What you need to know
- 1.8Heart structure, major vessels, atrial systoleThe stage of the cardiac cycle when heart muscle contracts., ventricular systole and cardiac diastoleThe stage of the cardiac cycle when heart muscle relaxes and the chambers fill with blood..
Details of myogenic stimulation are not required at IAS. This lesson focuses on chambers, vessels, pressures and valves, rather than the electrical conduction pathway.
By the end, you should be able to…
- Identify the four chambers, major vessels and valveA flap of tissue that closes when pressure behind it is higher, so blood can flow in only one direction..
- Explain why ventricular walls differ in thickness.
- Use pressure comparisons to determine flow and valve position throughout a cardiac cycleThe sequence of events in one heartbeat: atrial systole, ventricular systole and diastole..
Build your understanding
Trace the route through four chambers
The venae cavae bring blood from the body into the right atriumAn upper chamber of the heart. It receives blood from the veins and pumps it into the ventricle below.. Blood passes through the right atrioventricular (tricuspid) valveA flap of tissue that closes when pressure behind it is higher, so blood can flow in only one direction. into the right ventricleA lower chamber of the heart. Its thick muscular wall pumps blood into an artery., then through the pulmonary semilunar valve into the pulmonary arteryThe artery that carries deoxygenated blood from the right ventricle to the lungs. and towards the lungs.
Pulmonary veinA vessel that carries blood back to the heart at low pressure. It has a thin wall, a wide lumen and valves. bring blood from the lungs to the left atrium. It passes through the left atrioventricular (bicuspid or mitral) valve into the left ventricle, then through the aortic semilunar valve into the aortaThe main artery. It carries oxygenated blood from the left ventricle to the body. and towards the body. The septumThe wall of tissue that separates the left and right sides of the heart. separates the right and left sides.
Coronary arteryA vessel that carries blood away from the heart at high pressure. It has a thick wall with elastic fibres and smooth muscle. branch from the aorta and supply the heart muscle itself with oxygen and nutrients. Blood inside the chambers is not an adequate direct supply for the thick muscular wall.
Different walls for different pressures
Atria have relatively thin muscular walls because they move blood into nearby ventricles. Ventricles have thicker walls because they pump blood out of the heart. The left ventricular wall is thicker than the right because the systemic circuit requires a greater pressure to overcome its greater resistance.
The right ventricle pumps to the lungs at lower pressure. Both ventricles normally pump the same average volume per beat: a thicker left wall means greater pressure, not a greater long-term volume output. A sustained difference in their outputs would cause blood to accumulate in one circuit.
Valves respond to pressure differences
Blood flows from higher to lower pressure. Atrioventricular valves open when atrial pressure exceeds ventricular pressure and close when ventricular pressure rises above atrial pressure. Tendinous cords prevent these valves from inverting into the atria during ventricular contraction; they do not pull the valves open.
Semilunar valves open when ventricular pressure exceeds pressure in the aorta or pulmonary artery. They close when arterial pressure exceeds ventricular pressure, preventing return flow into the ventricles. Valve movements are passive responses to pressure differences.
One cardiac cycle
During cardiac diastoleThe stage of the cardiac cycle when heart muscle relaxes and the chambers fill with blood., the chambers relax. Once ventricular pressure falls below atrial pressure, the atrioventricular valves open and blood flows into the ventricles. Semilunar valves are closed. Much ventricular filling happens before atrial contraction.
During atrial systoleThe stage of the cardiac cycle when heart muscle contracts., the atria contract and raise their pressure, completing ventricular filling through open atrioventricular valves. The ventricles are still relaxed.
During ventricular systole, ventricular pressure rises, closing the atrioventricular valves. Initially both sets of valves are closed, so pressure rises without a change in ventricular volume. When ventricular pressure exceeds arterial pressure, the semilunar valves open and blood is ejected.
As ventricles relax, their pressure falls below arterial pressure and semilunar valves close. There is another brief interval with both sets closed. When ventricular pressure falls below atrial pressure, the atrioventricular valves reopen and filling resumes. Both sides of the heart follow this sequence together, at different pressures.
NOT: tendinous cords stop AV valves turning inside out. They do not pull them open.
- Flow is from higher pressure to lower pressure
- Left wall is thicker: higher pressure, not more volume
- Coronary arteries supply the heart muscle
- Both sides follow the same sequence at different pressures
Two traps this diagram blocks
- Do not say all valves are open in systole. During ejection the AV valves are shut.
