Transport in Animals
Large animals cannot rely on diffusion alone — substances would take far too long to reach cells deep inside the body. A circulatory system solves this by pumping blood, carrying substances rapidly to every tissue.
Most of this topic is structure-and-function: for every feature you learn, be ready to say what it does and how its structure suits that job.
1. The double circulatory system
A double circulatory system is one in which the blood passes through the heart twice for each complete circuit of the body.
The two circuits are:
| Circuit | Route |
|---|---|
| Pulmonary | Heart → lungs → heart |
| Systemic | Heart → rest of the body → heart |
The advantage: blood can be pumped again at high pressure after returning from the lungs. That means blood travels to the body faster, delivering oxygen and glucose more quickly and allowing a higher rate of respiration.
A single circulatory system (as in fish) loses pressure passing through the gills, so blood reaches the body more slowly.
2. The heart
Structures to know
| Structure | Function |
|---|---|
| Left and right atria | Upper chambers; receive blood returning to the heart |
| Left and right ventricles | Lower chambers; pump blood out of the heart |
| Septum | Wall separating the two sides, keeping oxygenated and deoxygenated blood separate |
| Valves | Prevent backflow of blood, keeping it moving in one direction |
| Coronary arteries | Supply the heart muscle itself with oxygen and glucose |
| Muscular wall | Contracts to pump blood |
The blood vessels attached to the heart
| Vessel | Carries blood | Blood type |
|---|---|---|
| Vena cava | From the body → right atrium | Deoxygenated |
| Pulmonary artery | Right ventricle → lungs | Deoxygenated |
| Pulmonary vein | Lungs → left atrium | Oxygenated |
| Aorta | Left ventricle → body | Oxygenated |
The pulmonary vessels are the exception to the usual rule. Arteries normally carry oxygenated blood and veins deoxygenated — but the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. Define arteries and veins by direction of flow, not by the blood they carry: arteries carry blood away from the heart; veins carry blood towards the heart.
The path of blood through the heart
Learn this as a fixed sequence:
vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body
Why the left ventricle wall is thicker
A very common question. The left ventricle pumps blood to the whole body, which is a much greater distance than the lungs. It therefore needs to generate a higher pressure, so it has more muscle in its wall.
The right ventricle only pumps blood to the lungs, which are close by — and lower pressure also avoids damaging the delicate capillaries there.
Do not say the thicker wall “stops the heart bursting.” The reason is that it must generate more force to push blood further.
Sides of a heart diagram
In a diagram, the heart is drawn as though you are facing the person. So the left side of the heart appears on the right of the diagram. Label from the vessels, not from which side of the page it is on.
3. Heart rate and exercise
During exercise, heart rate and breathing rate increase. The reasoning chain:
- Muscles contract more and need more energy
- So the rate of aerobic respiration increases
- More oxygen and glucose must be delivered, and more carbon dioxide removed
- So the heart beats faster and more strongly
Adrenaline also increases heart rate, preparing the body for activity.
After exercise, heart and breathing rates stay high for a while to supply oxygen to break down the lactic acid produced by anaerobic respiration.
4. Coronary heart disease
The coronary arteries supply the heart muscle. In coronary heart disease, fatty deposits build up in these arteries, narrowing them.
The consequences follow logically: less blood reaches the heart muscle → less oxygen and glucose → less respiration → the muscle cannot contract properly. If an artery becomes fully blocked, that region of muscle dies — a heart attack.
Risk factors:
| Risk factor | |
|---|---|
| Diet high in saturated fat and salt | Smoking |
| Lack of exercise | Stress |
| Being overweight | Age, sex and genetic factors |
Prevention and treatment: a balanced diet, regular exercise, not smoking, and reducing stress. Treatments include drugs, stents to widen the artery, and bypass surgery.
5. Blood vessels
| Artery | Vein | Capillary | |
|---|---|---|---|
| Direction | Away from the heart | Towards the heart | Links arteries to veins |
| Wall | Thick, muscular and elastic | Thin | One cell thick |
| Lumen | Narrow | Wide | Very narrow — one red blood cell wide |
| Valves | No | Yes | No |
| Pressure | High | Low | Falling |
How each structure suits its function:
- Arteries have thick, elastic, muscular walls to withstand the high pressure of blood leaving the heart, and to stretch and recoil to keep blood moving.
