The Cardiac Cycle & Cardiovascular Disease Revision Notes | A Level AQA Biology

1. The Heart as a Pressure Pump

Your heart beats roughly 70 times per minute, every minute of your life, without you thinking about it. What makes that possible is a precise sequence of pressure changes — and understanding that sequence is exactly what AQA examiners will ask you to analyse from graphs and data.

Before diving into the cycle itself, picture the heart's layout. It has four chambers: two atria at the top receiving blood, and two ventricles at the bottom pumping it out. The right side handles deoxygenated blood heading to the lungs via the pulmonary arteries; the left side handles oxygenated blood heading to the body via the aorta. The coronary arteries branch from the aorta immediately above the aortic valve and supply the heart muscle itself — a detail AQA specifically names.

2. Gross Structure of the Human Heart

The AQA specification requires you to name the blood vessels entering and leaving the heart, lungs and kidneys. The vessels you must know are: the vena cava (returning deoxygenated blood to the right atrium), the pulmonary artery (leaving the right ventricle to the lungs), the pulmonary vein (returning oxygenated blood from the lungs to the left atrium), the aorta (leaving the left ventricle to the body), the renal artery (to the kidneys) and the renal vein (from the kidneys). The coronary arteries are the only named arteries within the heart wall itself.

The left ventricular wall is noticeably thicker than the right. This is because the left ventricle must generate enough pressure to push blood around the entire systemic circulation, whereas the right ventricle only needs to push blood the short distance to the lungs. The atrial walls are thinner still — they only push blood the small distance into the ventricles below.

The semilunar valves (pulmonary and aortic) sit at the base of the pulmonary artery and aorta respectively. They open when ventricular pressure exceeds arterial pressure during systole, and snap shut during diastole when arterial pressure exceeds ventricular pressure. All valve movements are entirely passive — driven by pressure differences, not by muscle.

3. Pressure and Volume Changes During the Cardiac Cycle

This is the section most students find hardest — and the one most likely to appear as a data-analysis question. The trick is to read a cardiac cycle graph by asking one question at each moment: which chamber has the higher pressure right now? That tells you which valves are open and which are shut.

The cycle has three phases. During atrial systole, the atria contract, atrial pressure briefly exceeds ventricular pressure, the AV valves open, and the remaining blood is pushed into the ventricles. During ventricular systole, the ventricles contract hard, ventricular pressure rapidly exceeds both atrial pressure (closing the AV valves — producing the first heart sound, 'lub') and arterial pressure (opening the semilunar valves), and blood is ejected into the aorta and pulmonary artery. During diastole, all chambers relax, ventricular pressure drops below arterial pressure (closing the semilunar valves — producing the second heart sound, 'dub'), and blood flows passively from the veins into the atria and then into the ventricles.

When you look at a pressure-volume graph for the left ventricle, notice that ventricular volume reaches its maximum (end-diastolic volume) just before ventricular systole begins, and its minimum (end-systolic volume) just after the semilunar valve closes. The difference between these two values is the stroke volume — the volume of blood ejected per beat.

Worked example: Interpreting a Cardiac Cycle Pressure Graph

A graph shows left atrial pressure, left ventricular pressure and aortic pressure over one cardiac cycle. At point X, ventricular pressure rises above aortic pressure. At point Y, ventricular pressure falls below aortic pressure. Describe what happens to the aortic semilunar valve at points X and Y, and explain why.

1. At X: ventricular pressure > aortic pressure — Valves open when the pressure on the upstream side exceeds the downstream side.
2. Aortic semilunar valve opens at point X — Blood is pushed from the ventricle into the aorta. This is the start of ventricular ejection.
3. At Y: ventricular pressure < aortic pressure — The ventricle is beginning to relax; aortic pressure now exceeds ventricular pressure.
4. Aortic semilunar valve closes at point Y — Closure prevents backflow of blood from the aorta into the ventricle. This produces the second heart sound ('dub').
5. Sequence: valve opens (X) → blood ejected → valve closes (Y) → no backflow — The passive, pressure-driven opening and closing of valves ensures blood moves in one direction only — a key exam phrase.

Answer: The aortic semilunar valve opens at X (ventricular pressure exceeds aortic pressure) and closes at Y (aortic pressure exceeds ventricular pressure), maintaining unidirectional blood flow by preventing backflow into the ventricle.

4. Structure of Blood Vessels in Relation to Function

Now that you understand how the heart generates pressure, consider what happens to that pressure wave as it travels through the vessels. Each vessel type is structurally adapted to its role — and AQA expects you to link structure to function precisely.