- A heart diagram is often an anterior view, so the anatomical left can be on the right of the page. Use the vessels and the thicker wall.
From the diagram alone
Question
Ventricular pressure is above atrial pressure but below aortic pressure. State the positions of the left AV and aortic valves and explain one of them. [3]
Show a model answer
Both valves are closed. The left atrioventricular valve is closed because ventricular pressure is higher than atrial pressure, so blood cannot flow back into the atrium. The aortic valve is closed because aortic pressure is still higher than ventricular pressure.
| Stage | Atrioventricular valves | Semilunar valves |
|---|---|---|
| Ventricular filling, including atrial systole | Open | Closed |
| Early ventricular contraction before ejection | Closed | Closed |
| Ventricular ejection | Closed | Open |
| Early ventricular relaxation before filling | Closed | Closed |
Make your reasoning visible
Explain with a pressure comparison
Write “ventricular pressure exceeds atrial pressure, so the atrioventricular valveA flap of tissue that closes when pressure behind it is higher, so blood can flow in only one direction. closes”. Naming contraction alone does not explain how the valve moves.
Use curve intersections
On a pressure–time graph, look for where ventricular pressure crosses atrial or arterial pressure. Use the direction of the crossing to infer which valve opens or closes.
Do not swap the patient’s left and right
Heart diagrams often show an anterior view, so the left side of the heart appears on the right of the page. Identify vessels and wall thickness rather than relying on page position.
Avoid this misconception
The claim: All valves are open during systoleThe stage of the cardiac cycle when heart muscle contracts. because the heart is pumping.
The correction: During ejection, semilunar valves are open and atrioventricular valves are closed. At the start of ventricular systole there is a brief period when both sets are closed.
What makes the heart's valves open and shut?
Each heartbeat is a cardiac cycle: the atria contract, then the ventricles, then everything relaxes. The valves have no muscles of their own. They are flaps that get pushed open or shut by the blood, depending on which side has the higher pressure.
What the model shows
A heart cut open from the front beating in slow motion, next to a graph of the pressures in the left atrium, left ventricle and aorta, and the volume of blood in the ventricle.
What to look for
Where the pressure lines cross. Each crossing is a moment when a valve opens or closes. Look for the short times when both valves are shut.
What it tells us
Blood always flows from higher to lower pressure. A valve opens when the pressure behind it is higher, and shuts when the pressure in front is higher, which stops blood flowing backwards.
Be careful what you claim
The pressures come from a simple model, so the numbers are typical rather than exact. The drawing is a diagram, not to scale.
Try it in the model: press Play, then pause when the red ventricle line crosses above the purple atrium line. Which valves are shut?
The cardiac cycle: pressures and valves
Watch one heartbeat in slow motion beside the pressure graph. Every valve movement happens where two pressure lines cross.
What you see
- Left: a heart cut open from the front, so the heart's left side is on the right of the page. Blue blood is deoxygenated, red is oxygenated. The chambers shrink as blood leaves them.
- Valves turn green when open and red when closed. The banner names the phase.
- Right: pressure in the left atrium, left ventricle and aorta over one beat, which valves are open, and the volume of blood in the left ventricle. Yellow bands mark the times when both valves are closed.
How to use it
- Play in slow motion, step, or drag along the graph to scrub. Jump to any phase.
- Change the heart rate. Press New moment to test yourself on valve positions.
What it shows
- Valves open and close only because of pressure differences: AV valves compare atrium and ventricle; semilunar valves compare ventricle and artery.
- Atrial systole, ventricular systole (with a both-closed phase, then ejection) and cardiac diastole (both closed, then filling).
- The right side follows the same sequence at the same time, at lower pressures.
Limits of the model
- The pressures come from a simple model of the left side of the heart, so the values are illustrative (about 16 kPa in the aorta at its highest and 10 kPa at its lowest).
- The heart drawing is a diagram, not to scale. Tendinous cords and the electrical control of the beat are not shown here; the conduction pathway is in 7B 5.
- The right side is drawn with the same valve timing; its own pressures are not plotted.
Guided tour · step 1 of 9
- In Playback, just below these steps, set Speed to 1/10× (options: 1/10×, ¼×, ½×, 1×).
- Press Play and watch one whole beat.
- Follow the cursor along the graph in the simulator.
One cardiac cycle has atrial systole, ventricular systole and cardiac diastole. The graph shows the pressures that make blood and valves move.
Playback
One beat lasts 0.80 s. Stroke volume in this model: 70 cm³.
