AS Level PE 8386 · Area 3 · The Cardiovascular System

Structure and Function of the Heart

The heart is two pumps side by side, separated by the septum. Cambridge wants you to trace blood through both sides in order, name every valve it passes, and explain why the walls of the chambers differ in thickness. Learn the pathway once, properly, and most diagram questions answer themselves.

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Syllabus coverage

  • Explain the structure and functions of the features of the heart and the pathway of blood through the heart, including atria and ventricles (left and right), bicuspid and tricuspid valves, aortic and pulmonary semilunar valves, vena cava, aorta, pulmonary artery, pulmonary vein, septum and myocardium
  • Apply knowledge of the structure of the heart to a range of diagrams of the heart

Part of the AS Level PE 8386 syllabus 8386.

Vena cavaRight atriumRight ventricleLeft atriumLeft ventriclePulmonary arteryAortaPulmonary veinThe left ventricle wall is the thickest because it pumps blood around the whole body.
A labelled section through the heart showing the four chambers, the septum, the myocardium and the valves, with arrows tracing the pathway of blood.

Two pumps, four chambers

The heart is a muscular pump divided into four chambers: two atria (singular atrium) on top and two ventricles below. It is a double pump, because the right side sends blood to the lungs and the left side sends blood to the rest of the body.

The right side handles deoxygenated blood and is the pulmonary circuit; the left side handles oxygenated blood and is the systemic circuit. The two sides are separated by a thick muscular wall, the septum, which stops oxygenated and deoxygenated blood mixing.

The myocardium is the cardiac muscle that forms the walls of the heart and does the pumping. It is supplied with oxygen by the coronary arteries, which is why a performer can have a heart attack during exercise even though the heart itself is full of blood.

Chamber / featureBlood it handlesWhere it sends the blood
Right atriumDeoxygenatedTo the right ventricle
Right ventricleDeoxygenatedTo the lungs, through the pulmonary artery
Left atriumOxygenatedTo the left ventricle
Left ventricleOxygenatedTo the whole body, through the aorta
Septum—Separates the two sides so oxygenated and deoxygenated blood do not mix
Myocardium—Cardiac muscle that contracts to pump blood; it forms the walls of all four chambers

The pathway of blood, in order

Learn this as a fixed route, both sides, without skipping a step. Short-answer questions ask for the next structure in the sequence, and diagram questions label the same list.

Deoxygenated blood to the lungs: vena cava → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary artery → lungs.

Oxygenated blood to the body: pulmonary vein → left atrium → bicuspid valve → left ventricle → aortic semilunar valve → aorta → body.

  • Blood enters the atria first, always — the atria are receiving chambers and sit above the ventricles
  • Blood leaves the ventricles by an artery: the pulmonary artery from the right, the aorta from the left
  • Blood returns from the lungs by a vein: the pulmonary vein, the only vein carrying oxygenated blood
  • The vena cava is the only vein entering the heart carrying deoxygenated blood from the body
The most common error in pathway questions is starting at the lungs instead of the heart. Always write the route from the heart's own structures, and double-check the direction — vena cava into the right atrium, aorta out of the left ventricle.

The valves and why they are needed

Blood must travel through the heart in one direction only. The valves are the one-way gates that make this possible: they open to let blood through and snap shut to stop it flowing backwards.

There are two pairs of valves. Each pair works differently and Cambridge expects you to know which opening each one guards.

  • The tricuspid and bicuspid valves are the atrioventricular valves — they sit between an atrium and a ventricle
  • The pulmonary and aortic semilunar valves are the one-way exit valves — they sit between a ventricle and an artery
  • The bicuspid valve has two flaps and the tricuspid has three — a quick way to tell them apart on a diagram
ValvePositionJob
Tricuspid valveBetween the right atrium and right ventricleOpens to let deoxygenated blood into the right ventricle; closes to prevent backflow into the right atrium when the ventricle contracts
Bicuspid valveBetween the left atrium and left ventricleOpens to let oxygenated blood into the left ventricle; closes to prevent backflow into the left atrium when the ventricle contracts
Pulmonary semilunar valveAt the exit from the right ventricle into the pulmonary arteryOpens to let blood leave for the lungs; closes to prevent backflow from the pulmonary artery into the right ventricle
Aortic semilunar valveAt the exit from the left ventricle into the aortaOpens to let blood leave for the body; closes to prevent backflow from the aorta into the left ventricle

Why the wall thicknesses differ

The thickness of the myocardium varies across the four chambers, and the difference is not random — it matches the distance the blood has to travel.

The atria have thin walls. They only have to push blood a short distance into the ventricles directly below them. The right ventricle has a thicker wall than the atria, because it must push blood to the lungs. The left ventricle has the thickest wall in the heart, because it must generate enough pressure to push blood all the way around the body through the systemic circuit — the furthest distance and the greatest resistance.

This is the same reasoning you will meet again with Starling's law: a more powerful contraction builds a greater pressure, which forces out a larger volume of blood.

