HeartCambridge IGCSE Biology: Revision notes
Section 1
What are the key structures of the mammalian heart?
The mammalian heart is a muscular organ with four chambers: the left and right atria (singular: atrium) receive blood, and the left and right ventricles pump blood out of the heart. The septum is a thick muscular wall that divides the left and right sides of the heart, preventing oxygenated and deoxygenated blood from mixing.
One-way valves ensure blood flows in one direction:
- Atrioventricular valves (AV valves) lie between atria and ventricles; the right side has the tricuspid valve, the left side has the bicuspid (mitral) valve
- Semilunar valves lie at the exit of the ventricles into arteries (pulmonary artery and aorta)
The coronary arteries branch from the aorta and supply oxygenated blood to the heart muscle itself. The muscular walls are thicker in the left ventricle (which pumps blood to the whole body) than the right ventricle (which pumps only to the lungs), and ventricles have thicker walls than atria (which only need to push blood a short distance into ventricles).
Examiners expect you to name the specific valves and explain their position. Always state that atrioventricular valves lie between chambers and semilunar valves lie at the exits of ventricles.
The septum is like a wall in a house — it keeps the two sides separate so deoxygenated blood (right side) never mixes with oxygenated blood (left side).
Section 2
How does the heart function during the cardiac cycle?
The heart works through a coordinated sequence of contractions:
1. Atrial contraction (atrial systole):
- Both atria contract simultaneously, forcing blood down through the atrioventricular valves into the ventricles
- The semilunar valves remain closed
2. Ventricular contraction (ventricular systole):
- The ventricles contract, increasing pressure inside the chambers
- This pressure pushes the atrioventricular valves shut (preventing backflow into atria) and pushes the semilunar valves open
- Blood is forced out into the aorta (from left ventricle) and pulmonary artery (from right ventricle)
3. Relaxation (diastole):
- All heart chambers relax
- Pressure in the ventricles drops below that in the arteries, so semilunar valves snap shut
- Blood from veins begins refilling the atria, ready for the next cycle
The one-way valves are essential — they ensure blood only moves forward through the heart and prevents backflow.
Students often forget that atrioventricular valves close during ventricular contraction. Remember: when ventricles contract, the pressure rise forces AV valves shut (preventing backflow to atria) and semilunar valves open.
During atrial contraction, the tricuspid valve opens because atrial pressure exceeds ventricular pressure. During ventricular contraction, ventricular pressure rises above atrial pressure, so the tricuspid valve slams shut and the pulmonary semilunar valve opens instead.
Section 3
Why is the septum important in heart function?
The septum is a thick, muscular wall that completely separates the right and left sides of the heart. Its critical role is to keep oxygenated and deoxygenated blood from mixing.
- Right side of the heart: Receives deoxygenated blood from the body via the superior and inferior vena cava (into the right atrium) and pumps it to the lungs via the pulmonary artery
- Left side of the heart: Receives oxygenated blood from the lungs via the pulmonary veins (into the left atrium) and pumps it to the body via the aorta
If the septum were damaged or had a hole (called a septal defect), oxygenated and deoxygenated blood would mix in the heart chambers. This would reduce the amount of oxygen delivered to body cells, severely impairing health.
The left ventricle has a much thicker muscular wall than the right ventricle because it must generate greater pressure to pump blood around the entire body, whereas the right ventricle only pumps blood to the nearby lungs.
Examiners reward students who explain why the septum matters functionally, not just its position. Always state it prevents mixing of oxygenated and deoxygenated blood, which would reduce oxygen delivery to tissues.
Section 4
How is heart activity monitored and what affects heart rate?
