The heart is a muscular organ that keeps blood moving through the body, delivering oxygen and nutrients to tissues while carrying away carbon dioxide and other waste products. Its ability to perform this work depends on a coordinated system of four chambers, four main valves, and a network of blood vessels. Each part has a distinct role, and together they create a continuous circulation that supports nearly every function in the body.
The heart has two sides that work as a team. The right side sends oxygen-poor blood to the lungs, where it releases carbon dioxide and picks up oxygen. The left side receives oxygen-rich blood from the lungs and pumps it throughout the body. Valves keep blood moving in the correct direction, while electrical signals coordinate the contractions that drive each heartbeat.
Understanding how these structures fit together explains not only how the heart works but also why problems with its chambers, valves, or blood supply can affect the entire body.
The heart’s location and basic structure
The heart sits in the chest between the lungs, behind the breastbone and slightly to the left of the midline. It rests above the diaphragm, the muscular sheet that separates the chest from the abdomen. Although its size varies with a person’s body size and other factors, the heart is broadly comparable in size to a closed fist.
A protective, fluid-containing sac called the pericardium surrounds the heart. The small amount of fluid between its layers reduces friction as the heart expands and contracts with each beat.
The heart wall has three main layers. The epicardium forms the outer surface, the myocardium is the thick muscular layer responsible for pumping, and the endocardium lines the chambers and covers the valves. The myocardium is especially thick in the left ventricle because that chamber must generate enough pressure to send blood through the entire body.
A wall of tissue called the septum separates the heart’s right and left sides. This division prevents oxygen-poor blood returning from the body from mixing directly with oxygen-rich blood returning from the lungs. The right and left sides are connected through the pulmonary and systemic circulations, but blood normally travels between them through the lungs rather than across the septum.
The four chambers of the heart
The heart contains two upper chambers, called atria, and two lower chambers, called ventricles. The atria receive blood returning to the heart, while the ventricles pump blood out of it. This arrangement allows the heart to fill and empty in an organized sequence rather than relying on a single contraction to move blood through the entire circulation.
The right atrium receives blood from the body
The right atrium collects oxygen-poor blood returning from most of the body through two large veins: the superior vena cava, which brings blood from the head, neck, arms, and upper chest, and the inferior vena cava, which returns blood from the lower body. Blood from the heart muscle itself also drains into the right atrium, primarily through a vessel called the coronary sinus.
As the right atrium fills, its walls stretch. When the atrium contracts, it pushes blood through the tricuspid valve into the right ventricle. Most filling occurs passively as blood flows from the veins into the relaxed heart, with atrial contraction providing an additional push near the end of filling.
The right ventricle pumps blood to the lungs
The right ventricle receives blood from the right atrium and pumps it into the pulmonary artery through the pulmonary valve. The pulmonary artery divides into branches that carry blood to the lungs.
The right ventricle has a thinner muscular wall than the left ventricle because it pumps blood through the relatively short pulmonary circulation, where pressure is normally much lower than in the systemic circulation. Even so, it must generate enough force to move blood through the lung vessels so that gas exchange can occur.
The left atrium receives blood from the lungs
After blood passes through the lungs, it returns to the heart through the pulmonary veins, which empty into the left atrium. This blood has gained oxygen and released much of its carbon dioxide during its passage through the lung capillaries.
The left atrium transfers blood through the mitral valve into the left ventricle. As on the right side, blood flows into the relaxed ventricle for much of the filling period, and atrial contraction completes the process.
The pulmonary veins are an important exception to a common rule about blood vessels: veins carry blood toward the heart, not necessarily oxygen-poor blood. Pulmonary veins carry oxygen-rich blood, while the pulmonary arteries carry oxygen-poor blood.
The left ventricle pumps blood throughout the body
The left ventricle is the heart’s main pumping chamber for systemic circulation, which supplies the body’s organs and tissues. It ejects blood through the aortic valve into the aorta, the body’s largest artery. From there, branching arteries distribute blood to the brain, muscles, kidneys, digestive organs, and other tissues.
Because systemic circulation extends throughout the body and operates at higher pressure than pulmonary circulation, the left ventricle has a particularly thick muscular wall. Its strong contraction creates the pressure needed to propel blood through the arteries and sustain flow through the smaller vessels that serve individual tissues.
