AS 2 extended response, transport in mammals · 6 marks
Explain how pressure changes in the left side of the heart cause the atrioventricular and semilunar valves to open and close during one cardiac cycle.
Marked instantly, no account needed
AS · AS 2 Organisms and Biodiversity
The cardiac cycle: stages, pressures and valve timings
The short answer
The cardiac cycle has three stages: atrial systole, ventricular systole and diastole. Blood always moves down a pressure gradient, and every valve opens or closes purely because the pressure on one side exceeds the pressure on the other — that single idea answers most exam questions on the topic.
The three stages
| Stage | What contracts | Valves | Blood movement |
|---|---|---|---|
| Atrial systole | Atria contract, ventricles relaxed | Atrioventricular valves open, semilunar closed | Atria top the ventricles up |
| Ventricular systole | Ventricles contract | AV valves close (first heart sound), semilunar open | Blood forced into aorta and pulmonary artery |
| Diastole | All chambers relaxed | Semilunar close (second sound), AV valves open | Heart fills passively from the veins |
Reading the pressure graph
- The AV valve closes where the ventricular pressure curve crosses above the atrial curve.
- The semilunar valve opens where ventricular pressure crosses above aortic pressure, and closes when it drops back below.
- The flat, slightly falling aortic pressure between beats is elastic recoil of the aorta maintaining flow — worth a mark on its own.
- One full cycle on the x-axis gives heart rate: 60 ÷ cycle length in seconds.
Control of heartbeat
The sinoatrial node initiates the wave of excitation; it spreads over the atria, is delayed at the atrioventricular node so the atria empty first, then travels down the bundle of His and Purkyne fibres so the ventricles contract from the apex upwards. Every one of those clauses is a separate marking point.
Phrases that earn the marks
- pressure in the ventricle exceeds pressure in the atrium, so the AV valve closes
- down a pressure gradient
- delay at the atrioventricular node allows the atria to empty
- contraction from the apex upwards
- elastic recoil of the aorta maintains blood pressure
Where marks get lost
- Saying valves open 'to let blood through' instead of naming the pressure difference that opens them.
- Muddling systole (contraction) with diastole (relaxation).
- Forgetting that both sides of the heart contract at the same time.
- Quoting pressures off the graph with no units.
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Exam questions on cardiac cycle a level biology, marked
Two AS-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
AS 2 extended response, transport in mammals · 6 marks
Explain how pressure changes in the left side of the heart cause the atrioventricular and semilunar valves to open and close during one cardiac cycle.
Command word: Explain — how to answer itMark scheme — 6 creditworthy points
- Atrial systole: atrial pressure exceeds ventricular pressure, so the atrioventricular (bicuspid) valve opens
- Blood flows from the left atrium into the left ventricle
- Ventricular systole: ventricle contracts, ventricular pressure rises above atrial pressure
- Atrioventricular valve closes, preventing backflow into the atrium
- When ventricular pressure exceeds aortic pressure the semilunar valve opens and blood is ejected into the aorta
- During diastole ventricular pressure falls below aortic pressure, so the semilunar valve closes
Full-mark answer
During atrial systole the left atrium contracts and atrial pressure rises above ventricular pressure, so the bicuspid valve is pushed open and blood flows into the left ventricle. During ventricular systole the ventricle wall contracts, ventricular pressure rises above atrial pressure and the bicuspid valve closes, preventing backflow into the atrium. As the ventricle continues to contract, ventricular pressure rises above the pressure in the aorta, forcing the semilunar valve open so blood is ejected into the aorta. During diastole the ventricle relaxes and ventricular pressure falls below aortic pressure, so the semilunar valve closes and prevents blood returning to the ventricle.
Examiner insight: Every mark is a comparison of two pressures. Answers that say 'the valve opens to let blood through' with no pressure comparison score nothing, however fluent they read. This question carries QWC credit, so keep the four stages in order.
AS 2 calculation and application · 4 marks
A student measured a heart rate of 72 beats per minute and a stroke volume of 70 cm³. Calculate the cardiac output in dm³ min⁻¹ and explain one change that would occur during exercise.
Command word: Calculate — how to answer itMark scheme — 4 creditworthy points
- Cardiac output = stroke volume × heart rate = 70 × 72 = 5040 cm³ min⁻¹
- Converted correctly to 5.04 dm³ min⁻¹ (allow 5.0)
- During exercise heart rate and/or stroke volume increases, so cardiac output increases
- More oxygen and glucose delivered to respiring muscle / faster removal of carbon dioxide and lactate
Full-mark answer
Cardiac output = stroke volume × heart rate = 70 cm³ × 72 = 5040 cm³ min⁻¹ = 5.04 dm³ min⁻¹. During exercise both heart rate and stroke volume increase, so cardiac output rises. This delivers oxygen and glucose to the respiring muscle faster and removes carbon dioxide and lactate more quickly, maintaining aerobic respiration.
Examiner insight: The unit conversion is a mark in its own right; dividing by 1000 is where most marks are lost. Always show the substituted figures, because a correct method scores even when the arithmetic slips.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- How long is one cardiac cycle?
- About 0.8 seconds at a resting rate of 75 beats per minute: roughly 0.1 s atrial systole, 0.3 s ventricular systole and 0.4 s diastole.
- Why is the left ventricle wall thicker?
- It generates the higher pressure needed to push blood around the whole body, while the right ventricle only pumps to the nearby, low-pressure lungs.
- What causes the heart sounds?
- The 'lub' is the atrioventricular valves closing at the start of ventricular systole; the 'dup' is the semilunar valves closing at the start of diastole.
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