Syllabus coverage
- Describe the circulatory system as a system of blood vessels with a pump and valves to ensure one-way flow of blood
- Describe the single circulation of a fish and the double circulation of a mammal, and explain the advantages of a double circulation
- Identify in diagrams and images the structures of the mammalian heart, limited to the muscular wall, septum, left and right ventricles, left and right atria, one-way valves (atrioventricular and semilunar) and coronary arteries
- Describe the functioning of the heart in terms of the contraction of the muscles of the atria and ventricles and the action of the valves, and explain the relative thickness of the muscle walls of the ventricles and atria and the importance of the septum
- State that blood is pumped away from the heart in arteries and returns to the heart in veins, and that the activity of the heart may be monitored by ECG, pulse rate and listening to the sounds of valves closing
- Describe the structure of arteries, veins and capillaries, limited to the relative thickness of the wall, the diameter of the lumen and the presence of valves in veins, and explain how structure is related to function and to the pressure of the blood transported
- Identify the main blood vessels to and from the heart (vena cava, aorta, pulmonary artery and pulmonary vein), the lungs (pulmonary artery and pulmonary vein), the kidney (renal artery and renal vein) and the liver (hepatic artery, hepatic veins and hepatic portal vein)
- List the components of blood as red blood cells, white blood cells, platelets and plasma, and state the functions of each, including the role of haemoglobin, phagocytosis and antibody production, and the transport of blood cells, ions, nutrients, urea, hormones and carbon dioxide
- Describe the process of clotting as the conversion of fibrinogen to fibrin to form a mesh, state the roles of clotting as preventing blood loss and the entry of pathogens, describe coronary heart disease and its risk factors, and discuss the roles of diet and exercise in reducing the risk and the effect of physical activity on heart rate
Part of the IGCSE Biology syllabus 0610 / 0970.
A pump, tubes and valves: the whole system in one idea
The circulatory system is a mass transport system: it carries substances to and from every cell in the body. Cambridge defines it as a system of blood vessels with a pump and valves to ensure one-way flow of blood.
Each part has one job. The pump (the heart) generates the pressure that drives the blood. The tubes (arteries, veins and capillaries) carry it out to the tissues and back again. The valves stop blood running backwards, so the flow stays one-way even where the pressure is low.
Large, active animals have a small surface area to volume ratio, so diffusion alone cannot move substances fast enough to supply their cells. Blood solves the problem: it carries oxygen, glucose, amino acids, hormones and heat to the cells, and carries carbon dioxide, urea and other waste away from them.
- Pump: the heart, which keeps blood moving by contracting.
- Tubes: arteries take blood away from the heart, veins bring it back, and capillaries link the two in the tissues.
- Valves: found in the heart and in veins, they allow blood through in one direction only.
- One-way flow matters because a backwards flow would stop oxygen and glucose reaching the cells and waste being removed.
Single circulation in a fish, double circulation in a mammal
A fish has single circulation. Blood travels in one circuit: heart -> gills -> body -> heart. It passes through the heart only once in each complete circulation. The heart pumps deoxygenated blood to the gills, where it picks up oxygen, and that same blood then flows on around the body before returning to the heart.
The weakness of this system is pressure. Blood loses a great deal of pressure as it squeezes through the narrow capillaries of the gills, so it reaches the body tissues slowly and at low pressure. A fish can manage with this because it does not keep its body temperature constant, so its cells need oxygen at a modest rate.
A mammal has a double circulation. Blood passes through the heart twice in each complete circuit: once on the pulmonary circuit (heart -> lungs -> heart) and once on the systemic circuit (heart -> body -> heart).
- Advantage 1: blood can be pumped to the body at a higher pressure, because it is returned to the heart after the lungs rather than flowing straight on from the gills.
- Advantage 2: oxygenated and deoxygenated blood are kept completely separate by the septum, so the blood sent to the tissues always has a high oxygen concentration.
- Advantage 3: a higher concentration of oxygen arriving at the tissues allows a higher rate of aerobic respiration, which supports a high metabolic rate and the constant body temperature of a mammal.