Although the right and left ventricles differ in wall thickness and the pressure they generate, they normally pump the same volume of blood over time. If one side consistently pumps less than the other, blood can accumulate behind the underperforming chamber, potentially contributing to congestion in the lungs or the rest of the body.
The four heart valves and how they work
The heart’s valves act as one-way gates. They open when the pressure behind them becomes greater than the pressure ahead and close when the pressure relationship reverses. Their movements are largely passive: pressure differences drive the opening and closing rather than muscles actively pulling the valve leaflets apart or together.
Four main valves direct blood through the heart. Two are located between the atria and ventricles, and two sit at the exits of the ventricles.
The tricuspid valve
The tricuspid valve lies between the right atrium and right ventricle. It usually has three leaflets, or flexible flaps of tissue, that open as blood flows into the right ventricle.
When the right ventricle contracts, pressure inside it rises and pushes the valve leaflets together. This prevents blood from flowing backward into the right atrium. The leaflets are supported by fibrous cords called chordae tendineae, which attach to small muscular projections inside the ventricle called papillary muscles. These structures help prevent the valve leaflets from bulging backward excessively during ventricular contraction.
The pulmonary valve
The pulmonary valve sits between the right ventricle and the pulmonary artery. It typically consists of three pocket-shaped leaflets, known as semilunar cusps.
When the right ventricle contracts, pressure forces the valve open so blood can enter the pulmonary artery. As the ventricle relaxes and pressure inside it falls, a brief tendency for blood to flow backward fills the pockets of the leaflets, bringing them together and closing the valve.
Unlike the tricuspid and mitral valves, the pulmonary valve does not use chordae tendineae or papillary muscles. Its shape and the pressure of the blood moving through it allow it to open and close.
The mitral valve
The mitral valve, also called the bicuspid valve, separates the left atrium from the left ventricle. It has two leaflets that open to allow oxygen-rich blood to enter the left ventricle.
During ventricular contraction, the mitral valve closes to prevent blood from being forced back into the left atrium. Chordae tendineae and papillary muscles help stabilize the leaflets against the high pressure generated by the left ventricle.
Because the left ventricle works at higher pressure than the right ventricle, the mitral valve must withstand substantial mechanical stress. Damage or dysfunction can allow blood to leak backward or make it difficult for blood to move into the ventricle.
The aortic valve
The aortic valve lies between the left ventricle and the aorta. Like the pulmonary valve, it generally has three semilunar leaflets.
When the left ventricle contracts, pressure opens the aortic valve, allowing blood to enter the aorta. When the ventricle relaxes, pressure in the aorta helps fill the valve’s leaflets and closes the opening, preventing blood from returning to the ventricle.
The aortic valve is essential to maintaining forward blood flow into the systemic circulation. Its closure also helps sustain pressure in the aorta between heartbeats, supporting continued blood movement through the body’s arteries even while the ventricles relax.
How blood flows through the heart
Blood circulation follows a repeating route through the body, lungs, and heart. The right side sends blood to the lungs for gas exchange, and the left side sends it back out to the body. The valves ensure that blood follows this route in the intended direction.
The full pathway is:
- From the body to the right atrium: Oxygen-poor blood returns through the superior and inferior venae cavae and enters the right atrium.
- Into the right ventricle: Blood passes through the tricuspid valve as the ventricle fills.
- From the heart to the lungs: The right ventricle contracts, opening the pulmonary valve and sending blood into the pulmonary arteries.
- Through the lungs: Blood travels through small vessels surrounding the air sacs, or alveoli. Oxygen crosses from inhaled air into the blood, while carbon dioxide moves from the blood into the alveoli to be exhaled.
- Back to the left atrium: Oxygen-rich blood returns through the pulmonary veins.
- Into the left ventricle: Blood passes through the mitral valve as the left ventricle fills.
- From the heart to the body: The left ventricle contracts, opening the aortic valve and pushing blood into the aorta.
- Through the body’s tissues: Arteries branch into smaller vessels and capillaries, where oxygen and nutrients are delivered and waste products enter the blood. Veins then carry the blood back toward the heart.
This circuit repeats continuously. The heart does not manufacture oxygen or directly supply every cell with nutrients; instead, it maintains the flow that allows the lungs, blood vessels, and tissues to exchange gases and substances efficiently.
