IGCSE Biology | Answers | 25 Questions
Give the definition of the circulatory system.
What is meant by the term double circulation?
Name the four chambers of the human heart.
- Right atrium
- Right ventricle
- Left atrium
- Left ventricle
What is the function of the septum in the heart?
Name the two types of valve found in the heart.
- Atrioventricular (AV) valves — between the atria and ventricles
- Semilunar valves — at the exits of the ventricles (in the aorta and pulmonary artery)
What are the coronary arteries, and why are they important to the heart?
The coronary arteries are small arteries that run over the surface of the heart.
State where blood goes after it is pumped out of the left ventricle.
State where blood goes after it is pumped out of the right ventricle.
List three methods that can be used to monitor the activity of the heart.
- Electrocardiogram (ECG) — records the electrical signals that trigger each heartbeat
- Pulse rate — counting the pulse at the wrist or neck gives the heart rate in beats per minute
- Listening to sounds of valves closing — hearing the sounds made by the valves snapping shut
State two possible risk factors for coronary heart disease.
Any two from:
- Diet
- Sedentary lifestyle (lack of exercise)
- Stress
- Smoking
- Genetic predisposition
- Age
- Gender
Explain why the left ventricle has a thicker muscular wall than the right ventricle.
The left ventricle pumps blood to supply the entire body. Blood must travel a long distance through all the body’s organs and tissues, so it needs to be pushed at high pressure.
A thicker muscular wall contracts with greater force, producing this higher pressure.
The right ventricle only pumps blood to the nearby lungs. This is a shorter distance and requires much less pressure, so a thinner wall is sufficient.
Explain why the walls of the ventricles are thicker than the walls of the atria.
The ventricles pump blood out of the heart — to the lungs or to the body. This requires strong contractions to generate enough pressure to push blood through the entire circulation.
The atria only push blood a very short distance — down into the ventricles directly below them. Very little force is needed for this, so the atrial walls are much thinner.
Describe what happens to the atrioventricular (AV) valves and the semilunar valves when the ventricles contract.
When the ventricles contract, pressure inside them rises sharply.
- The AV valves snap shut — the high pressure in the ventricles pushes them closed, preventing blood from flowing back into the atria.
- The semilunar valves are forced open — the high pressure pushes them open, allowing blood to be pumped out into the aorta (left) and pulmonary artery (right).
A student measures her pulse rate at rest, then again immediately after 3 minutes of jogging.
(a) State the expected result. (b) Give two reasons why the pulse rate changes in this way.
- During exercise, muscles use oxygen and glucose more quickly, so more oxygenated blood needs to be delivered to them.
- Muscles produce more carbon dioxide during exercise, so the heart beats faster to remove waste products from the muscles more quickly.
Explain how the valves in the heart ensure that blood flows in one direction only.
Valves act like one-way doors.
When blood pushes forward (in the correct direction), the pressure forces the valve to open, allowing blood through.
When blood tries to flow backwards, it pushes against the back of the valve and forces it shut, blocking the backflow.
Describe how coronary heart disease develops. Include in your answer what plaque is and what effect it has on blood flow to the heart muscle.
Over time, fatty deposits build up inside the walls of the coronary arteries, making them narrower.
The narrowed arteries mean that less blood can flow through to the heart muscle.
The heart muscle receives less oxygen and glucose.
Explain how smoking increases the risk of developing coronary heart disease.
Chemicals in cigarette smoke damage the walls of the arteries, including the coronary arteries. This damage makes it easier for fatty deposits to build up on the artery walls, narrowing them.
Smoking also raises blood pressure, which puts extra strain on the artery walls and the heart, increasing the risk of further damage and blockage.
State the names of the two circulations that make up the double circulation. For each one, state whether blood pressure is high or low, and explain why.
Low pressure. The lungs are close to the heart and contain very delicate, thin-walled capillaries. High pressure would damage these capillary walls and disrupt gas exchange.
High pressure. Blood must travel long distances to reach all parts of the body. High pressure gives blood the force it needs to reach distant tissues.
Describe the complete path of blood through the heart, starting from when deoxygenated blood enters the right atrium. Include the names of the valves it passes through.
- Deoxygenated blood enters the right atrium from the body.
- The right atrium contracts, pushing blood through the AV valve into the right ventricle.
- The right ventricle contracts, pushing blood through the semilunar valve into the pulmonary artery, which carries it to the lungs.
- In the lungs, blood picks up oxygen and loses carbon dioxide. Oxygenated blood returns to the left atrium.
- The left atrium contracts, pushing blood through the AV valve into the left ventricle.
- The left ventricle contracts, pushing blood through the semilunar valve into the aorta, which carries it to the rest of the body.
State three lifestyle risk factors for coronary heart disease. Choose one and explain how it increases the risk.
Three lifestyle risk factors (any three from):
- Diet
- Sedentary lifestyle
- Stress
- Smoking
A sedentary lifestyle means very little physical activity. This leads to weight gain, which puts extra strain on the heart. It also reduces cardiovascular fitness, meaning the heart is weaker and less efficient. Both of these increase the likelihood of fatty deposits building up in the coronary arteries.
The left and right ventricles pump the same volume of blood per beat. However, blood pressure in the aorta is much higher than in the pulmonary artery.
(a) Explain why blood pressure is higher in the aorta. (b) Explain how wall thickness accounts for this. (c) Why must pulmonary pressure stay low?