The heart: four chambers and a one-way route
The heart is a double pump with four chambers. The right atrium receives deoxygenated blood from the body through the vena cava. The right ventricle pumps it to the lungs through the pulmonary artery.
The left atrium receives oxygenated blood from the lungs through the pulmonary vein. The left ventricle pumps it to the whole body through the aorta.
The septum is the muscular wall that divides the left side of the heart from the right. Its job is to stop oxygenated and deoxygenated blood mixing: if they mixed, the blood sent to the tissues would carry less oxygen, so the cells could not respire aerobically as fast.
The route through the heart is worth learning as a single chain, because examiners often ask you to trace a blood cell or to label a diagram: vena cava -> right atrium -> right ventricle -> pulmonary artery -> lungs -> pulmonary vein -> left atrium -> left ventricle -> aorta -> body.
| Vessel | Direction | Blood carried |
|---|---|---|
| Vena cava | Into the heart (right atrium) | Deoxygenated - from the body |
| Pulmonary artery | Out of the heart (right ventricle) | Deoxygenated - to the lungs |
| Pulmonary vein | Into the heart (left atrium) | Oxygenated - from the lungs |
| Aorta | Out of the heart (left ventricle) | Oxygenated - to the body |
The cardiac cycle, the valves and how the heart rate is controlled
One complete cardiac cycle is one heartbeat. First the atria contract, raising the pressure and pushing blood through the atrioventricular valves into the ventricles. These are the tricuspid valve on the right and the bicuspid valve on the left.
Then the ventricles contract. The pressure rises sharply and slams the atrioventricular valves shut - this is the first heart sound, the 'lub' a doctor hears through a stethoscope - so blood cannot flow back into the atria. The semilunar valves in the aorta and the pulmonary artery open, and blood is forced out of the heart.
When the ventricles relax, the semilunar valves snap shut to stop blood falling back from the arteries - the second heart sound, the 'dup' - and the heart fills with blood again ready for the next beat.
Wall thickness follows the pressure the chamber must produce. The left ventricle wall is much thicker than the right because it pumps blood all round the body and must generate a high pressure; the right ventricle only pumps blood to the lungs, which is a short distance at low pressure. The atria have thinner walls than the ventricles because they only push blood the short distance into the ventricles.
The heart rate is set by a group of cells in the wall of the right atrium, the pacemaker, which sends out regular electrical impulses that spread through the heart muscle and make it contract. Adrenaline, released during exercise or excitement, speeds the pacemaker up. The activity of the heart can be monitored with an ECG, by taking the pulse rate, or by listening to the sounds of the valves closing.
- Atria contract: atrioventricular valves open, blood flows into the ventricles.
- Ventricles contract: atrioventricular valves close (no backflow to atria), semilunar valves open, blood leaves through the aorta and pulmonary artery.
- Ventricles relax: semilunar valves close (no backflow from the arteries), the heart refills.
- Left ventricle thick, right ventricle thinner, atria thinnest - the reason is always how far the blood must be pumped and at what pressure.
Arteries, veins and capillaries: structure matched to function
Arteries carry blood away from the heart, where the pressure is highest. An artery therefore has a thick muscular and elastic wall to withstand that pressure and to stretch with every heartbeat, and a narrow lumen to keep the blood moving fast. The elastic recoil of the wall helps push the blood along, and it is this expansion and recoil that you feel as a pulse.
Veins carry blood back to the heart at low pressure. Their walls are thin and their lumen is wide, which keeps resistance low. Veins contain valves that prevent backflow, and the blood is helped along by the squeezing of the muscles around them as you move.
Capillaries are the smallest vessels and link arteries to veins. Their wall is one cell thick, so the diffusion distance is short, and their lumen is extremely narrow - about the width of a red blood cell - so blood flows slowly and red cells pass through in single file. There are millions of them, giving a huge total surface area. These features suit their function: the exchange of substances between the blood and the tissues. Oxygen and glucose diffuse out into the cells, and carbon dioxide and urea diffuse in from them.
