Blood has four main components: plasma (liquid), red blood cells (carry oxygen), white blood cells (fight infection), and platelets (help clotting). Each has a specific job.
Check: Can you name all four components of blood?
Flowchart
Use the pathway to build a clear mental model. Open each node and answer the check question.
1) Learn the four components of blood
2) Understand red blood cells
Red blood cells contain haemoglobin, which binds to oxygen in the lungs and releases it to tissues. They have no nucleus to make more room for haemoglobin. Their biconcave shape increases surface area.
Check: Why do red blood cells have no nucleus?
3) Understand white blood cells and platelets
White blood cells are part of the immune system - they engulf pathogens or produce antibodies. Platelets are cell fragments that help blood clot when you get a cut, preventing blood loss.
Check: What do white blood cells and platelets do?
4) Understand plasma
Plasma is the yellow liquid that carries everything: blood cells, nutrients (glucose, amino acids), hormones, antibodies, and waste products (carbon dioxide, urea). It's about 55% of blood volume.
Check: What substances does plasma transport?
Explanation
What is Blood Made Of?
Blood might look like a simple red liquid, but it's actually a complex tissue with four main components, each with specific functions. An adult has about 5 litres of blood circulating in their body.
1. Plasma (55% of blood)
Plasma is the straw-coloured liquid part of blood. It's mostly water (about 90%) and transports:
- Blood cells - red cells, white cells, and platelets float in plasma
- Nutrients - glucose, amino acids, vitamins, and minerals from digested food
- Waste products - carbon dioxide (dissolved) and urea to be removed
- Hormones - chemical messengers like insulin and adrenaline
- Antibodies - proteins that help fight infection
2. Red Blood Cells (Erythrocytes)
Red blood cells are the most numerous cells in blood - about 5 million per cubic millimetre! Their job is to transport oxygen.
- Contain haemoglobin - a red protein that binds to oxygen in the lungs and releases it in tissues
- No nucleus - removed to make more space for haemoglobin
- Biconcave disc shape - like a doughnut without a hole, increasing surface area for gas exchange
- Flexible - can squeeze through narrow capillaries
3. White Blood Cells (Leucocytes)
White blood cells are part of your immune system. There are fewer of them than red cells, but they're vital for fighting disease.
- Phagocytes - engulf and digest bacteria and other pathogens
- Lymphocytes - produce antibodies that target specific pathogens
- Have a nucleus - unlike red blood cells
- Can change shape - to squeeze through capillary walls into tissues
4. Platelets (Thrombocytes)
Platelets are tiny cell fragments (not whole cells) that help blood clot.
- Stick together at wound sites to form a plug
- Release chemicals that trigger the clotting process
- Help form scabs - the fibrin mesh that seals wounds
Key Vocabulary
- Plasma: The liquid part of blood that transports substances.
- Haemoglobin: The red protein in red blood cells that carries oxygen.
- Phagocyte: A white blood cell that engulfs pathogens.
- Antibody: A protein produced by lymphocytes that targets specific pathogens.
- Platelet: A cell fragment that helps blood clot.
Why does this matter?
Blood tests can reveal a lot about your health. Doctors count blood cells, measure haemoglobin levels, and check for antibodies. Conditions like anaemia (too few red cells), leukaemia (cancer of white cells), and haemophilia (clotting problems) all relate to blood components.
Examples
Example 1: When You Cut Yourself
When you get a small cut, you see blood clotting in action. Platelets rush to the wound and stick together, forming a plug. They release chemicals that cause fibrin (a protein in plasma) to form a mesh over the wound. This dries to become a scab, protecting the wound while it heals underneath. If you didn't have platelets, even small cuts could be dangerous.
Example 2: Why You Feel Tired When Ill
When you have an infection, your white blood cells work overtime to fight it. Your body diverts energy to producing more white cells and antibodies. This is partly why you feel tired when you're ill - your immune system is using a lot of resources. When you recover, your antibodies remain, giving you immunity to that specific pathogen.
Example 3: Blood Donation in the UK
NHS Blood and Transplant relies on 1.6 million donations each year. Donated blood is separated into components: red cells, plasma, and platelets are stored separately and given to patients who need them. Someone having surgery might need red cells; a cancer patient might need platelets; a burn victim might need plasma. One donation can help multiple people!
Example 4: Anaemia and Iron
Anaemia is a condition where you don't have enough healthy red blood cells. Without enough haemoglobin, your blood can't carry sufficient oxygen, making you feel tired and breathless. Iron is essential for making haemoglobin, which is why eating iron-rich foods (like red meat, spinach, and fortified cereals) helps prevent anaemia.
Practice
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Common Mistakes
Mistake 1: Confusing red and white blood cells
Red blood cells carry oxygen using haemoglobin; white blood cells fight infection. Red cells have no nucleus and are much more numerous. White cells have a nucleus and are part of the immune system. Don't mix up their functions!
Mistake 2: Saying red blood cells carry carbon dioxide
Red blood cells mainly carry oxygen. While a small amount of carbon dioxide binds to haemoglobin, most CO2 is transported dissolved in plasma. The main job of red cells is oxygen transport - that's what haemoglobin is specialised for.
Mistake 3: Calling platelets "cells"
Platelets are cell fragments, not complete cells. They break off from larger cells in bone marrow. They don't have a nucleus and are much smaller than red or white blood cells. However, they're still crucial for blood clotting.
Mistake 4: Forgetting what plasma does
Students often focus on blood cells but forget plasma. Plasma is the liquid that carries everything - not just cells, but also nutrients, hormones, antibodies, and waste. It makes up over half of blood volume and is essential for transport.
Summary
- Plasma (55%) - liquid that transports blood cells, nutrients, hormones, antibodies, and waste
- Red blood cells - contain haemoglobin to carry oxygen; no nucleus; biconcave shape
- White blood cells - fight infection; phagocytes engulf pathogens, lymphocytes make antibodies
- Platelets - cell fragments that help blood clot at wound sites
- Each component has a specific structure suited to its function
Skills Checklist
- I can name the four components of blood.
- I can describe the function of each blood component.
- I can explain why red blood cells have no nucleus.
- I can describe how white blood cells fight infection.
- I can explain how platelets help wounds heal.