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

This guide covers statements B2.2a to B2.2e of OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), for first assessment in 2018. The statements are in module B2.2, The challenges of size. No statement in this unit is printed in bold, so everything here is assessed in both Foundation and Higher Tier papers. Plant transport (B2.2f to B2.2j) has its own pages. For condensed recall see the revision notes, and for exam-style questions use the practice set. Course pages: OCR GCSE Biology hub, checklist and free diagnostics. For the wider topic see Scaling Up.

What this unit covers

Statement	What you must be able to do
B2.2a	Give the reasons large multicellular organisms need both exchange surfaces and a transport system, using surface area : volume ratio. Calculate surface area, volume and the ratio. Refer to diffusion distances.
B2.2b	Describe the substances moved into and out of different organisms, matching each to the organism’s needs. The list is oxygen, carbon dioxide, water, urea, mineral ions and dissolved food molecules.
B2.2c	Describe the human circulatory system: the link to the gaseous exchange system, why mammals need a double circulation, and how the vessels are arranged.
B2.2d	Explain how the heart and blood vessels are adapted to their functions: cardiac muscle, valve names, chambers, vessels in and out of the heart, and wall thickness, lumen and valves in vessels.
B2.2e	Explain how red blood cells and plasma are adapted to transport.

The mathematical outcomes are surface area : volume ratios, rates, and plotting and interpreting graphs.

B2.2a: Why size creates a problem

Every living cell needs oxygen and food molecules, and has to get rid of carbon dioxide and other wastes. These substances cross the cell surface by diffusion. A very small organism gets by on diffusion alone because its surface is large compared with the amount of living material inside, and no cell is far from the surface.

Make an organism bigger and the picture changes. Volume grows faster than surface area, so the ratio of surface area to volume falls. The surface can no longer supply the whole body. Cells in the middle are far from the outside, so diffusion is too slow to reach them.

A large multicellular organism therefore needs two things:

• An exchange surface, a specialised organ with a large surface area, such as lungs, gills or the small intestine wall, where substances cross between the organism and its surroundings.
• A transport system, such as the blood in a circulatory system, which delivers substances from the exchange surface to all the cells, so that no cell depends on diffusion across a long distance.

Calculating surface area, volume and the ratio

For a cuboid with sides a, b and c:

• Surface area = 2(ab + bc + ac), which is 6 × side² for a cube.
• Volume = a × b × c, which is side³ for a cube.
• Ratio = surface area ÷ volume, written as a : 1 or as a single number.

Worked example 1: cubes of different size. Each cube stands for an imaginary organism.

Side (mm)	Surface area (mm²)	Volume (mm³)	SA : V
1	6	1	6 : 1
6	216	216	1 : 1
12	864	1728	0.5 : 1
24	3456	13824	0.25 : 1

Check the 12 mm row. Surface area = 6 × 12 × 12 = 864 mm². Volume = 12 × 12 × 12 = 1728 mm³. Ratio = 864 ÷ 1728 = 0.5. Each time the side doubles, the ratio halves. The 24 mm cube is 24 times bigger in side than the 1 mm cube, and its ratio is 24 times smaller.

Worked example 2: shape matters as well as size. Two blocks each have a volume of 60 mm³.

Flat block, 12 × 5 × 1 mm. Surface area = 2(60 + 5 + 12) = 154 mm². Ratio = 154 ÷ 60 = 2.57, so 2.6 : 1 to 2 significant figures.

Compact block, 5 × 4 × 3 mm. Surface area = 2(20 + 12 + 15) = 94 mm². Ratio = 94 ÷ 60 = 1.57, so 1.6 : 1.

The flat shape has the larger ratio, and no cell is more than 0.5 mm from a surface. This is why flatworms can exchange gases across their body surface, and why leaves are thin and flat. A large active animal cannot do this, so it folds its exchange surface inside the body (lungs, villi) and uses blood to link them to the cells.

Practical link. A common investigation soaks cubes of agar or gelatine containing an indicator in acid, then times how long the colour takes to change throughout. Small cubes change colour faster because the acid has less distance to travel and the ratio is larger.

B2.2b: What is moved, and why

Learn six substances: oxygen, carbon dioxide, water, urea, mineral ions and dissolved food molecules. What moves in which direction depends on what the organism needs.

Organism	Taken in	Given out	Reason
Single-celled organism in pond water	Oxygen, dissolved food molecules, water	Carbon dioxide, wastes	Cell respiration needs oxygen and glucose. Everything crosses the cell surface.
Fish	Oxygen (from water at the gills), dissolved food molecules (from the gut)	Carbon dioxide (at the gills), ammonia and other wastes	Large body, so blood carries gases and food between exchange surfaces and cells.
Human	Oxygen (lungs), dissolved food molecules, water and mineral ions (small intestine)	Carbon dioxide (lungs), urea (in urine), water	Cells need oxygen and glucose for respiration, amino acids for protein, mineral ions for example for haemoglobin.
Plant	Carbon dioxide (leaves), water and mineral ions (roots)	Oxygen (from photosynthesis), water vapour	Photosynthesis needs carbon dioxide and water. Mineral ions such as nitrate are needed to make amino acids.

Urea is a nitrogen-containing waste. Liver cells produce it when amino acids are in excess, plasma delivers it to the kidneys, and it leaves the body dissolved in urine. Carbon dioxide is a waste of respiration, and it is also a raw material in plants, so the direction depends on the organism.