The main vessels to the organs are worth knowing by name. The heart is served by the vena cava, aorta, pulmonary artery and pulmonary vein. The lungs are served by the pulmonary artery and pulmonary vein. The kidney is supplied by the renal artery and drained by the renal vein. The liver is unusual: it receives oxygenated blood through the hepatic artery and nutrient-rich blood from the gut through the hepatic portal vein, and blood leaves it through the hepatic veins.
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Wall thickness | Thick, muscular and elastic | Thin | One cell thick |
| Lumen | Narrow | Wide | Very narrow - one red blood cell wide |
| Valves | None (the semilunar valves sit at the heart, not in the vessel) | Present, to prevent backflow | None |
| Blood pressure | High | Low | Falls from arterial to venous along its length |
| Main function | Carry blood away from the heart | Carry blood back to the heart | Exchange of substances with the tissues |
The components of blood and what each one does
Blood is a tissue made of cells floating in a liquid. Just over half of it is plasma, a pale yellow liquid. Plasma carries the blood cells and dissolves ions, nutrients, urea, hormones and carbon dioxide, and it also distributes heat around the body.
Red blood cells transport oxygen. Each one is packed with haemoglobin, which combines with oxygen in the lungs to form oxyhaemoglobin and releases the oxygen again in the tissues, where the oxygen concentration is low. Two adaptations make them efficient: they are biconcave, a flattened disc with a dent on each side, which gives a large surface area to volume ratio for fast oxygen uptake, and they have no nucleus, which leaves more room for haemoglobin.
White blood cells defend the body against pathogens. Phagocytes engulf and digest microorganisms by phagocytosis; lymphocytes produce antibodies that stick to pathogens and help destroy them.
Platelets are tiny cell fragments that help blood to clot. When a vessel is damaged, platelets trigger the conversion of fibrinogen, a soluble protein in the plasma, into fibrin, an insoluble protein that forms a mesh. The mesh traps blood cells and forms a clot. Clotting has two roles: it prevents further blood loss from the damaged vessel, and it seals the wound so that pathogens cannot enter the body.
Exercise, heart rate and coronary heart disease
During physical activity the muscles contract more often and respire faster. They need more oxygen and glucose, and they produce more carbon dioxide, so the heart must deliver and remove these faster. The heart rate rises - sometimes to twice the resting value - and the heart beats more strongly, so more blood is pumped each minute.
You can investigate this with a pulse. Press two fingers on the wrist (over the radial artery) or on the side of the neck (over the carotid artery) until you feel the beat, then count the beats for 15 seconds and multiply by 4, or count for a full minute, to get the rate in beats per minute. Measure the pulse at rest, immediately after a fixed amount of exercise, and then every minute until it returns to normal. That time is the recovery rate.
A trained athlete has a lower resting heart rate. Their heart muscle is stronger, so each beat pushes out more blood (a larger stroke volume), which means fewer beats per minute are needed to supply the same amount of blood. Their heart rate also returns to normal faster after exercise.
Coronary heart disease is a disease of the coronary arteries, the vessels that supply the heart muscle itself with blood. If they become narrowed or blocked by fatty deposits, the heart muscle receives less oxygen. During exertion it cannot respire aerobically fast enough, so the person feels chest pain; if an artery becomes completely blocked, part of the heart muscle dies - a heart attack. The risk factors include a diet high in saturated fat, a lack of exercise, stress, smoking, genetic predisposition, age and sex. A diet low in saturated fat and salt, and regular exercise, both reduce the risk: they lower blood cholesterol and blood pressure and keep the heart muscle strong.
| Risk factor | How it raises the risk of coronary heart disease |
|---|---|
| Diet high in saturated fat | Raises blood cholesterol, so fatty deposits build up in the coronary arteries |
| Lack of exercise | Weakens the heart muscle and is linked to higher blood pressure and cholesterol |
| Smoking | Narrows the arteries and raises blood pressure and heart rate |
| Stress | Raises blood pressure, which puts extra strain on the arteries and heart |
| Genetic predisposition, age and sex | Cannot be changed - these factors are inherited or fixed, but they still raise the risk |
Key terms you must be able to define
- Circulatory system
- A system of blood vessels with a pump (the heart) and valves to ensure one-way flow of blood around the body.
