composition of blood: plasma, red cells, white cells and platelets, and the function of each.
2. 02Explain how haemoglobin transports oxygen around the body.
3. 03Outline how blood clots and why clotting matters.
4. 04Explain the ABO and Rh blood group systems and the rules of transfusion compatibility.
5. 05Identify the main risk factors for cardiovascular disease and how it can be prevented.

Blood is the only tissue in the body that is a liquid, and an adult carries about five litres of it. Though it looks like a simple red fluid, blood is in fact a busy suspension: a straw-coloured liquid called plasma in which billions of cells are carried. Spin a blood sample in a centrifuge and it separates into layers that reveal its make-up at a glance. Just over half the volume — about 55% — is plasma. A thin whitish layer in the middle, the buffy coat, holds the white blood cells and the platelets. And the bottom 45% or so is a dense, dark-red packing of red blood cells. Between them these components carry out the three great jobs of the blood: transport, defence and repair, all in one flowing tissue.