The pulmonary and systemic circulations are connected in series, meaning blood passing through one must also pass through the other. In a healthy, stable circulation, the amount of blood pumped by the right and left ventricles is approximately equal over time. The blood may spend different amounts of time in different parts of the circulation, but the two sides must maintain a compatible output to prevent fluid from accumulating in either circuit.
The cardiac cycle: how one heartbeat works
A heartbeat involves more than the heart muscle squeezing and relaxing. It is a repeating sequence called the cardiac cycle, which includes chamber filling, coordinated contraction, blood ejection, and relaxation.
During diastole, the ventricles relax and fill with blood. As pressure in the ventricles falls below pressure in the atria, the tricuspid and mitral valves open. Blood flows into the ventricles, initially without requiring the atria to contract. Near the end of filling, the atria contract and move additional blood into the ventricles.
During systole, the ventricles contract. Pressure rises inside them, causing the tricuspid and mitral valves to close. For a brief moment, the ventricular pressure increases while both the inlet and outlet valves are closed. Once ventricular pressure exceeds the pressure in the pulmonary artery or aorta, the pulmonary and aortic valves open and blood is ejected.
As the ventricles relax, their pressure falls. The pulmonary and aortic valves close when pressure in the arteries becomes greater than pressure inside the ventricles. Once ventricular pressure drops below atrial pressure, the tricuspid and mitral valves open again, and the next filling phase begins.
These phases overlap in time, and the sequence is continuous rather than a series of isolated events. At a normal resting heart rate, the entire cycle occurs in less than a second, with the exact duration depending on the rate of the heartbeat. As heart rate rises during exercise, the cycle becomes shorter, especially the filling period.
The familiar heart sounds are closely associated with valve closure. The first heart sound, often described as “lub,” occurs mainly when the mitral and tricuspid valves close at the start of ventricular contraction. The second sound, “dub,” occurs mainly when the aortic and pulmonary valves close near the beginning of ventricular relaxation. Additional sounds can occur in some circumstances and may be normal in certain settings or indicate an underlying problem in others.
How the heart’s electrical system coordinates each beat
The heart contains specialized cells that generate and conduct electrical impulses. These signals coordinate the timing of atrial and ventricular contractions, allowing the chambers to fill and pump in the proper sequence.
The normal electrical impulse begins in the sinoatrial node, or SA node, a small group of specialized cells in the right atrium. Often called the heart’s natural pacemaker, the SA node generates impulses spontaneously. The impulses spread across both atria, causing them to contract.
The signal then reaches the atrioventricular node, or AV node, near the junction between the atria and ventricles. The AV node briefly delays conduction, giving the ventricles time to fill before they contract. The impulse then travels through the bundle of His, its right and left bundle branches, and a network of Purkinje fibers that distributes the signal throughout the ventricular muscle.
This conduction system helps the ventricles contract in a coordinated way, pushing blood toward the pulmonary artery and aorta rather than producing poorly timed or inefficient contractions.
The electrical system is influenced by the autonomic nervous system. Sympathetic activity can increase heart rate and the force of contraction during exercise or stress. Parasympathetic activity, particularly through the vagus nerve, slows the heart rate during rest. Hormones and other physiological signals also affect the heart’s rhythm and pumping strength.
Electrical coordination is distinct from the mechanical work of the valves. Electrical impulses trigger muscle contraction, while pressure changes produced by that contraction move blood and open or close the valves.
How the heart muscle receives its own blood supply
Although blood passes through the heart’s chambers, the heart muscle cannot rely directly on that blood to meet all its oxygen needs. Instead, it receives a dedicated supply through the coronary arteries, which branch from the beginning of the aorta.
The main coronary arteries divide into smaller vessels that deliver oxygen-rich blood to the myocardium. After oxygen and nutrients are used, blood drains through cardiac veins, which return much of it to the right atrium through the coronary sinus.
The heart muscle requires a steady supply of oxygen because it works continuously. During periods of increased demand, such as exercise, coronary blood flow can increase to help meet the muscle’s needs.
If a coronary artery becomes significantly narrowed or blocked, part of the heart muscle may receive too little oxygen. This condition is called myocardial ischemia. It can cause chest discomfort or other symptoms, although some people experience few or no obvious warning signs. If blood flow is interrupted long enough to damage or kill heart muscle cells, a heart attack, or myocardial infarction, occurs.
