Blood has four parts: plasma, red blood cells, white blood cells and platelets. Each has a job that follows from its structure, and each answer should say both.
This lesson is part of transport in humans and animals. It follows on from comparing blood vessels, because the vessels carry what this lesson describes.
What does each component do, and why?
The structure gives you the job. If you know the structure, you can rebuild the function.
| Component | Key structure | Job |
|---|---|---|
| Plasma | Watery liquid | Carries dissolved food, carbon dioxide, hormones and urea |
| Red blood cell | Biconcave, no nucleus, contains haemoglobin | Carries oxygen |
| White blood cell | Has a nucleus, can change shape | Fights infection |
| Platelet | Small cell fragment | Helps blood clot at a wound |
Worked example: following one trip
A runner’s muscle cell uses glucose and oxygen, and produces carbon dioxide. Explain how blood helps each.
- Glucose enters the blood in the small intestine and dissolves in plasma. The plasma carries it to the muscle.
- Oxygen binds to haemoglobin in red blood cells in the lungs. In the muscle, where the oxygen level is low, it is released and diffuses into the cell.
- Carbon dioxide diffuses from the muscle into the plasma and is carried to the lungs, where it leaves the blood.
Notice that each sentence names a component, a substance and a place. Aim for the same pattern in your own answers.
The mistake that loses marks
The usual slip is “red blood cells carry oxygen” with no reason, or “the blood carries oxygen” without naming the component. Both give away marks.
| Weak | Strong |
|---|---|
| Blood carries oxygen | Red blood cells carry oxygen bound to haemoglobin |
| Red blood cells have no nucleus | Red blood cells have no nucleus, so there is room for more haemoglobin |
| White blood cells protect us | White blood cells engulf pathogens or produce antibodies |
A short note on the word carry
Carry has different meanings here. Plasma carries dissolved substances. Red blood cells carry oxygen bound to a pigment. Keep both ideas separate in your answer.
Check yourself
A cut on the knee stops bleeding after a few minutes, and the wound stays free of infection. Name the two components mainly involved and state the job of each.
Answer
Platelets help the blood clot and seal the cut, which stops bleeding. White blood cells destroy pathogens that enter through the cut, which prevents infection.
What to study next
Put the components into the diagrams in interpreting cardiovascular diagrams without medical diagnosis. Then try the transport in humans and animals practice set.
If you want a teacher to question you on structure and function until the links are secure, see online one-to-one Biology tuition.