- Veins have a wide lumen to reduce resistance at low pressure, and valves to prevent backflow — blood is helped along by the contraction of surrounding skeletal muscles.
- Capillaries are one cell thick, giving a short diffusion distance for the rapid exchange of substances — oxygen and glucose out to the tissues, carbon dioxide and waste in.
6. Blood
| Component | Structure | Function |
|---|---|---|
| Red blood cells | Biconcave disc; no nucleus; contain haemoglobin | Transport oxygen |
| White blood cells | Have a nucleus | Defence against pathogens |
| Platelets | Cell fragments | Blood clotting |
| Plasma | Straw-coloured liquid | Transports blood cells, nutrients, carbon dioxide, urea, hormones and heat |
Red blood cells
Haemoglobin combines with oxygen in the lungs to form oxyhaemoglobin, and releases it in respiring tissues where oxygen concentration is low.
Their adaptations:
- Biconcave shape — a large surface area to volume ratio for faster oxygen absorption
- No nucleus — more room for haemoglobin
- Small and flexible — can squeeze through capillaries
White blood cells
| Type | Function |
|---|---|
| Phagocytes | Engulf and digest pathogens (phagocytosis); have a lobed nucleus |
| Lymphocytes | Produce antibodies; have a large round nucleus |
Lymphocytes make antibodies; phagocytes engulf. These are swapped constantly. The hook is in the name: phagocyte → phagocytosis → engulfing.
Clotting
Platelets trigger a series of reactions in which the soluble protein fibrinogen is converted into insoluble fibrin, which forms a mesh of fibres trapping red blood cells to form a clot.
The clot prevents blood loss and stops pathogens entering the wound.
The direction matters: fibrinogen → fibrin. Writing it the other way round is marked wrong.
7. Mistakes that cost marks
Defining arteries as “carrying oxygenated blood.” They carry blood away from the heart — the pulmonary artery carries deoxygenated blood.
Saying the thick left ventricle wall stops it bursting. It generates more force for a greater distance.
Saying phagocytes produce antibodies. Lymphocytes do.
Writing “fibrin → fibrinogen.” It is fibrinogen → fibrin.
Saying red blood cells have no nucleus “so they are smaller.” The reason is more space for haemoglobin.
Labelling the heart from the page rather than the vessels.
Saying capillaries are thin “so blood flows faster.” They are one cell thick for a short diffusion distance for exchange.
Frequently asked questions
What is a double circulatory system? One where blood passes through the heart twice per circuit — once to the lungs, once to the body. It allows blood to be re-pressurised, so it travels round the body faster.
Which side of the heart has the thicker wall, and why? The left, because it pumps blood to the whole body and needs to generate higher pressure.
Which blood vessel carries deoxygenated blood away from the heart? The pulmonary artery.
Why do veins have valves but arteries don’t? Blood in veins is at low pressure, so valves are needed to prevent it flowing backwards. Arterial blood is at high pressure and keeps moving forwards.
Why do red blood cells have no nucleus? To leave more room for haemoglobin, so each cell carries more oxygen.
What is the difference between a phagocyte and a lymphocyte? Phagocytes engulf and digest pathogens; lymphocytes produce antibodies.
What causes a heart attack? A blocked coronary artery stops blood reaching part of the heart muscle, so it receives no oxygen and dies.
Quick revision checklist
- I can define a double circulatory system and give its advantage
- I can label the heart: atria, ventricles, septum, valves, coronary arteries
- I can name the four vessels and say which blood each carries
- I can write the path of blood through the heart in order
- I can explain why the left ventricle wall is thicker, correctly
- I can explain why heart rate rises during exercise, as a chain
- I can describe coronary heart disease, its risk factors and treatments
- I can compare arteries, veins and capillaries across all five rows
- I can link each vessel’s structure to its function
- I can name the four components of blood and their functions
- I can give three adaptations of red blood cells with reasons
- I know lymphocytes make antibodies and phagocytes engulf
- I can describe clotting, with fibrinogen → fibrin the right way round
These notes cover topic 9 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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