Coronary circulation is therefore essential to the heart’s own function. Even if the chambers and valves are structurally sound, impaired blood supply can weaken the muscle and reduce its ability to pump effectively.
How blood pressure and blood vessels support circulation
The heart creates pressure by contracting, but blood vessels help determine how that pressure is distributed and how blood moves through the body. Arteries carry blood away from the heart, veins return it, and capillaries allow exchanges between blood and tissues.
Arteries have relatively thick, elastic walls that withstand the pressure generated by ventricular contraction. The aorta and its larger branches stretch as blood enters them and recoil as the heart relaxes. This elastic recoil helps maintain forward flow between beats.
Smaller arteries and arterioles regulate how much blood reaches different tissues. Their muscular walls can narrow or widen, changing resistance to blood flow. This adjustment helps direct circulation according to the body’s needs and contributes to the regulation of blood pressure.
Veins operate at lower pressure and can hold substantial amounts of blood. Valves in many veins, especially in the limbs, help prevent backward flow. Skeletal muscle contractions also assist venous return by squeezing veins and moving blood toward the heart.
Blood pressure reflects the interaction between cardiac output, the volume of blood pumped per minute, and the resistance offered by the systemic blood vessels. Cardiac output depends on both heart rate and the amount of blood ejected with each beat, called stroke volume. A change in either heart rate or stroke volume can alter cardiac output, while changes in blood vessel diameter influence the pressure required to sustain circulation.
These relationships explain why the heart and blood vessels must work together. A strong heartbeat alone cannot ensure adequate tissue circulation if blood vessels are severely constricted, blood volume is too low, or the heart cannot fill properly.
What happens when chambers or valves do not work properly
The heart’s parts depend on one another, so a problem in one area can affect the entire circulatory system. Some disorders interfere with filling, others weaken pumping, and still others disrupt the one-way flow maintained by the valves.
A valve can develop stenosis, meaning its opening becomes narrowed, or regurgitation, meaning it allows blood to leak backward. Stenosis forces the chamber behind the valve to generate greater pressure to move blood through the narrowed opening. Regurgitation causes some blood to travel in the wrong direction, reducing the amount of blood delivered efficiently to the next part of the circulation.
The consequences depend on which valve is affected and how severe the problem is. Mitral valve disease, for example, can interfere with movement from the left atrium into the left ventricle or allow blood to flow back into the atrium during contraction. Aortic valve stenosis can obstruct the left ventricle’s outflow, increasing the workload on the heart muscle.
Chamber disorders can also impair circulation. If the ventricles become weakened, stiff, or excessively thickened, they may be unable to pump or fill normally. Heart failure is a clinical syndrome in which the heart cannot meet the body’s needs adequately without elevated filling pressures. It does not necessarily mean the heart has stopped working; it means its pumping or filling function is impaired enough to cause symptoms or signs such as breathlessness, fatigue, or fluid buildup.
Problems with the electrical system can disturb the timing or rate of contractions. These abnormalities, called arrhythmias, range from relatively harmless variations to conditions that significantly reduce cardiac output or threaten life. Because the heart depends on coordinated electrical activity, an abnormal rhythm can affect circulation even when the chambers and valves are structurally normal.
The heart’s anatomy also helps explain why a defect in the septum can matter. An opening between the right and left sides may allow blood to pass between chambers when it normally would not. The direction and amount of flow depend on the size of the opening and the pressure difference between the chambers. Some defects cause few problems, while others place additional strain on the heart or lungs.
Why the heart’s design is so effective
The heart’s efficiency comes from the integration of its chambers, valves, muscle, electrical conduction system, and blood vessels. The atria receive blood and help complete ventricular filling. The ventricles provide the force needed to move blood through two connected circuits. The valves maintain forward flow, while electrical signals ensure that contraction occurs in the right order.
The separation between pulmonary and systemic circulation allows the right ventricle to pump blood through the lungs at relatively low pressure while the left ventricle generates the higher pressure needed to supply the body. The coronary arteries sustain the muscle that performs this work, and the blood vessels adjust distribution according to changing demands.
Every heartbeat depends on these systems functioning together. When they do, oxygen reaches tissues, carbon dioxide is carried away, and blood pressure supports the delivery of nutrients throughout the body. The heart’s anatomy is therefore more than a collection of chambers and valves: it is a coordinated mechanical and electrical system that keeps the body’s circulation moving continuously.