- Single circulation
- A circulation in which blood passes through the heart only once in a complete circuit, as in a fish: heart -> gills -> body -> heart.
- Double circulation
- A circulation in which blood passes through the heart twice in a complete circuit, as in a mammal: once on the pulmonary circuit to the lungs and once on the systemic circuit to the body.
- Septum
- The muscular wall that separates the left and right sides of the heart, preventing oxygenated and deoxygenated blood from mixing.
- Atrioventricular valve
- A one-way valve between an atrium and a ventricle that closes when the ventricle contracts, preventing blood from flowing back into the atrium.
- Semilunar valve
- A one-way valve in the aorta and the pulmonary artery that closes when the ventricles relax, preventing blood from flowing back into the heart.
- Haemoglobin
- The red pigment in red blood cells that combines with oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues.
- Fibrin
- An insoluble protein formed from fibrinogen during clotting; it forms a mesh that traps blood cells to make a clot.
Applied to the real world
Cambridge sets biology in unfamiliar contexts - a hospital, a farm, a water-treatment plant. Use these as models for how to apply the science, then write your own.
A person with narrowed coronary arteries feels a tight pain in the chest when they climb stairs or hurry for a bus. The coronary arteries supply the heart muscle itself, so when they are narrowed the muscle cannot get oxygen fast enough for the extra work. During exertion the heart muscle cannot respire aerobically quickly enough, and the pain is the signal that its cells are short of oxygen.
A pulse is felt where an artery runs close to the surface over a bone - at the wrist over the radial artery, or at the side of the neck over the carotid artery. Each time the left ventricle contracts it pushes a surge of blood into the arteries, and the elastic wall of the artery stretches and springs back. That expansion and recoil is the pulse, and its rate is exactly the heart rate.
A trained runner may have a resting heart rate of 50 beats per minute, while an untrained person of the same age sits at about 70. Training makes the heart muscle thicker and stronger, so each contraction pushes out a larger volume of blood. The same amount of blood is therefore delivered in fewer beats, and the athlete's heart rate also returns to normal faster after exercise.
When a knife cuts your finger, blood escapes from the damaged vessel - so the cut bleeds. Platelets arriving at the damage cause the soluble plasma protein fibrinogen to be converted into fibrin, which forms a mesh across the wound. The mesh traps red blood cells and forms a clot, which stops further blood loss and seals the cut so that pathogens cannot get in.
Flashcards
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Exam-style questions
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Exam technique for this topic
- Arteries carry blood away from the heart and veins carry it back - the definition is about direction, not about oxygen. Never write that arteries always carry oxygenated blood, because the pulmonary artery is the exception.
- Never state a structural feature on its own. Link it to its reason: not 'the left ventricle wall is thick' but 'the left ventricle wall is thick because it pumps blood all round the body and must generate a high pressure'.
- Learn the route through the heart as a chain you can recite (vena cava -> right atrium -> right ventricle -> pulmonary artery -> lungs -> pulmonary vein -> left atrium -> left ventricle -> aorta). Many questions ask you to trace a blood cell or to label a diagram.
- In the cardiac cycle, name each valve and say what it stops: the atrioventricular valves close when the ventricles contract to prevent backflow into the atria, and the semilunar valves close when the ventricles relax to prevent backflow from the arteries.
- For exercise questions, give the purpose behind the faster heart rate - more oxygen and glucose carried to the muscles and more carbon dioxide carried away. 'The heart beats faster to pump more blood' on its own rarely scores.
- In the pulse investigation, name the independent variable (exercise), the dependent variable (pulse rate) and at least one control variable (same person, same exercise, same duration), and state how the pulse was counted, for example beats in 15 seconds multiplied by 4.
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Where this fits in IGCSE Biology
This page is one topic of Cambridge IGCSE Biology 0610 / 0970. IGCSE Biology is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE Biology syllabus page.