Arteries carry blood away from the heart under high pressure. Their walls contain a thick layer of elastic fibres (in the tunica media) that stretch during systole and recoil during diastole, smoothing out the pressure pulse. They also have a significant layer of smooth muscle, allowing vasoconstriction to regulate blood distribution. The lumen is relatively narrow compared to the wall thickness.

Arterioles are smaller branches of arteries with a proportionally greater smooth muscle component. By contracting or relaxing, arterioles act as the main regulators of blood flow into capillary beds — a process called vasoconstriction and vasodilation. This is how the body redirects blood to active muscles during exercise.

Veins return blood to the heart at low pressure. Their walls are thinner with less elastic tissue and less smooth muscle, but they have a wider lumen. Crucially, veins contain semilunar pocket valves at intervals along their length. These prevent backflow when the surrounding skeletal muscles squeeze the vein during movement — the so-called skeletal muscle pump.

5. Required Practical — Dissection of a Mass Transport Organ

Required Practical 5 asks you to dissect an animal or plant gas exchange or mass transport system. For the cardiac cycle topic, dissecting a mammalian heart (typically a sheep or pig heart) is the standard approach. Beyond the practical itself, you need to be able to evaluate the method and identify the structures you observe.

6. Cardiovascular Disease: Risk Factors, Evidence and Evaluation

Cardiovascular disease (CVD) encompasses conditions such as coronary heart disease, atherosclerosis and stroke. Atherosclerosis involves the build-up of fatty plaques (atheromas) within arterial walls, narrowing the lumen and reducing blood flow. If a plaque ruptures, a blood clot (thrombus) can form, potentially blocking a coronary artery and causing a myocardial infarction (heart attack).

AQA requires you to analyse data linking risk factors to CVD incidence, and — critically — to evaluate conflicting evidence and distinguish between correlation and causation. These are higher-order skills that carry significant marks in extended-response questions.

Established risk factors for CVD include: high blood pressure (hypertension), high blood cholesterol (particularly LDL cholesterol), smoking, a diet high in saturated fat, physical inactivity, obesity, type 2 diabetes, age, sex and genetic predisposition. Some of these are modifiable (lifestyle choices); others are non-modifiable (age, genetics, sex).

Where the evidence becomes genuinely complex — and where AQA specifically asks you to evaluate — is in cases where studies conflict. For example, some studies suggest moderate alcohol consumption is associated with reduced CVD risk, while others find no protective effect once confounding variables (such as socioeconomic status and diet) are controlled for. A well-constructed evaluation acknowledges both sides of the evidence, identifies methodological limitations, and reaches a reasoned conclusion.

The data above show a positive correlation between cigarette consumption and relative CVD risk. Notice the dose-response relationship — risk increases progressively with the number of cigarettes smoked per day. A dose-response relationship strengthens the case for causation, because it is harder to explain by chance or confounding alone. However, to establish causation fully, you would also need a plausible biological mechanism (which exists: smoking damages arterial endothelium, promotes atheroma formation and raises blood pressure) and evidence from controlled experimental studies.

7. Exam Technique and Mark-Winning Strategies

The cardiac cycle and CVD topics generate several predictable question types. Knowing how examiners award marks — and where students routinely drop them — will sharpen your responses significantly.

For pressure-volume graph questions, always read the axes carefully before writing anything. Examiners frequently ask you to identify the moment a valve opens or closes: your answer must reference the specific pressure relationship (e.g. 'the AV valve closes when ventricular pressure exceeds atrial pressure'), not just state that the valve closes. Describing the event without the pressure reason typically earns only one of two available marks.

For structure-function questions on blood vessels, the mark scheme rewards precise anatomical language. Saying a vein has 'thin walls' earns one mark; saying it has 'thin walls with little elastic tissue and little smooth muscle, and a wide lumen' earns the full set. Always link the structural feature to the functional consequence — for example, 'elastic fibres in the artery wall recoil after systole, maintaining pressure during diastole.'

For evaluate questions on CVD risk factors, use the Point-Evidence-Explanation-Counter-Conclusion (PEEC) structure. State a point, cite the type of evidence (e.g. large-scale epidemiological study, randomised controlled trial), explain the biological mechanism, acknowledge a counter-argument or limitation, and reach a reasoned conclusion. Avoid writing 'this proves' — use 'this suggests' or 'this is consistent with a causal relationship' to reflect the nature of scientific evidence accurately.