State the exchange functions of capillaries
Connect blood transport with delivery to cells and removal of their wastes.
Section B9.3 · Blood vessels
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall diffusion and respiration.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which vessel is the main local exchange surface?
What you will learn
- Delivery ends at a tissue exchange surface
- Distinguish body exchange from lung exchange
Delivery ends at a tissue exchange surface
Capillaries form networks close to body cells. They allow exchange between blood and tissues: oxygen and nutrients such as glucose can move from blood towards cells, while carbon dioxide and other wastes can move from cells towards blood. The heart transports blood around the system; a capillary network connects that transport with local exchange.
For oxygen in a respiring body tissue, the supplied model shows a higher concentration on the blood side than the cell side. Net diffusion follows that gradient. Carbon dioxide produced by respiration has the opposite net direction in this model. Do not assume every substance moves in the same direction at every organ.
Distinguish body exchange from lung exchange
At the lungs, oxygen moves into the blood from the air-exchange surface and carbon dioxide moves out of blood towards the air. Capillaries still carry out exchange, but the directions differ from a respiring body tissue. Use the stated location and gradient rather than memorising that oxygen always leaves blood.
Blood cells normally remain in the capillary lumen during this exchange model. Oxygen and dissolved substances cross the wall; whole red blood cells do not routinely pass through it into tissues. The schematic is for explaining exchange, not for calculating real vessel dimensions or diagnosing a specimen.
Sources for this lesson
Pause and explain
What is the net oxygen direction at the supplied respiring body tissue?
Worked example
At a supplied body tissue, oxygen is more concentrated in blood and carbon dioxide in cells. State both net movements.
Show the worked solution
- Oxygen moves down its gradient from blood towards cells.
- Carbon dioxide moves down its gradient from cells towards blood.
Answer Oxygen: blood → cells. Carbon dioxide: cells → blood.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State two materials delivered from blood to body cells in this model.
Give me a hint
Choose respiration materials.
Compare my reasoning
- Oxygen is delivered for aerobic respiration.
- Glucose is a transported nutrient used by cells.
Oxygen and glucose.
Look for these in your work
- Name oxygen
- Name a nutrient
State the movement of carbon dioxide at body tissue and at lungs.
Give me a hint
Identify where carbon dioxide is produced and removed.
Compare my reasoning
- Respiring body cells produce carbon dioxide.
- At lungs it moves from blood towards the air-exchange surface.
Body: cells → blood. Lungs: blood → air-exchange surface.
Look for these in your work
- Distinguish the two locations
- Give both directions
Why is “capillaries release whole red blood cells to feed tissues” incorrect?
Give me a hint
Separate the carrier from the material it carries.
Compare my reasoning
- Red blood cells normally stay within the lumen.
- Oxygen and dissolved nutrients cross the capillary wall towards cells.
Capillaries exchange oxygen and dissolved materials; red blood cells normally remain in the blood.
Look for these in your work
- Keep cells within the lumen
- Describe exchanged materials
Common mistakes
- Saying every substance leaves blood.
- Making red blood cells normally leave the lumen.
What can you explain now?
What is the net oxygen direction at the supplied respiring body tissue?
Compare with the explanation
Blood → cells
The supplied oxygen gradient favours delivery.
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Make it stick
Trace oxygen and carbon dioxide across a body-tissue capillary, then reverse the context to lungs.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.