OCR GCSE Biology A: Exchange surfaces and the human circulatory system (J247) – Revision Notes

Quick recall for OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), first assessment 2018, statements B2.2a to B2.2e. Nothing in this unit is Higher Tier only. Full explanations are in the study guide, and you can test yourself with the practice set. See also the OCR GCSE Biology hub, the checklist, the wider Scaling Up revision notes and the free diagnostics for a quick check.

The core argument (B2.2a)

Step	Statement
1	Cells need oxygen and food and must lose carbon dioxide and wastes.
2	As size increases, volume rises faster than surface area, so SA : V falls.
3	Diffusion distance to inner cells becomes long, so diffusion alone is too slow.
4	Solution: an exchange surface (large area, thin, good blood supply) plus a transport system (blood).

Formulae

Shape	Surface area	Volume	Ratio
Cube, side s	6s²	s³	6 ÷ s
Cuboid with edges a, b, c	2(ab + ac + bc)	a × b × c	SA ÷ V

Method in steps

1. Work out every face area and add them (or use the formula).
2. Work out volume.
3. Divide surface area by volume.
4. Give units: area in mm², volume in mm³, ratio with no unit, or written as a : 1.
5. Comment: larger ratio means faster exchange per unit volume.

Worked reminder. Cube of side 8 mm: SA = 6 × 64 = 384 mm². V = 512 mm³. Ratio = 384 ÷ 512 = 0.75, so 0.75 : 1. A cube of side 16 mm has SA = 1536 mm², V = 4096 mm³, ratio 0.375 : 1. Doubling the side halves the ratio.

Shape example. Both blocks 60 mm³: flat 12 × 5 × 1 mm has SA 154 mm², ratio 2.6 : 1. Compact 5 × 4 × 3 mm has SA 94 mm², ratio 1.6 : 1. Flat means higher ratio and shorter diffusion distance.

Substances moved (B2.2b)

Substance	Direction and reason
Oxygen	In, for aerobic respiration. Out of plants after photosynthesis.
Carbon dioxide	Out of animals (waste of respiration). Into plants (raw material for photosynthesis).
Water	In, as a reagent, solvent and for turgor. Lost as water vapour by plants, and in urine and sweat in humans.
Dissolved food molecules	In from the gut in animals (glucose, amino acids). Moved around in blood plasma.
Mineral ions	In by plant roots (for example nitrate for amino acids). Absorbed from the human gut.
Urea	Nitrogen-containing waste produced by liver cells, taken by plasma to the kidneys, lost in urine.

Always link the substance to what that organism needs.

Human circulatory system (B2.2c)

• Left side of heart to body: aorta, arteries, capillaries, veins, vena cava, back to the right side.

• Right side of heart to lungs: pulmonary artery, lung capillaries, pulmonary vein, back to the left side.

• Double circulation: heart to lungs and back, then heart to body and back.

• Link to gaseous exchange: blood is oxygenated in alveolar capillaries and loses carbon dioxide there.

• Why double: blood loses pressure in the lung capillaries so it is returned to the heart to be pumped hard again. Keeps oxygenated and deoxygenated blood apart. Lungs receive a lower pressure that does not damage their fragile capillaries.

Heart and vessels (B2.2d)

Name	Job
Right atrium	Receives deoxygenated blood from the vena cava
Right ventricle	Pumps to the lungs via the pulmonary artery
Left atrium	Receives oxygenated blood from the pulmonary vein
Left ventricle	Thick wall. Pumps to the body via the aorta
Tricuspid (right), bicuspid (left)	Valves between atrium and ventricle
Semilunar valves	In the aorta and pulmonary artery exits
Coronary arteries	Supply the cardiac muscle with oxygen and glucose

Cardiac muscle contracts and relaxes regularly without tiring. Valves stop backflow.

Vessel	Wall	Lumen	Valves	Why
Artery	Thick, muscle and elastic fibres	Narrow	None	Wall stretches and recoils under high pressure
Vein	Thin	Wide	Present	Low pressure, so valves stop blood slipping back
Capillary	One cell thick	Very narrow	None	Short diffusion distance, large combined area

Slow flow in capillaries: the total cross-sectional area of all capillaries is far larger than that of the aorta (the specification says about 1000 times), so flow is slow and there is time for diffusion. Do not blame narrow width alone.

