Blood: composition, plasma and red blood cellsSpec 2.59, 2.60, 2.61
Blood is a tissue made of a liquid, plasma, with red blood cells, white blood cells and platelets suspended in it.
| Component | Description | Main job |
|---|---|---|
| Red blood cells | Biconcave discs with no nucleus, full of haemoglobin | Transport oxygen |
| White blood cells | Larger cells with a nucleus. Phagocytes and lymphocytes are two types | Defend the body against pathogens |
| Platelets | Small fragments of cells with no nucleus | Help the blood to clot |
| Plasma | Pale yellow liquid | Transports blood cells and dissolved substances |
(opens full size in a new tab)Plasma
Plasma is mostly water. It transports substances around the body, dissolved or carried in it.
| Substance | Transported from | Transported to |
|---|---|---|
| Carbon dioxide | Respiring cells | The lungs |
| Digested food (such as glucose and amino acids) | The small intestine | All the cells of the body |
| Urea | The liver, where it is made | The kidneys, to be removed in urine |
| Hormones | The glands that make them | Their target organs |
| Heat energy | Warm, active organs such as the liver and muscles | Other parts of the body, including the skin |
Red blood cells
Red blood cells transport oxygen from the lungs to all the cells of the body. Three adaptations help them to do this.
- The biconcave disc shape gives a large surface area, so oxygen can diffuse in and out quickly.
- They contain haemoglobin, a red pigment that combines with oxygen in the lungs to form oxyhaemoglobin, and releases it in the tissues.
- They have no nucleus, which leaves more room inside the cell for haemoglobin.
Common mistake:
Red blood cells carry oxygen. Haemoglobin is the substance inside them that binds to oxygen. Do not say that the cells 'carry haemoglobin'.