Launching your learning experience…
Detailed notes on Transport in Animals for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Mammals run a DOUBLE circulation — blood passes through the heart twice per loop. Faster, higher-pressure delivery than fish.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Fish: blood through heart once. Mammals: twice. Doubling re-pressurises blood for the body.
Single circulation (fish).
Double circulation (mammals, birds).
| Feature | Single (fish) | Double (mammal) |
|---|---|---|
| Times through heart per circuit | Once | Twice |
| Heart pumps per loop | One | Two (right then left) |
| Pressure to body | Low (dropped at gills) | High (re-pressurised) |
| Speed of O₂ delivery | Slower | Faster |
| Suits | Lower-energy lifestyle | High metabolism (warm-blooded) |
Worked qualitative. Why do mammals have a double circulation but lizards have a partial one?
Cambridge tip. Memorise the four-step loop for each. Single = 4 stops; double = 5 (heart appears twice).
Two named loops within the double circulation.
Pulmonary circulation.
Systemic circulation.
Sub-loops within systemic.
Worked qualitative. Why does the LEFT VENTRICLE have a thicker wall than the right?
Cambridge tip. Cambridge often draws the heart and asks for a vessel name. The pulmonary VESSELS confuse most students — pulmonary artery (deoxy) and pulmonary vein (oxy). Memorise.
Two pumps, two pressures — blood reaches body cells faster and at higher pressure.
Three big advantages:
1. Higher blood pressure to body.
2. Faster delivery of O₂ and nutrients.
3. No mixing.
Why mammals couldn't survive on single circulation.
Worked qualitative. Why are athletes' resting heart rates LOWER than non-athletes'?
Cambridge tip. When asked WHY double circulation, give "higher pressure to body" + "faster O₂ delivery" + "no mixing". Three points = top marks.
Verbatim phrases and definitions Cambridge mark schemes credit.
Circulatory systems appear Paper 4 (5-7 marks). Common formats: contrast single/double, label heart vessels, justify the advantage. Examiner reports flag students mislabelling the pulmonary artery/vein and saying blood mixes in the heart.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (9.1); 0610/42 May/Jun 2024 — Q9 (circulatory systems); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on circulatory systems, written exactly the way a tutor would explain them at the board.
Question
Compare single circulation (e.g. fish) with double circulation (e.g. mammals).
Step-by-step solution
Step 1
Single circulation: the blood passes through the heart ONCE per complete circuit (heart → gills → body → heart). Found in fish.
Step 2
Double circulation: the blood passes through the heart TWICE per complete circuit (heart → lungs → heart → body → heart). Found in mammals.
Step 3
In single circulation the blood pressure drops a lot as it passes through the gills, so it flows slowly to the body.
Step 4
In double circulation the blood returns to the heart to be re-pressurised before going to the body, so it flows quickly at high pressure.
Answer
Single (fish): blood through the heart once per circuit. Double (mammal): blood through the heart twice per circuit, re-pressurised, so it reaches the body faster.
Examiner tip
Cambridge often shows diagrams of both and asks you to identify them. Learn the loop order.
Question
State what the pulmonary circulation and the systemic circulation each connect.
Step-by-step solution
Step 1
Pulmonary circulation: from the RIGHT side of the heart → the LUNGS → back to the LEFT side of the heart.
Step 2
Systemic circulation: from the LEFT side of the heart → the rest of the BODY → back to the RIGHT side of the heart.
Step 3
Each full cycle passes through the heart twice — once for each circuit — which is what makes it a double circulation.
Answer
Pulmonary: heart ↔ lungs (for gas exchange). Systemic: heart ↔ rest of the body (for delivery and collection).
Question
Starting at the lungs, describe the path of blood around the body in a double circulation.
Step-by-step solution
Step 1
Oxygenated blood leaves the lungs and travels in the pulmonary vein to the left atrium.
Step 2
It passes into the left ventricle, which pumps it through the aorta to the body.
Step 3
At the body, oxygen is delivered to the cells, so the blood becomes deoxygenated, and returns in the vena cava to the right atrium.
Step 4
It passes into the right ventricle, which pumps it through the pulmonary artery back to the lungs to be re-oxygenated.
Answer
Lungs → left atrium → left ventricle → body (O₂ delivered) → right atrium → right ventricle → lungs. Two loops, two passes through the heart.
Question
Explain why mammals keep oxygenated and deoxygenated blood separate as it passes through the heart.
Step-by-step solution
Step 1
The right side of the heart carries DEOXYGENATED blood (to the lungs); the left side carries OXYGENATED blood (to the body).
Step 2
A wall of muscle, the septum, separates the two sides so the blood does not mix.
Step 3
If the blood mixed, the blood sent to the body would carry less oxygen.
Step 4
Less oxygen to the cells means less aerobic respiration and less energy released — which would not support the high metabolic rate of an active mammal.
Answer
The septum keeps the sides separate so fully oxygenated blood goes to the body, maximising oxygen delivery for the high energy needs of a mammal.
Question
Explain why a double circulation can deliver oxygen to the body more efficiently than a single circulation.
Step-by-step solution
Step 1
In a single circulation, blood loses a lot of pressure passing through the narrow capillaries of the gills, so it travels on to the body slowly and at low pressure.
Step 2
In a double circulation, blood returns to the heart after the lungs and is pumped again, restoring a high pressure.
Step 3
So the blood reaches the body tissues at high pressure and flows quickly, delivering oxygen and nutrients faster.
Step 4
Faster delivery (and faster removal of waste) supports a higher rate of respiration in the cells, releasing more energy.
Answer
After the lungs, the blood is re-pumped by the heart, so it reaches the body at high pressure and flows fast — delivering oxygen quickly enough to support a high metabolic rate. A single circulation cannot do this.
