What blood is made of
Blood is a liquid tissue with four main parts. The liquid part is plasma, a pale yellow fluid that is mostly water. Floating in the plasma are three kinds of blood cells: red blood cells, white blood cells and platelets.
| Component | Main job |
|---|---|
| Plasma | Carries dissolved food, wastes, hormones and heat |
| Red blood cells | Carry oxygen |
| White blood cells | Defend against pathogens |
| Platelets | Help blood to clot |
Red blood cells and oxygen
Red blood cells are packed with a red pigment called haemoglobin, which joins with oxygen in the lungs and releases it to the body cells. To fit in as much haemoglobin as possible, a mature red blood cell has no nucleus, and its biconcave (double-dished) shape gives a large surface area for taking in oxygen.
Key idea
No nucleus + biconcave shape = more room for haemoglobin and a bigger surface for oxygen.
Defence and blood groups
White blood cells protect the body: some engulf germs while others make antibodies. Platelets are cell fragments that clump together at a wound to form a clot and stop bleeding. Human blood also comes in groups such as A, B, AB and O. Group O has no A or B antigens, which is why it is often called the universal donor.
Remember
- Plasma is the liquid; the cells float in it.
- Only red blood cells carry oxygen, using haemoglobin.