Blood (B2.2e)

Part	Adaptation	Reason
Red blood cell	Biconcave disc	Large surface area, flexible in capillaries
Red blood cell	No nucleus	More space for haemoglobin
Red blood cell	Haemoglobin	Picks up oxygen at the lungs and gives it up in the tissues
Plasma	Mostly water, a solvent	Carries dissolved carbon dioxide, glucose, amino acids, mineral ions, urea, hormones

Rates and graphs

Rate = amount ÷ time. A pulse of 41 beats in 30 s is 41 × 2 = 82 beats per minute. On a graph of heart rate against time, the gradient is the change in rate per unit time, and a plateau means the rate has levelled off.

Must-know distinctions

Pair	Difference
Exchange surface and transport system	The surface is where substances cross into or out of the body. The transport system carries them between the surface and the cells.
Artery and vein	Arteries carry blood away from the heart at high pressure. Veins carry it towards the heart at low pressure and have valves.
Pulmonary and systemic circuit	Pulmonary goes heart to lungs to heart. Systemic goes heart to body to heart.
Atrium and ventricle	Atria receive blood. Ventricles pump it out.
Haemoglobin and oxyhaemoglobin	Haemoglobin is the protein. Oxyhaemoglobin is haemoglobin combined with oxygen.
Coronary artery and pulmonary artery	Coronary arteries feed the heart muscle. The pulmonary artery takes blood to the lungs.

Worked reminder: an agar cube result

Cubes of agar with indicator are placed in acid. After a set time the acid has reached 2 mm in from every face. For a 10 mm cube the unaffected core is 10 − 4 = 6 mm across, so its volume is 6 × 6 × 6 = 216 mm³. The cube volume is 1000 mm³, so 21.6% is unaffected and 78.4% has been reached. For a 20 mm cube the core is 16 mm across: 4096 mm³ of 8000 mm³, so 51.2% unaffected and 48.8% reached.

Conclusion: the smaller cube is reached more completely in the same time because it has the larger SA : V ratio and a shorter distance from surface to centre. This is the model for why large organisms need transport systems.

Answer frames

Explain why a large organism needs a transport system. (1) Volume increases faster than surface area, so the ratio falls. (2) Distance from the surface to inner cells is long. (3) Diffusion is too slow to supply oxygen and food and remove waste. (4) So a transport system carries substances to the cells.

Explain how an adaptation suits its function. Use the pattern feature, then effect, then function. Example: thick elastic wall, so it stretches and recoils, so it withstands high pressure and keeps blood moving.

Describe the path of blood. Name each chamber and vessel in order and say whether the blood is oxygenated or deoxygenated at each stage. Mention the lungs, where oxygen is picked up.

Explain why mammals have a double circulation. (1) Blood loses pressure in the lung capillaries. (2) It returns to the heart to be pumped again. (3) The body therefore receives blood at high pressure. (4) The lungs receive it at a lower pressure, which protects their thin capillary walls.

Explain how red blood cells are adapted. Give three features (biconcave shape, no nucleus, haemoglobin) and attach one reason to each. Do not stop at the name of the feature.

Quick self-test

1. What happens to SA : V when an organism gets bigger?
2. Calculate the SA : V of a cube of side 10 mm.
3. Name the vessel that carries blood from the heart to the lungs.
4. Which heart chamber has the thickest wall, and why?
5. Name two features that let veins carry blood at low pressure.
6. Why is a capillary wall one cell thick?
7. Why do red blood cells lack a nucleus?
8. Which substance made in the liver is carried in plasma to the kidneys?
9. Name the valves between the atria and ventricles.
10. Why does blood slow down in capillaries?

Answers

1. It falls, because volume increases faster than surface area.
2. SA = 600 mm², V = 1000 mm³, ratio 0.6 : 1.
3. Pulmonary artery.
4. Left ventricle, because it pumps blood at high pressure around the whole body.
5. Thin wall with a wide lumen, and valves.
6. It gives a short diffusion distance for exchange.
7. More room for haemoglobin, so more oxygen carried.
8. Urea.
9. Tricuspid (right) and bicuspid (left).
10. Their total cross-sectional area is much larger than the aorta’s.

Where marks are usually lost

• Saying the ratio “increases” as size increases.
• Giving the ratio without showing surface area and volume.
• Leaving out units, or writing cm² for volume.
• Calling the pulmonary vein deoxygenated, or the pulmonary artery oxygenated.
• Mixing left and right, or saying the left ventricle is thick “to pump harder” without saying around the whole body.
• Naming a feature but not explaining its function, such as “biconcave” with no mention of surface area.
• Writing that oxygen is carried by plasma.
• Blaming narrow capillaries alone for slow flow.

Official syllabus

These notes follow OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for first assessment in 2018, published by OCR: statements B2.2a to B2.2e.