Examiner tip
The key idea is re-pressurising the blood after the lungs, giving fast, high-pressure flow to the body.
Question
Blood leaving the left ventricle is at a much higher pressure than blood leaving the right ventricle. Explain why.
Step-by-step solution
Step 1
The left ventricle pumps blood through the systemic circulation — all the way around the whole body, a long route with high resistance.
Step 2
It must therefore generate a high pressure so the blood reaches every part of the body.
Step 3
The right ventricle pumps blood only to the nearby lungs (the pulmonary circulation), a short route.
Step 4
The lung capillaries are also delicate, so a lower pressure is needed to avoid damaging them. This is why the left ventricle wall is thicker than the right.
Answer
The left ventricle pumps blood around the whole body (long route), needing high pressure; the right ventricle pumps only to the nearby lungs, needing lower pressure. Hence the left side works at a higher pressure.
Examiner tip
Link the pressure difference to the distance/resistance of each circuit and to the thicker left ventricle wall.
High-scoring sample answers for circulatory systems on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State what is meant by a double circulation. (1 mark)
Model answer
A circulatory system in which the blood passes through the heart twice for each complete circuit of the body.
Why this scores
One mark for 'blood passes through the heart twice per circuit'.
Name the two circuits of a double circulation and state what each connects. (2 marks)
Model answer
The pulmonary circulation connects the heart to the lungs. The systemic circulation connects the heart to the rest of the body.
Why this scores
One mark each: pulmonary = heart to lungs; systemic = heart to body.
Describe the path taken by the blood around a double circulatory system, starting at the lungs. (3 marks)
Model answer
Oxygenated blood leaves the lungs and travels to the left side of the heart, which pumps it out to the body. At the body the oxygen is used, so the blood becomes deoxygenated and returns to the right side of the heart, which pumps it back to the lungs to be oxygenated again.
Why this scores
Three marks for the correct sequence: lungs → left heart → body → right heart → lungs.
Explain the advantage of a double circulation over a single circulation. (4 marks)
Model answer
In a single circulation, the blood loses much of its pressure as it passes through the gills, so it flows slowly to the body. In a double circulation, the blood returns to the heart after passing through the lungs and is pumped again, so it is re-pressurised to a high pressure. This means the blood reaches the body tissues quickly and at high pressure, delivering oxygen and nutrients faster, which supports the high metabolic rate of an active, warm-blooded mammal.
Why this scores
Four marks: single loses pressure at gills/flows slowly; double re-pumps blood; higher pressure/faster flow to body; supports high metabolic rate.
Explain why it is important that oxygenated and deoxygenated blood do not mix in the mammalian heart. (5 marks)
Model answer
The right side of the heart carries deoxygenated blood to the lungs, while the left side carries oxygenated blood to the body, and the two are kept apart by the septum. If the two mixed, the blood pumped to the body would carry less oxygen. This would mean the body cells receive less oxygen for aerobic respiration, so less energy would be released. Because mammals are active and warm-blooded and have a high metabolic rate, they need a constant, full supply of oxygen, so keeping the blood fully oxygenated makes the delivery of oxygen as efficient as possible.
Why this scores
Five marks: right = deoxygenated/left = oxygenated; septum keeps them apart; mixing would lower oxygen to body; less respiration/energy; mammals need high oxygen supply for high metabolic rate.
Compare single and double circulation, and explain why a double circulation is more suitable for an active mammal. (6 marks)
Model answer
In a single circulation, such as that of a fish, the blood passes through the heart only once per circuit: the heart pumps blood to the gills for oxygenation, and the same blood then flows straight on to the body before returning to the heart. Because the blood loses a lot of its pressure at the gills, it flows to the body slowly and at low pressure. In a double circulation, such as that of a mammal, the blood passes through the heart twice per circuit: once in the pulmonary circuit (heart → lungs → heart) and once in the systemic circuit (heart → body → heart). After the lungs, the blood returns to the heart to be pumped again, so it leaves for the body at high pressure and flows quickly. This faster, high-pressure delivery brings oxygen and nutrients to the cells, and removes waste, more quickly, supporting a high and constant rate of respiration. Since a mammal is active and warm-blooded and so has a high metabolic rate, the double circulation is much better suited to meeting its high energy demands than a single circulation would be.
Why this scores
Up to 6 marks: single = once through the heart; pressure drops at gills/slow flow; double = twice through the heart (pulmonary + systemic); re-pressurised after lungs; high-pressure/fast delivery; supports high metabolic rate of an active mammal.
Definitions to memorise and the exact keywords mark schemes credit for circulatory systems answers — sharpened from recent examiner reports for the 2026 0610 sitting.
Circulatory system in which blood passes through the heart twice per complete circuit — once for the lungs (pulmonary) and once for the rest of the body (systemic).
Loop carrying blood from the right side of the heart to the lungs and back to the left side of the heart.
Loop carrying blood from the left side of the heart, around the body, and back to the right side.
Solid wall of muscle separating the right and left sides of the heart, preventing mixing of oxygenated and deoxygenated blood.
The traps other students keep falling into on circulatory systems questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Veins usually carry deoxygenated blood — except this one.
How to avoid it
The PULMONARY VEIN is the exception — it carries OXYGENATED blood from the lungs to the heart. The pulmonary artery carries deoxygenated blood.
Why it happens
Both pass through the same organ.
How to avoid it
The SEPTUM keeps them apart. Right side = deoxygenated, left side = oxygenated; they do not mix in a healthy heart.
Why it happens
Both terms describe how often blood visits the heart.
How to avoid it
DOUBLE = blood passes through the heart TWICE per circuit; SINGLE = once. The mammal heart has 4 chambers; the fish heart effectively 2.
The things students keep getting wrong in this sub-topic, answered.