B2.2c: The human circulatory system

Humans have a double circulation, meaning that blood goes through the heart twice during one full circuit of the body.

• Pulmonary circuit (heart to lungs and back). Deoxygenated blood leaves the right ventricle in the pulmonary artery and goes to the lungs. In the capillaries around the alveoli, oxygen moves into the blood and carbon dioxide moves out. Oxygenated blood returns to the left atrium in the pulmonary vein.
• Systemic circuit (heart to body and back). Oxygenated blood leaves the left ventricle in the aorta and travels in arteries to organs. In organ capillaries it gives up oxygen and picks up carbon dioxide. Deoxygenated blood returns to the right atrium in the vena cava.

The link to the gaseous exchange system is the pulmonary circuit. The lungs supply oxygen to the blood, the blood carries it to the heart, and the heart sends it to the cells.

Why a double circulation?

1. Blood loses pressure as it is forced through the narrow capillaries of the lungs. Returning it to the heart lets it be pumped again at high pressure to the rest of the body.
2. Oxygenated and deoxygenated blood stay separate, so the blood sent to the cells is carrying a full load of oxygen.
3. The lungs get blood at a lower pressure than the body, which protects the delicate capillary walls in the alveoli, while the body gets a high pressure that moves blood quickly to supply a high rate of respiration.

Compare this with a fish, where blood passes through the heart once and goes straight from gills to body, with a large loss of pressure on the way.

B2.2d: Heart and blood vessels

The heart

The heart is a muscular pump with four chambers: two atria on top that receive blood, and two ventricles below that pump blood out. A wall called the septum separates the left side from the right.

Vessel	Joins	Carries
Vena cava	Body to right atrium	Deoxygenated blood
Pulmonary artery	Right ventricle to lungs	Deoxygenated blood
Pulmonary vein	Lungs to left atrium	Oxygenated blood
Aorta	Left ventricle to body	Oxygenated blood
Coronary arteries	Branch from the aorta over the heart surface	Oxygenated blood to the heart muscle

Valves make blood flow one way. The atrioventricular valves sit between each atrium and the ventricle below it. The right side has the tricuspid valve and the left side has the bicuspid valve. The semilunar valves lie at the exits of the ventricles, in the pulmonary artery and the aorta.

Cardiac muscle is the tissue of the heart wall. It contracts and relaxes regularly without tiring, and it needs a steady supply of oxygen and glucose, which the coronary arteries provide. The wall of the left ventricle is thicker than that of the right, because it must generate enough pressure to push blood around the whole body. The right ventricle only pumps to the nearby lungs.

Blood vessels

Feature	Artery	Vein	Capillary
Direction of flow	Away from heart	Towards heart	Links arteries to veins
Wall	Thick, with muscle and elastic fibres	Thin	One cell thick
Lumen	Narrow compared with wall	Wide	Very narrow
Valves	None	Present	None
Blood pressure	High	Low	Falls along the vessel

• Arteries carry blood at high pressure. The thick elastic wall stretches as blood surges through and recoils afterwards, which helps push blood on and stops the wall bursting.
• Veins carry blood at low pressure, so a thick wall is unnecessary. The wide lumen reduces resistance to flow, and valves stop backflow. Squeezing by nearby muscles helps move blood.
• Capillaries are where exchange occurs. A wall one cell thick gives a very short diffusion distance.

Why does blood move slowly in capillaries? Not because each one is narrow. What matters is the combined cross-sectional area of all the capillaries, which the specification states is about 1000 times that of the aorta. The same blood flow spreads over a much bigger total area, so it slows down, giving time for diffusion.

Worked example 3: wall to lumen ratio. An artery has a wall 1.2 mm thick and a lumen 4.8 mm across. A vein has a wall 0.3 mm thick and a lumen 6.0 mm across.

Artery wall ÷ lumen = 1.2 ÷ 4.8 = 0.25. Vein wall ÷ lumen = 0.3 ÷ 6.0 = 0.05. The artery’s ratio is five times the vein’s, which fits an artery carrying blood at high pressure and a vein needing a wide channel at low pressure.

Worked example 4: a rate. A pulse is counted as 38 beats in 30 seconds. In beats per minute, 38 × 2 = 76.

B2.2e: Blood cells and plasma

Red blood cells carry oxygen. Their features:

• Biconcave disc shape. This gives a large surface area compared with volume, so oxygen diffuses in and out quickly. The shape also makes the cell flexible, so it can squeeze through narrow capillaries.
• No nucleus. This leaves more room inside for haemoglobin.
• Haemoglobin. This iron-containing protein binds oxygen in the lungs, where there is plenty, forming oxyhaemoglobin. In respiring tissues, where oxygen is scarce, it lets the oxygen go.

Plasma is a pale yellow liquid, mostly water. As a solvent it dissolves and transports carbon dioxide, dissolved food molecules such as glucose and amino acids, mineral ions, urea and hormones. Blood cells and platelets are carried in it.

Common errors

• Writing that a bigger organism has a bigger surface area to volume ratio. It is smaller.
• Calling the pulmonary vein deoxygenated. It is the exception to “veins carry deoxygenated blood”.
• Mixing up left and right. The left ventricle is the thick-walled chamber.
• Explaining slow capillary flow by narrow diameter alone. Use total cross-sectional area.
• Writing “oxygen is carried in plasma”. Oxygen is carried by haemoglobin in red cells.

Official syllabus

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