ChamberRelative wall thicknessWhy
Right atriumThinPumps blood only as far as the right ventricle next door
Left atriumThinPumps blood only as far as the left ventricle next door
Right ventricleThicker than the atria, thinner than the leftPumps blood to the lungs, which is a short, low-resistance circuit
Left ventricleThickest wall in the heartPumps blood around the whole body, the longest distance and highest resistance, so the greatest pressure is needed
Never answer this with "because the left side is bigger". The mark is for the consequence: a thicker myocardium produces a more forceful contraction, generating the higher pressure needed to send blood around the whole body.

Working from a diagram

The syllabus says candidates must be able to apply this knowledge to a range of diagrams of the heart, and that includes diagrams drawn the other way round from the textbook.

Use two reliable orientation cues. First, atria on top, ventricles below — the heart is never inverted. Second, the left ventricle is the thickest-walled chamber, so the side with the thickest wall is the left side, which means that side handles oxygenated blood from the pulmonary vein.

If the diagram shows an arrow of blood flow, follow it and identify the structure it crosses: an arrow entering a chamber from a vein, or leaving through an artery, tells you which chamber you are looking at.

  • Find the thickest wall → that is the left ventricle
  • Find the vessel entering the top-right chamber from the body → vena cava
  • Find the two vessels connecting the heart to the lungs → pulmonary artery (out) and pulmonary vein (in)
  • Find the valve between an atrium and a ventricle → tricuspid on the right, bicuspid on the left

Key terms you must be able to define

Atrium
An upper chamber of the heart that receives blood — the right atrium receives deoxygenated blood from the vena cava, the left atrium receives oxygenated blood from the pulmonary vein.
Ventricle
A lower chamber of the heart that pumps blood out — the right ventricle to the lungs through the pulmonary artery, the left ventricle to the body through the aorta.
Tricuspid valve
The valve between the right atrium and right ventricle that prevents backflow of deoxygenated blood into the right atrium.
Bicuspid valve
The valve between the left atrium and left ventricle that prevents backflow of oxygenated blood into the left atrium.
Pulmonary semilunar valve
The valve at the exit of the right ventricle into the pulmonary artery, preventing backflow from the artery into the ventricle.
Aortic semilunar valve
The valve at the exit of the left ventricle into the aorta, preventing backflow from the aorta into the left ventricle.
Vena cava
The large vein that carries deoxygenated blood from the body into the right atrium.
Aorta
The large artery that carries oxygenated blood from the left ventricle to the body.
Pulmonary artery
The artery that carries deoxygenated blood from the right ventricle to the lungs.
Pulmonary vein
The vein that carries oxygenated blood from the lungs into the left atrium.
Septum
The muscular wall dividing the left and right sides of the heart, preventing oxygenated and deoxygenated blood from mixing.
Myocardium
The cardiac muscle forming the walls of the heart; its thickness varies between chambers according to the pressure each must generate.

Applied to your sport

Cambridge requires you to apply this knowledge to physical activities. Use these as models, then write your own for the sport you play.

Cross-country running

The left ventricle has the thickest myocardium so it can force oxygenated blood around the whole body. In a sustained run, the working leg muscles need a continuous supply, which is why the left ventricle works hardest of the four chambers.

Competitive swimming

The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, where it is oxygenated before returning through the pulmonary vein to the left atrium — the same cycle supports every stroke of a long swim.

Basketball

The valves stop blood flowing backwards as chamber pressures change. When a player's heart rate rises sharply during a fast break, the tricuspid and bicuspid valves close on every ventricular contraction to keep blood moving in one direction.

Olympic weightlifting

A maximal lift demands a large and rapid rise in blood flow to the working muscles, so the myocardium of the left ventricle must generate high pressure to push blood through the aorta against the resistance of the systemic circuit.

Flashcards

Click a card to flip it. Mark honestly — the tracker uses this.

Q1Which chamber receives deoxygenated blood from the vena cava?
The right atrium.
Q2Name the valve between the right atrium and right ventricle.
The tricuspid valve.
Q3Name the valve between the left atrium and left ventricle.
The bicuspid valve.
Q4Which vessel carries blood from the right ventricle to the lungs?
The pulmonary artery — carrying deoxygenated blood.
Q5Which vessel carries oxygenated blood from the lungs to the heart?
The pulmonary vein — into the left atrium.
Q6Which vessel carries oxygenated blood from the left ventricle to the body?
The aorta, through the aortic semilunar valve.
Q7What is the function of the septum?
To separate the left and right sides of the heart so oxygenated and deoxygenated blood do not mix.
Q8What is the myocardium?
The cardiac muscle forming the walls of the heart; its thickness differs between chambers.
Q9Which chamber has the thickest wall and why?
The left ventricle — it must generate the highest pressure to pump blood around the whole body, the longest distance against the greatest resistance.
Q10Why do the atria have thin walls?
They only pump blood a short distance into the ventricles directly below them, so little pressure is needed.
Q11What is the function of the valves in the heart?
To prevent backflow, so blood flows through the heart in one direction only.
Q12Give the four valves in the order blood meets them from the right atrium.
Tricuspid → pulmonary semilunar → bicuspid → aortic semilunar.
0 of 12 marked as known

Exam-style questions

8 questions, 22 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.