The left ventricle pumps blood to the entire body. Blood must travel long distances through many organs and tissues and overcome resistance in the vessels. This requires a much higher driving pressure.
The right ventricle only pumps blood to the nearby lungs, so far less pressure is needed to move the blood that short distance.
The left ventricle has a much thicker muscular wall than the right ventricle. When it contracts, it generates greater force, producing the high pressure that drives blood around the entire body.
The right ventricle has a thinner wall and contracts with less force, producing a lower pressure that is just sufficient to move blood to the lungs.
The lungs contain millions of tiny, very thin-walled capillaries where gas exchange takes place. High blood pressure in these capillaries would damage or burst the delicate walls, preventing gas exchange and causing fluid to leak into the lungs.
This question is about one complete heartbeat.
(a) Describe the sequence of events. (b) Role of AV valves. (c) Role of semilunar valves.
- Atria fill with blood — blood flows from the veins into the relaxed atria. The AV valves are open and blood begins to fill the ventricles too.
- Atria contract — the atria squeeze, pushing the remaining blood through the open AV valves into the ventricles.
- Ventricles contract — pressure rises sharply. The AV valves snap shut (preventing backflow into the atria). The semilunar valves are forced open. Blood is pushed into the aorta (left) and pulmonary artery (right).
- Ventricles relax — pressure drops. The semilunar valves snap shut, stopping blood flowing back from the arteries into the ventricles. The cycle begins again.
The AV valves sit between each atrium and its ventricle. They are open during the filling phase, allowing blood to flow from the atria into the ventricles — both during passive filling and when the atria contract.
When the ventricles contract, the high pressure inside the ventricles forces the AV valves shut, preventing blood from flowing backwards into the atria.
The semilunar valves are located at the exits of the ventricles (in the aorta and pulmonary artery). They are forced open when the ventricles contract and push blood into the arteries.
When the ventricles relax, pressure inside them drops. Blood in the arteries would flow back, but this reverse pressure forces the semilunar valves shut, preventing backflow into the ventricles.
This question is about the double circulation in humans.
(a) Describe the pulmonary circulation. (b) Describe the systemic circulation. (c) Explain the advantage of two separate circuits at different pressures.
Deoxygenated blood leaves the right ventricle through the semilunar valve and travels via the pulmonary artery to the lungs. In the lungs, carbon dioxide leaves the blood and oxygen enters (gas exchange). The now oxygenated blood returns to the left atrium.
Oxygenated blood leaves the left ventricle through the semilunar valve and travels via the aorta to all the body’s organs and tissues. Oxygen and nutrients are delivered to cells, and carbon dioxide and waste are collected. Deoxygenated blood returns to the right atrium.
In a double circulation, blood returns to the heart between the two loops. The heart can then pump blood at different pressures for each circuit:
- Low pressure to the lungs — protecting the delicate lung capillaries from damage.
- High pressure to the body — ensuring blood reaches all distant tissues efficiently.
If there were only one circuit, the same pressure would have to serve both the lungs and the body. Either the lungs would be damaged by high pressure, or the body would not receive enough blood flow at low pressure.
A class is investigating the effect of physical activity on heart rate.
(a) Describe the investigation and fair test. (b) Explain why heart rate increases. (c) Explain why a fit athlete recovers faster.
What to measure: Heart rate (pulse rate) in beats per minute, counted at the wrist or neck.
Method:
- Measure and record the resting heart rate (count pulse for 60 seconds).
- Perform a set exercise (e.g., stepping on and off a step) for a fixed time (e.g., 2 minutes).
- Measure heart rate immediately after stopping, then every minute until it returns to resting.
- Repeat to get reliable results and calculate an average.
Fair test: Keep the same type of exercise, same duration, and same person each time. Only change the stage being measured (before vs. after exercise).
During exercise, muscles contract more rapidly and need more oxygen and glucose. They also produce more carbon dioxide as a waste product.
The heart beats faster, speeding up blood circulation. This delivers more oxygen to the muscles and removes waste products more quickly.
Heart rate will gradually decrease and return to the resting rate.
After exercise stops, the muscles no longer need extra oxygen and glucose, and less carbon dioxide is being produced. As demand for blood returns to normal, the heart no longer needs to beat as fast and the heart rate slowly returns to its resting level.
Coronary heart disease (CHD) is a major cause of death worldwide.
(a) List all CHD risk factors. (b) How do diet and exercise reduce risk? (c) Why should someone with a genetic predisposition still follow a healthy lifestyle?
- Diet
- Sedentary lifestyle
- Stress
- Smoking
- Genetic predisposition
- Age
- Gender
Diet:
- Eating less fat reduces the build-up of fatty deposits in the coronary arteries, keeping them wider and reducing the risk of blockage.
- Eating more fruit and vegetables and maintaining a healthy body weight reduces the strain on the heart.
Exercise:
- Regular physical activity strengthens the heart muscle, so it pumps blood more efficiently.
- Exercise helps maintain a healthy body weight and reduces the build-up of fatty deposits in the coronary arteries, lowering the risk of blockage.
A genetic predisposition means the person has a naturally higher tendency to develop fatty deposits in the coronary arteries, increasing the risk of blockage.
However, lifestyle factors like diet and exercise can slow or reduce this build-up. A healthy diet (eating less fat) and regular exercise help maintain a healthy body weight and keep the coronary arteries as clear as possible.
