How the Heart Works: Chambers, Valves, and the Cardiac Cycle
The heart is a four-chambered muscular pump. Learn how blood flows through its chambers and valves, how it keeps its rhythm, and what a heartbeat actually is.
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The heart is a four-chambered muscular pump that moves blood around the body, supplying every tissue with oxygen and nutrients and carrying away carbon dioxide and waste. It is about the size of a clenched fist and weighs roughly 300 grams. At rest it beats 60 to 100 times a minute, around 100,000 times a day, and pumps about five litres of blood each minute, which adds up to billions of beats in a lifetime.
What are the four chambers?
The upper two chambers, the right and left atria, receive blood returning to the heart. The lower two, the right and left ventricles, pump blood out. The right side of the heart sends oxygen-poor blood to the lungs, which is the pulmonary circulation. The left side sends oxygen-rich blood from the lungs to the rest of the body, the systemic circulation. The two circuits run in series, so the output of one feeds the other. The left ventricle has the thickest wall, about a centimetre, because it must generate far higher pressure than the right to push blood through the whole body.
How does blood flow through the heart?
- Oxygen-poor blood returns from the body through two large veins, the superior and inferior vena cava, into the right atrium.
- It passes through the tricuspid valve into the right ventricle.
- The right ventricle pumps it through the pulmonary valve into the pulmonary arteries, which carry it to the lungs, where it picks up oxygen and releases carbon dioxide.
- Oxygen-rich blood returns through the pulmonary veins into the left atrium.
- It passes through the mitral valve into the left ventricle.
- The left ventricle pumps it through the aortic valve into the aorta, the main artery, which branches to supply the whole body.
What do the valves do?
Four valves keep blood moving in one direction. They have no muscles of their own: they open and close passively as pressure changes on either side. The familiar heartbeat sound of lub-dub is the sound of the valves closing. The first sound comes when the mitral and tricuspid valves shut at the start of ventricular contraction, and the second when the aortic and pulmonary valves shut as the ventricles relax. Valve disease comes in two basic forms: stenosis, in which a valve is too narrow to open fully, and regurgitation, in which it leaks. Either can produce an audible murmur.
What is the cardiac cycle?
Each heartbeat alternates between diastole, when the heart relaxes and the chambers fill, and systole, when the ventricles contract and eject blood. At 75 beats per minute one cycle lasts about 0.8 seconds, roughly 0.3 seconds of systole and 0.5 seconds of diastole. The left ventricle ejects around 70 millilitres with each beat, called the stroke volume. Cardiac output, the volume pumped per minute, equals heart rate times stroke volume: 70 beats per minute times 70 millilitres gives about 4.9 litres a minute, close to the body's entire blood volume every minute.
What keeps the heartbeat going?
Heart muscle generates its own electrical signals, so the heart will keep beating even if its nerves are cut. The natural pacemaker is the sinoatrial node, a small cluster of cells in the wall of the right atrium that fires 60 to 100 times a minute. Its signal spreads across the atria, making them contract. It then pauses for about a tenth of a second at the atrioventricular node, which gives the ventricles time to fill. Finally it races down special conducting fibres so that the ventricles contract from the bottom upward, squeezing blood toward the exits. The electrocardiogram (ECG) records this electrical activity.
The electrical signal in each muscle cell is an action potential, although the cardiac version has a long plateau, driven by calcium, that keeps the muscle contracted long enough to eject blood. The nervous system adjusts the rate: sympathetic nerves speed it up and the vagus nerve slows it down, a classic example of homeostasis. Constant pumping demands a steady supply of ATP, which is why the heart is packed with mitochondria that run cellular respiration.
How does the heart feed itself?
Blood inside the chambers does not nourish the heart muscle. Instead the coronary arteries, which branch from the aorta just above the aortic valve, wrap around the heart's surface and supply it. If a coronary artery becomes blocked, the muscle it feeds is starved of oxygen, which is a heart attack. Heart disease is the leading cause of death worldwide. Its main risk factors are high blood pressure, high cholesterol, smoking, a sedentary life, and diabetes (see diabetes and insulin), most of which can be reduced by changes in lifestyle and by treatment.
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anatomy cardiovascular circulation heart physiology