StateAO11 mark
Name the valve that prevents blood flowing back from the left ventricle into the left atrium.

Mark scheme — tick each point you actually made

Examiner tip: One mark for the exact name. "Heart valve" or "the valve on the left" scores nothing.
IdentifyAO12 marks
Identify the two blood vessels that connect the heart to the lungs.

Mark scheme — tick each point you actually made

Examiner tip: Name each vessel and its direction of travel. Reversing them is the most common error in this topic.
DescribeAO13 marks
Describe the pathway of deoxygenated blood from the body until it reaches the lungs.

Mark scheme — tick each point you actually made

Examiner tip: Three structures in order, three marks. Write the route as a sequence rather than a description so nothing is left out.
ExplainAO22 marks
Explain the function of the septum in the heart of a performer during exercise.

Mark scheme — tick each point you actually made

Examiner tip: The second mark is the consequence for the performer. Saying "it separates the sides" twice will only earn one mark.
ExplainAO12 marks
Explain why the atria have thinner walls than the ventricles.

Mark scheme — tick each point you actually made

Examiner tip: Compare the two, always. The mark is for the difference in the distance the blood must travel.
ExplainAO23 marks
Explain why the wall of the left ventricle is thicker than the wall of the right ventricle, and what this means for a performer during a game.

Mark scheme — tick each point you actually made

Examiner tip: Two marks for the explanation and one for the consequence. Finish every wall-thickness answer with what it means for the performer.
ExplainAO24 marks
Explain how the structure of the heart ensures that blood flows in one direction only, and why this matters during a match.

Mark scheme — tick each point you actually made

Examiner tip: Four marks, four named valves/steps: name each valve and say which backflow it prevents. The final mark is for the consequence for the performer.
AnalyseAO25 marks
Analyse how the structure of the heart allows a cross-country runner to maintain a supply of oxygenated blood to the working muscles throughout a race.

Mark scheme — tick each point you actually made

Examiner tip: An analyse question wants structure linked to function throughout, and a sense of the whole circuit. Work through pulmonary circuit → left side → separation → valves, and finish with the benefit to the runner.

Exam technique for this topic

  • Learn the pathway in order as a single sequence. Questions often ask for "the next structure" or "the chamber blood enters after the pulmonary vein", so the order is the mark.
  • For wall thickness questions, always link thickness to pressure needed and distance travelled: thicker myocardium → stronger contraction → higher pressure → blood pushed further.
  • On a diagram question, use the two orientation rules — atria above ventricles, thickest wall is the left ventricle — before you label anything.
  • Never swap the pulmonary artery and pulmonary vein. The artery leaves the heart for the lungs (deoxygenated); the vein returns from the lungs (oxygenated). This is the most commonly reversed pair in the topic.
  • Use the syllabus names exactly: bicuspid and tricuspid valves, aortic and pulmonary semilunar valves. "Heart valve" on its own will not score.

Why students lose marks here

A student wroteDeoxygenated blood enters the left atrium from the vena cava and is pumped to the lungs.
Why it loses marksThe sides of the heart have been swapped. Deoxygenated blood enters the right atrium, and it is the right ventricle that pumps it to the lungs. Two structures are wrong, so no marks can be given even though the valves named might have been right.
The answer that scoresDeoxygenated blood enters the right atrium from the vena cava, passes through the tricuspid valve into the right ventricle, and is then pumped through the pulmonary semilunar valve into the pulmonary artery to the lungs.
A student wroteThe left ventricle is thicker because it is on the left side and bigger.
Why it loses marksThis repeats the question. The mark is for the reason: the left ventricle must generate enough pressure to push blood around the whole body, which is the longest route and meets the greatest resistance.
The answer that scoresThe left ventricle has the thickest myocardium because it pumps oxygenated blood around the whole body through the aorta; this is the longest distance and the greatest resistance to flow, so the most forceful contraction and highest pressure are needed.
A student wroteThe pulmonary vein carries deoxygenated blood away from the heart to the lungs.
Why it loses marksDirection and oxygen state have both been reversed. The pulmonary vein returns oxygenated blood from the lungs to the left atrium; it is the pulmonary artery that takes deoxygenated blood from the right ventricle to the lungs.
The answer that scoresThe pulmonary vein carries oxygenated blood from the lungs into the left atrium. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs.
A student wroteThe valves push the blood through the heart.
Why it loses marksValves are passive. They do not contract or pump — they open and close in response to pressure changes, and their function is to prevent backflow so blood flows in one direction.
The answer that scoresThe valves have no pumping action. They open and close according to the pressure difference between chambers, and their function is to prevent backflow, ensuring blood travels through the heart in one direction only.