SECTION 6.2
Valves and coronary circulation
Trace blood through the heart valves in order and describe the coronary arteries that supply the heart muscle itself.
Learn
The heart pumps in one direction only, and valves make that possible. Without them, blood would flow backwards each time a chamber relaxed, and the pumping would be wasted. There are four valves: two atrioventricular valves between the atria and ventricles, and two semilunar valves at the exits of the ventricles. Blood passes through them in a fixed order, and tracing that order is the most reliable way to learn cardiac anatomy. A second circulation serves the heart itself: the coronary arteries branch from the base of the aorta and deliver oxygen to the myocardium, while cardiac veins return the blood to the right atrium through the coronary sinus. This section follows blood through the four valves and then describes the coronary vessels, whose blockage causes angina and heart attack.
Key Concepts & Topics
Why valves are needed
A pump works only if the fluid moves in one direction. When a chamber contracts, pressure inside rises and blood is pushed forward; when it relaxes, pressure falls and blood would flow backward unless something prevents it. Valves are flaps of endocardium reinforced with connective tissue that open and close passively, driven only by pressure differences across them. They do not contract and do not use ATP. When pressure on the upstream side is higher, the valve opens; when pressure on the downstream side becomes higher, it closes. This simple mechanism ensures that blood always moves forward through the heart and the circuits. When a valve is narrowed or leaks, the heart must work harder and the direction of flow is no longer perfectly controlled.
For example: When you listen to a heart, the closing of valves produces the sounds: the first sound is the atrioventricular valves closing, the second is the semilunar valves closing.
The four valves and their supports
The two atrioventricular (AV) valves sit between an atrium and a ventricle. The right AV valve is the tricuspid valve, with three cusps; the left is the mitral or bicuspid valve, with two. Their cusps are anchored to the ventricular wall by tendinous cords called chordae tendineae, which attach to muscular projections called papillary muscles. When the ventricle contracts, the papillary muscles also contract and pull the cords taut, so the cusps meet and do not blow back into the atrium. The two semilunar valves sit at the exits of the ventricles and each has three cusps shaped like half-moons. The pulmonary valve guards the exit of the right ventricle into the pulmonary trunk, and the aortic valve guards the exit of the left ventricle into the aorta. Both semilunar valves open when ventricular pressure exceeds arterial pressure and snap shut when it falls.
For example: If a papillary muscle ruptures, the AV valve it supports can blow back into the atrium, allowing blood to flow backward and causing sudden breathlessness.
Tracing blood through the valves in order
The order of the valves is fixed and follows the two circuits. Deoxygenated blood enters the right atrium and passes through the tricuspid valve into the right ventricle. When the right ventricle contracts, blood is ejected through the pulmonary valve into the pulmonary trunk and reaches the lungs. After gas exchange, oxygenated blood returns to the left atrium and passes through the mitral valve into the left ventricle. When the left ventricle contracts, blood is ejected through the aortic valve into the aorta. Notice that an AV valve is always followed by a semilunar valve on the same side, and that the names of the valves match the chamber or vessel they guard. Learning the sequence in this way makes it possible to work out where a murmur is arising and which chamber is overloaded.
For example: A murmur heard between the first and second heart sounds, at the apex, often arises from the mitral valve during the pumping phase.
The coronary arteries
The myocardium cannot take oxygen from the blood inside the chambers, because the endocardium is too thick a barrier for useful diffusion and the muscle is far too thick. The heart therefore has its own blood supply, the coronary circulation. The right and left coronary arteries arise from the base of the aorta, just above the aortic valve, so they receive blood when the ventricle relaxes and the valve cusps fill. The left coronary artery divides into the anterior interventricular artery, also called the left anterior descending artery, which supplies the front of both ventricles and the interventricular septum, and the circumflex artery, which supplies the left atrium and the side of the left ventricle. The right coronary artery supplies the right atrium and right ventricle and usually gives off the posterior interventricular artery, which supplies the back of the heart.
For example: The left anterior descending artery supplies a large part of the left ventricle, so its blockage is particularly dangerous and is sometimes called a widow-maker lesion.
When coronary supply fails
The coronary arteries behave largely as end arteries, with little effective alternative supply, so a blockage threatens the muscle it feeds. Narrowing by atherosclerosis reduces flow, especially when demand rises during exercise, and produces angina: chest pain that appears with effort and eases with rest. A sudden complete blockage causes myocardial infarction, in which the deprived muscle dies and is replaced by scar tissue that cannot contract. Because the coronary arteries fill mainly during diastole, a very fast heart rate shortens the filling time and can worsen ischaemia. The cardiac veins run alongside the arteries and drain into the coronary sinus, which empties into the right atrium, so the heart’s own used blood rejoins the circulation there.