Heart activity can be assessed in three main ways:
1. Electrocardiogram (ECG):
- Records the electrical activity of the heart as it contracts
- Produces a characteristic trace showing atrial and ventricular contractions
- Useful for detecting irregular heart rhythms and damage
2. Pulse rate:
- Counts the number of times the heart beats per minute (bpm)
- Can be measured by feeling an artery (e.g. radial artery in the wrist or carotid artery in the neck)
- Normal resting heart rate is typically 60–100 bpm
3. Listening to valve sounds:
- Using a stethoscope, distinct sounds indicate valve closure
- Abnormal sounds (murmurs) may indicate faulty valves
Effect of physical activity on heart rate:
During and immediately after physical exercise, heart rate increases significantly. This happens because:
- Muscles require more oxygen and glucose during exercise
- The body detects increased metabolic demand and raised CO₂ levels
- The nervous system increases the frequency and force of heart contractions
- After exercise, heart rate gradually returns to resting level as muscles relax and metabolic demand drops
Regular physical activity can lower resting heart rate (cardiac fitness) because the heart becomes more efficient at pumping blood.
When describing how physical activity affects heart rate, use the phrase 'increased oxygen demand' — this is the key mechanism examiners expect. State that heart rate increases during exercise and returns to resting level when exercise stops.
A student measures resting pulse rate (70 bpm), exercises for 5 minutes (heart rate increases to 150 bpm), then rests and records how long it takes to return to 70 bpm. This shows the heart's response to increased metabolic demand and recovery after exercise.
Section 5
What is coronary heart disease and how can it be prevented?
Coronary heart disease (CHD) occurs when the coronary arteries become narrowed or blocked by atherosclerosis (build-up of fatty deposits), restricting blood flow to the heart muscle. This can cause a heart attack (myocardial infarction) if blood supply to part of the heart is completely cut off.
Risk factors for coronary heart disease:
| Risk Factor | How It Contributes |
|---|---|
| Poor diet (high saturated fat/salt, low fibre) | Increases cholesterol and blood pressure, promoting atherosclerosis |
| Lack of exercise | Reduces cardiac fitness; leads to weight gain and high cholesterol |
| Smoking | Damages artery walls and promotes blood clotting |
| Stress | Raises blood pressure and heart rate, damaging arteries |
| Genetic predisposition | Family history increases risk; some people inherit high cholesterol |
| Age | Risk increases with age; more common in older people |
| Sex | Higher risk in men; women's risk increases after menopause |
Prevention through diet and exercise:
- Healthy diet: Reduce saturated fat intake (lower cholesterol levels), increase fibre, reduce salt (lowers blood pressure), maintain healthy weight
- Regular exercise: Improves cardiac fitness, reduces cholesterol, maintains healthy weight, lowers resting blood pressure and stress
- Lifestyle changes: Stop smoking, manage stress through relaxation techniques
Adopting these changes significantly reduces the risk of developing coronary heart disease.
Examiners expect you to name at least three risk factors and explain the mechanism (e.g. 'smoking damages artery walls' rather than just 'smoking is bad'). Always include both diet and exercise as preventative measures.
Students sometimes confuse risk factors with symptoms. Remember: risk factors are conditions that make CHD more likely (e.g. smoking, poor diet); they do not directly cause a heart attack but increase probability.
Must Know
- The heart has four chambers (left/right atria and ventricles) separated by the septum, which prevents mixing of oxygenated and deoxygenated blood
- Atrioventricular valves lie between atria and ventricles; semilunar valves lie at the exits of ventricles — both are one-way and prevent backflow
- The left ventricle has a much thicker wall than the right ventricle because it pumps blood around the entire body; ventricles have thicker walls than atria because they generate greater pressure
- During the cardiac cycle: atria contract → ventricles contract (forcing AV valves shut and semilunar valves open) → all chambers relax (semilunar valves shut, atria refill)
- Coronary arteries supply oxygen-rich blood to the heart muscle; blockage causes coronary heart disease and may result in a heart attack
- Physical activity increases heart rate (due to increased oxygen demand by muscles) and regular exercise improves cardiac fitness, reducing resting heart rate and lowering risk of coronary heart disease
- Risk factors for CHD include poor diet, lack of exercise, smoking, stress, genetic predisposition, age and sex; both diet and regular exercise reduce CHD risk
That's the notes covered.
Carry on to the next subtopic.