What is blood made of, and what does each part do?
Describe the components of human blood and explain the functions of red blood cells, white blood cells, platelets and plasma
A scaffolded answer to the N(A)-Level Biology outcome on human blood. The four main components, what red and white cells, platelets and plasma do, and how red blood cells are adapted to carry oxygen.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
This outcome wants you to name the four main parts of blood and explain what each one does. You should be able to say which part carries oxygen, which fights disease, which helps clotting, and which carries dissolved substances. You should also be able to describe how a red blood cell is adapted for its job. The marks reward naming each component and linking it clearly to its function.
The answer
What blood is made of
Blood is a tissue made of a liquid called plasma with three kinds of cell-sized parts floating in it:
- Red blood cells carry oxygen.
- White blood cells fight disease.
- Platelets help the blood to clot.
- Plasma is the pale yellow liquid that carries everything else.
Red blood cells
Red blood cells carry oxygen from the lungs to all the body cells. They are adapted for this job in three ways. They contain haemoglobin, a red pigment that joins with oxygen. They have a biconcave shape (a disc dented on both sides), which gives a large surface area for oxygen to diffuse in and out. And they have no nucleus, leaving more room inside for haemoglobin, so each cell carries more oxygen.
White blood cells
White blood cells protect the body against disease. There are two main jobs. Some white blood cells engulf and digest germs, swallowing bacteria whole. Others make antibodies, chemicals that destroy germs or make them clump together so they are easier to deal with. Unlike red blood cells, white blood cells keep their nucleus.
Platelets
Platelets are tiny fragments that help the blood to clot. When a blood vessel is cut, the platelets help form a clot, a kind of plug, that seals the wound. This stops too much blood being lost and stops germs from entering.
Plasma
Plasma is the liquid part of blood. It carries dissolved substances around the body, including digested food such as glucose and amino acids, waste such as carbon dioxide and urea, and hormones. It also carries heat, which helps spread warmth around the body.
Examples in context
Example 1. Why people at high altitude make more red blood cells. High in the mountains the air has less oxygen, so the body makes extra red blood cells to capture as much oxygen as possible. This shows the job of red blood cells: the harder it is to get oxygen, the more carriers the body needs.
Example 2. Why a vaccine uses white blood cells. A vaccine contains a harmless version of a germ. The white blood cells make antibodies against it, and the body remembers how, so if the real germ arrives later it is destroyed quickly. This is the antibody-making job of white blood cells in action.
Try this
Q1. Name the part of the blood that carries oxygen. [1 mark]
- Cue. Red blood cells (using haemoglobin).
Q2. State the function of platelets. [1 mark]
- Cue. They help the blood to clot, sealing wounds and stopping blood loss.
Q3. Explain why a red blood cell has no nucleus. [2 marks]
- Cue. Having no nucleus leaves more room inside the cell for haemoglobin, so it can carry more oxygen.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksDescribe three ways a red blood cell is adapted to carry oxygen efficiently.Show worked answer →
Adaptation 1: a red blood cell contains haemoglobin, a red pigment that joins with oxygen to carry it.
Adaptation 2: it has a biconcave shape (a flattened disc dented on both sides), which gives a large surface area for oxygen to diffuse in and out quickly.
Adaptation 3: it has no nucleus, leaving more room inside the cell for haemoglobin, so it can carry more oxygen.
What markers reward: three correct adaptations each linked to carrying oxygen (haemoglobin to bind oxygen; biconcave shape for large surface area; no nucleus for more haemoglobin). Just naming the cell as red is not an adaptation.
Original4 marksExplain the role of white blood cells and platelets in protecting the body.Show worked answer →
White blood cells defend the body against disease. Some white blood cells engulf and digest germs (bacteria and other microbes), while others make chemicals called antibodies that destroy germs or stick them together.
Platelets help the blood to clot. When a blood vessel is cut, platelets help form a clot that seals the wound. This stops too much blood being lost and stops germs getting in.
What markers reward: white blood cells linked to fighting disease (engulfing germs and/or making antibodies) and platelets linked to clotting to seal wounds. Saying platelets carry oxygen is a common error; that is the red blood cells.
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