For example: A patient with angina may be given a drug that slows the heart, because a slower rate lengthens diastole and gives the coronary arteries more time to fill.
Key facts to remember
- Valves open and close passively with pressure differences and keep blood flowing in one direction.
- The atrioventricular valves are the tricuspid (right) and mitral (left); their cusps are held by chordae tendineae attached to papillary muscles.
- The semilunar valves are the pulmonary valve at the right ventricular exit and the aortic valve at the left ventricular exit.
- Blood order: right atrium → tricuspid → right ventricle → pulmonary valve → lungs → left atrium → mitral → left ventricle → aortic valve → aorta.
- The right and left coronary arteries arise from the aortic base; the left divides into the anterior interventricular and circumflex arteries, and cardiac veins drain into the coronary sinus.
Example · Trace a drop through the valves. It enters the right atrium from the inferior vena cava, passes the tricuspid valve into the right ventricle, and is ejected through the pulmonary valve into the pulmonary trunk. After exchanging gases in the lungs, it returns to the left atrium and passes the mitral valve into the left ventricle. It is then ejected through the aortic valve into the aorta. Just above that valve, some blood enters the coronary arteries and supplies the heart muscle itself before the rest continues to the body. Four valves, two circuits and one self-supplying organ.
Read full notes
Read one topic at a time. These notes explain the mechanisms and relationships behind the key facts.
Valves: one-way flow
Valves solve a mechanical problem: a pump that ejects into a pressure reservoir will lose its output if the fluid returns while the pump refills. The heart has two sets of valves to prevent this, one at each inlet and one at each outlet. Because they open and close only in response to pressure, they require no nerve supply and no energy, and they operate reliably for a lifetime. Their design also explains their failure modes. A valve whose cusps do not seal properly leaks, so blood moves backward and the chamber must pump extra volume. A valve whose cusps stiffen does not open fully, so the chamber must generate higher pressure. Both problems make the heart work harder, and both produce characteristic sounds that a clinician can hear.
The four valves and their supports
The AV valves are more complex than they first appear. Their cusps are not rigid doors but flexible flaps that must withstand the full pressure of ventricular contraction without being pushed into the atrium. The chordae tendineae and papillary muscles provide the necessary support: the papillary muscles contract with the ventricle, tightening the cords at exactly the moment the pressure rises. This is why rupture of a papillary muscle causes sudden, severe backward flow. The semilunar valves are simpler, with three cusps that meet in the middle when pressure reverses. Their cusps also fill with blood during diastole, which is how the coronary arteries receive their supply, and their closure produces the second heart sound.
Tracing blood through the valves in order
The sequence of valves is a reliable framework because it never changes: tricuspid, pulmonary, mitral, aortic. Each valve belongs to one side of the heart and to one circuit, and each is positioned so that the pressure on its upstream side normally exceeds the pressure downstream while it must be open. Learning the sequence helps in two practical ways. First, it lets you predict where blood will be when a chamber contracts, and therefore which chamber is overloaded if a valve fails. Second, it links the positions on the chest where each valve is best heard, because sound travels from the valve to the chest wall in predictable directions. This is why a murmur heard at the apex usually involves the mitral valve, while one heard at the upper right sternal border usually involves the aortic valve.
The coronary arteries
The coronary arteries are the heart’s own fuel line, and their anatomy explains their importance. The left coronary artery supplies most of the left ventricle, the chamber that does the heaviest work, so its branches are the most critical. The anterior interventricular artery runs down the front of the heart and supplies a large area of both ventricles and the septum; the circumflex wraps around the left side. The right coronary artery supplies the right side and often the back of the left ventricle. Because the coronary vessels fill during diastole, anything that shortens diastole, such as a very fast heart rate, reduces supply. Because they are largely end arteries, a blockage cannot be easily compensated, which is why coronary disease is so often symptomatic and dangerous.
When coronary supply fails
The consequences of coronary disease depend on how much muscle is deprived and for how long. Narrowing reduces flow, and the mismatch between supply and demand becomes obvious when demand rises, producing angina on exertion that eases with rest. If the plaque ruptures and a clot forms, flow may stop completely, and the muscle downstream dies: this is myocardial infarction. Dead muscle is replaced by scar tissue that cannot contract, so the heart’s pumping capacity falls permanently. Restoring flow quickly, by drugs or by opening the vessel, limits the damage, which is why recognising the symptoms and acting fast matters. Cardiac veins then drain the used blood into the coronary sinus and back to the right atrium, completing the heart’s own circuit.
Explore the diagram
Start with the whole view, then select a numbered structure. Compare the additional illustrations to connect what a structure looks like with what it does.
Two valve mechanisms
Reduced motion · use the slider to explore each state.
Pressure differences move the leaflets; papillary muscles do not pull them open.
Educational sequence · slowed and simplified, not real-time physiology
The heart’s own blood supply
Reduced motion · use the slider to explore each state.
Arises from the right aortic sinus and follows the right AV groove.
Educational sequence · slowed and simplified, not real-time physiology
Connect the illustrations to the key ideas
AV VALVES · tricuspid (right) and mitral (left)
Cusps held by chordae tendineae and papillary muscles prevent backflow into the atria.
SEMILUNAR VALVES · pulmonary (right exit) and aortic (left exit)
They open when ventricular pressure exceeds arterial pressure and close when it falls.
CORONARY CIRCULATION · coronary arteries from the aortic base; veins to the coronary sinus
The heart muscle receives its own blood supply, mainly during diastole.
Core Concepts Defined
- Heart valve
- A passive flap that keeps blood flowing in one direction through the heart.
- Atrioventricular (AV) valve
- A valve between an atrium and a ventricle; tricuspid on the right and mitral on the left.
- Tricuspid valve
- The right AV valve, with three cusps.
- Mitral (bicuspid) valve
- The left AV valve, with two cusps.
- Semilunar valve
- A three-cusped valve at a ventricular exit: pulmonary or aortic.
- Pulmonary valve
- The semilunar valve guarding the right ventricular exit.
- Aortic valve
- The semilunar valve guarding the left ventricular exit.
- Cusp
- A flap of a heart valve.
- Chordae tendineae
- Tendinous cords that anchor AV valve cusps to papillary muscles.
- Papillary muscle
- A muscular projection in the ventricle that tenses the chordae tendineae.
- Coronary artery
- An artery arising from the aortic base that supplies the heart muscle.
- Left coronary artery
- The coronary artery dividing into the anterior interventricular and circumflex arteries.
- Anterior interventricular artery (LAD)
- A major branch supplying the front of the ventricles and the interventricular septum.
- Circumflex artery
- A branch supplying the left atrium and the side of the left ventricle.
- Right coronary artery
- The artery supplying the right atrium and ventricle, usually giving the posterior interventricular artery.
- Coronary sinus
- The vein that drains the heart muscle into the right atrium.
- Angina
- Chest pain caused by inadequate coronary flow, typically on effort and relieved by rest.
- Myocardial infarction
- Death of heart muscle caused by prolonged loss of coronary blood supply.
- Ischaemia
- Reduced blood supply to a tissue, which lowers oxygen and energy available to it.
Go deeper · worked example
Tracing blood through the valves, then into the coronaries
- Enter the right side. Deoxygenated blood from the body enters the right atrium through the venae cavae.
- Pass the first valve. Blood flows through the tricuspid valve into the right ventricle; the chordae tendineae and papillary muscles will stop the cusps blowing back when the ventricle contracts.
- Pass the second valve and reach the lungs. The right ventricle ejects blood through the pulmonary valve into the pulmonary trunk, which carries it to the lungs for gas exchange.
- Pass the third and fourth valves. Oxygenated blood returns to the left atrium, passes the mitral valve into the left ventricle, and is ejected through the aortic valve into the aorta.
- Supply the heart itself. Just above the aortic valve, the right and left coronary arteries receive blood during ventricular relaxation and deliver it to the myocardium; cardiac veins return it through the coronary sinus to the right atrium.
Explain it yourself · Explain why the heart needs its own coronary circulation when its chambers are full of blood.
Reveal a model explanation
Blood inside the chambers is separated from the myocardium by the endocardium, and the muscle is far too thick for oxygen to reach its cells by diffusion from the cavity. Only a small fraction of the muscle lies close enough to the blood for diffusion to help. The myocardium therefore depends on the coronary arteries, which deliver oxygenated blood deep into the muscle, and on the cardiac veins, which remove the used blood. This supply is also why the coronary arteries behave as end arteries: if one blocks, the muscle it feeds cannot obtain oxygen from the chamber blood and becomes ischaemic.
Check
Answer three questions from this section — one easy, one medium and one hard. If you miss one, you get a fresh set with explanations for the last attempt.
Practise this sectionOpen all questions and filter by difficulty or Bloom levelReferences and review status
OpenStax, Anatomy and Physiology 2e, 19.1 Heart Anatomy (valves and coronary circulation); Tortora & Derrickson, Principles of Anatomy and Physiology, 12e, ch. 20. Author review only; medical and French-language expert review pending.
Original teaching schematics. English and French wording is author-reviewed for study; anatomy and French-language expert review is pending. Checks are practice only, not a graded examination.
Practice progress is saved on this device for the current learner.