Test yourself
Correct in a row: 0
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
In an illustrative left-heart pressure trace, atrial pressure is 1 kPa, ventricular pressure is 5 kPa and aortic pressure is 10 kPa. State the valve positions and explain them.
Show suggested answer
The left atrioventricular valve is closed because ventricular pressure exceeds atrial pressure. The aortic semilunar valve is closed because aortic pressure exceeds ventricular pressure. Both valves can be closed briefly during contraction or relaxation; the pressure values alone do not establish which.
Practice question
Retrieval and explanation · suggested answer
Explain why the left ventricle has a thicker wall than the right ventricle.
Show suggested answer
It must generate a higher pressure to drive blood through the higher-resistance systemic circuit. Its thicker muscle wall produces a more forceful contraction. This does not mean it normally pumps a greater average volume than the right ventricle.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A pressure trace shows ventricular pressure rising while ventricular volume stays constant. Later, pressure is still rising but volume begins to fall. Explain what has changed.
Show a hint
Use the valve positions before and after the change.
Explore the reasoning
Initially both valve sets are closed, so contracting muscle raises pressure without ejecting blood. Once ventricular pressure exceeds arterial pressure, the semilunar valve opens. Blood is ejected and ventricular volume falls, while the atrioventricular valve remains closed.
Recall this lesson
Lesson recall questions
Question 1 of 5: List the four major vessels attached to the mammalian heart and state whether each carries oxygenated or deoxygenated blood.
Cardiac cycle and heart structure
Practice question
Retrieval and explanation · suggested answer
List the four major vessels attached to the mammalian heart and state whether each carries oxygenated or deoxygenated blood.
Lesson: 1B 4: The Mammalian Heart
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Seeing flow rather than guessing from a heartbeat
1B · The Mammalian HeartDoppler ultrasound can show blood flow through heart chambers and valves. This helps clinicians investigate valve function, linking images and flow information to the pressure-and-valve model you have…Specification 1.8Read & use the scienceClose article
Why this matters
Doppler ultrasound can show blood flow through heart chambers and valves. This helps clinicians investigate valve function, linking images and flow information to the pressure-and-valve model you have learned. Doppler measurements use changes in reflected ultrasound to investigate movement. In cardiac assessment, a moving structure or a detectable heartbeat does not by itself establish that blood passes through valves normally. Flow measurements add evidence about direction and speed, which must be interpreted with the anatomy and the rest of the examination. Students do not need the ultrasound physics here. The useful biological question is whether pressure changes, valve action and blood movement agree with the proposed explanation.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.8 know the cardiac cycle (atrial systole, ventricular systole and cardiac diastole) and relate the structure and operation of the mammalian heart, including the major blood vessels, to its function
Use the science
A drama shows a character checking a pulse and declaring every heart valve healthy. Why is that conclusion too strong?
Explore a possible response
A pulse provides evidence of pulsatile arterial flow, but does not directly show whether each valve closes completely or whether some blood flows backwards. Assessing valve function requires more specific evidence.
Explore the source
A replacement valve is a biological trade-off
1B · The Mammalian HeartA replacement heart valve must open and close under changing pressure, restoring directional flow. Choosing a valve also brings a longer-term trade-off. NHS information distinguishes durable mechanical valves…Specification 1.8 · 1.11 · 1.20Read & use the scienceClose article
Why this matters
A replacement heart valve must open and close under changing pressure, restoring directional flow. Choosing a valve also brings a longer-term trade-off. NHS information distinguishes durable mechanical valves from tissue valves; mechanical valves carry a clotting risk that requires anticoagulant treatment, while tissue valves may eventually need replacement. A cardiac team therefore combines the mechanics you study with evidence about blood clotting, treatment risks and a patient’s circumstances. The most durable material is not automatically the best choice for every person. This is a useful example of a device solving one biological problem while creating another that needs management.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.8 know the cardiac cycle (atrial systole, ventricular systole and cardiac diastole) and relate the structure and operation of the mammalian heart, including the major blood vessels, to its function
- 1.11 understand the blood clotting process (thromboplastin release, conversion of prothrombin to thrombin and fibrinogen to fibrin) and its role in cardiovascular disease (CVD)
- 1.20 know the benefits and risks of treatments for cardiovascular disease (CVD) (antihypertensives, statins, anticoagulants and platelet inhibitors)
Use the science
Explain why a valve could prevent backflow successfully but still require anticoagulant treatment.
Explore a possible response
Directional flow and clot formation are different properties. Contact with a mechanical valve can increase clot risk even when its opening and closing function is effective.