Blood components
Match plasma, red cells, white cells and platelets to their jobs in the blood.
On this page
Blood Circulation · about 25–35 min
What you need to understand
Plasma transports; red cells carry oxygen; platelets clot; white cells defend.
Definitions
- plasma
the liquid part of blood that transports dissolved substances
For example: Plasma carries digested food and carbon dioxide.- platelet
a blood component that helps clotting
For example: Platelets help reduce blood loss after a cut.- red blood cell
a blood cell specialised to transport oxygen
For example: Red blood cells carry oxygen from lungs to tissues.- white blood cell
a blood cell involved in defence
For example: White blood cells can form antibodies or destroy pathogens.
Key idea
- LookA cut stops bleeding after a clot forms.
- ThinkPlasma transports; red cells carry oxygen; platelets clot; white cells defend.
- DoSelect the blood component from its transported material or defence function.
Review the full blood circulation topic picture
Explanation
Blood contains a liquid called plasma, with blood cells and platelets suspended in it. Plasma carries dissolved substances: digested food substances such as glucose travel to cells, while carbon dioxide is carried towards the lungs for removal. It is a transport medium, not waste liquid.
Red blood cells carry most of the blood’s oxygen. Oxygen binds to a substance in them called haemoglobin. At the lungs, they gain oxygen; at body tissues, they release it for cells to use in respiration.
Platelets are small cell fragments that help blood clot. At a damaged vessel, they gather and help a clot form with proteins from plasma. The clot seals the damaged area and reduces blood loss. Platelets do not replace the red cells’ oxygen-transport role.
White blood cells help defend the body against pathogens, organisms or agents that cause disease. Some white cells engulf and destroy pathogens. Others produce antibodies , proteins that recognise specific targets on pathogens and help the body deal with them. Different white cells do different jobs; not every white cell performs both roles.
Defence can also affect transplanted tissue, such as a donated kidney. The
immune system may recognise cells from another person as foreign and attack
them. White blood cells and antibodies can take part in this
tissue rejection. The donated tissue need not be infected:
rejection is a response to foreign tissue. The NHS Blood and Transplant explanation
Pause and say it: Plasma transports; red cells carry oxygen; platelets clot; white cells defend.
Common mistake
Tempting wrong idea: “The blood is red, so red blood cells must be what stops a cut bleeding.”
Why it fails: Colour does not identify the job. Platelets help form the clot that reduces blood loss; red blood cells mainly carry oxygen.
Use this instead: Plasma carries dissolved substances, platelets help clotting, red cells carry oxygen and white cells provide defence. Match the function described to the component, rather than guessing from colour.
Practical work
What to show: Select the blood component from its transported material or defence function.
Before you finish: Match a component to its function rather than its appearance.
Practical link: The topic investigation is taught in “Pulse response and recovery”
6. Worked Examples
Modelled example 1
Select a component
Problem
A cut stops bleeding after a clot forms. Identify the blood component that helps this happen and explain how its function reduces blood loss.
Study the worked solution
Reason from the evidence
Method
Identify the function shown by the clot, then match that function to a blood component.
Reason
Each blood component has a distinct main role, so the observed response is better evidence than the colour of blood.
Working
- The required function is clotting.
- Platelets help initiate clot formation.
- Red blood cells instead transport oxygen.
State the conclusion
Working
Platelets are the component most directly linked to the clotting response.
Common misconception 2
Blood is more than red cells
Learner response
“Red blood cells fight infection, white blood cells carry oxygen, and plasma is only waste liquid.” Correct all three claims.
Match each component to its main role
View solution step by step
Reassign the functions
Method
Match oxygen transport, defence and dissolved-substance transport correctly.
Reason
Different blood components have structures suited to different jobs.
Working
Red blood cells carry oxygen. White blood cells help defend against pathogens. Plasma transports blood cells and dissolved substances such as nutrients, hormones, carbon dioxide and urea.
Challenge 3
Predict effects from a blood test
Problem
A blood test shows that a patient has far fewer red blood cells and platelets than usual. Predict one possible effect of each shortage and explain it using the component’s function.
Give two function → effect links
Hints
Hint 1: red cells
What substance do they transport?
Hint 2: platelets
Which response helps stop bleeding?
View solution step by step
Use each missing function
Method
Predict an oxygen-transport effect and a clotting effect.
Reason
A shortage reduces the job performed by that component.
Working
Fewer red blood cells may reduce oxygen delivery to tissues, contributing to tiredness or breathlessness. Fewer platelets may slow clot formation, so bleeding may take longer to stop.
Guided practice
Try it with support
Choose the blood component for each job: oxygen transport, clotting, dissolved-food transport and antibody formation.
Use each of the four components once.
Check the guided answer
Answer: Red blood cells transport oxygen, platelets help clotting, plasma transports dissolved food, and white blood cells contribute to antibody formation.
Check: White blood cells also destroy pathogens and are involved in tissue rejection.
Practise and continue
Practise this
Explain why plasma is not merely water with no transport function.
Need a hint?
Name two dissolved substances it carries.
Check your answer
Answer: Plasma carries dissolved digested food substances and wastes such as carbon dioxide around the body.
Check: Match a component to its function rather than its appearance.
Think like a scientist
Why would counting only red blood cells give incomplete information about blood function?
Check the reasoning
Red blood cells address oxygen transport but omit plasma transport, platelet clotting and white-cell defence.
Remember: A model or count must match the biological question being asked.
One-minute check
Hide the page. Give the main jobs of plasma, red blood cells, white blood cells and platelets.
Explain why white blood cells can be involved in rejection of a donated organ even when it is not infected.
Correct this claim: “Red blood cells stop bleeding because blood is red.”
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
Last reviewed: