The human heart pumps blood by repeatedly contracting and relaxing its muscular walls. Each contraction pushes blood forward through the circulatory system, delivering oxygen and nutrients to tissues and carrying carbon dioxide and other waste products away. Four chambers, four sets of valves, electrical signals, and a coordinated network of blood vessels work together to keep this process running continuously.
The heart does more than move blood. It maintains the pressure needed to circulate blood throughout the body, adjusts its activity to meet changing demands, and ensures that blood traveling to the lungs can pick up oxygen before returning to the tissues.
The heart’s four chambers work as two pumps
The heart is a muscular organ located in the chest, slightly left of center, between the lungs. Its interior is divided into four chambers: two upper chambers called the atria and two lower chambers called the ventricles.
The right atrium receives blood returning from the body. This blood has delivered much of its oxygen to tissues and carries relatively little oxygen compared with blood arriving from the lungs. The right ventricle pumps it to the lungs, where it releases carbon dioxide and takes up oxygen.
The left atrium receives oxygen-rich blood from the lungs. The left ventricle then pumps that blood into the aorta, the body’s largest artery, which distributes it to the rest of the body.
The heart therefore operates as two connected pumps. The right side sends blood through the pulmonary circulation, the pathway between the heart and lungs. The left side sends blood through the systemic circulation, which supplies nearly every other part of the body.
A muscular wall called the septum separates the right and left sides of the heart. This separation keeps oxygen-poor blood from mixing directly with oxygen-rich blood under normal conditions.
Although the two sides pump the same volume of blood over time, the left ventricle must generate substantially higher pressure. Blood traveling through the entire body encounters more resistance than blood moving through the short route to the lungs. The left ventricle has a thicker muscular wall to meet this greater demand.
How blood moves through the heart
Blood follows a specific route through the heart and lungs. Each chamber receives blood from one part of the circulation and passes it to the next.
Blood returning from the body enters the right atrium through two large veins: the superior vena cava, which carries blood from the upper body, and the inferior vena cava, which carries blood from the lower body. The right atrium passes the blood through the tricuspid valve into the right ventricle.
When the right ventricle contracts, blood moves through the pulmonary valve into the pulmonary arteries and travels to the lungs. In tiny blood vessels called capillaries, carbon dioxide moves from the blood into the air spaces of the lungs, while oxygen moves from the air into the blood.
The oxygen-rich blood then returns to the left atrium through the pulmonary veins. From there, it passes through the mitral valve into the left ventricle.
When the left ventricle contracts, blood flows through the aortic valve into the aorta. Branching arteries carry it toward organs and tissues throughout the body. In tissue capillaries, oxygen and nutrients move into surrounding cells, while carbon dioxide and other waste products enter the blood.
The blood then returns through veins to the right atrium, and the cycle begins again.
This circulation is continuous rather than a series of isolated trips. Blood moves through the lungs and body in a connected loop, with the heart providing the pressure that sustains forward flow.
Why the heart needs valves
The heart’s four valves help keep blood moving in the correct direction. They open when pressure behind them becomes greater than pressure ahead of them and close when the pressure difference reverses.
The tricuspid valve lies between the right atrium and right ventricle. The mitral valve lies between the left atrium and left ventricle. These are called atrioventricular valves because they separate the atria from the ventricles.
The pulmonary valve sits between the right ventricle and the pulmonary artery, while the aortic valve sits between the left ventricle and the aorta. These are the semilunar valves.
During ventricular relaxation, the atrioventricular valves open, allowing blood to enter the ventricles. When the ventricles contract, rising pressure closes these valves, preventing blood from flowing backward into the atria. At the same time, pressure opens the pulmonary and aortic valves so blood can leave the heart.
As the ventricles relax, the pressure inside them falls. The pulmonary and aortic valves close, preventing blood in the arteries from flowing back into the ventricles.
The familiar heartbeat sounds are produced mainly by the closing of these valves and the vibrations that follow. The first sound, often described as “lub,” occurs when the mitral and tricuspid valves close. The second sound, “dub,” occurs when the aortic and pulmonary valves close.
Healthy valves do not need to be actively pulled open or shut by muscles. They respond to pressure differences, making forward blood flow efficient and limiting backflow.
How the heart contracts and relaxes
Each heartbeat has two broad phases: systole and diastole. Systole is the contraction phase, when the ventricles eject blood. Diastole is the relaxation phase, when the ventricles fill with blood.
During diastole, the ventricular walls relax, allowing pressure inside the ventricles to fall. The atrioventricular valves open as blood pressure in the atria exceeds that in the ventricles. Most ventricular filling occurs passively, as blood flows down this pressure gradient.
Near the end of diastole, the atria contract and push additional blood into the ventricles. This final contribution to filling can become particularly important when the heart beats rapidly or when the ventricles are less able to fill easily.
During systole, the ventricles contract and pressure inside them rises. At first, all four valves are closed. The ventricular muscles are generating pressure, but the volume of blood inside the chambers has not yet changed substantially. This brief phase is called isovolumetric contraction.
When ventricular pressure exceeds the pressure in the pulmonary artery or aorta, the corresponding outflow valve opens. Blood is then ejected into the arteries.
After ejection, the ventricular muscles relax. The outflow valves close as arterial pressure becomes greater than ventricular pressure. For a short period, all valves are again closed while the ventricles relax and their pressure falls. This is called isovolumetric relaxation.
When ventricular pressure drops below atrial pressure, the atrioventricular valves open and filling begins again.
The atria and ventricles do not contract at exactly the same time. The atria contract first, helping complete ventricular filling, and the ventricles then contract to pump blood into the lungs and body. This sequence allows the heart to fill and empty efficiently with each beat.
How electrical signals control the heartbeat
The heart can generate its own electrical signals. Specialized cells initiate and conduct these signals, coordinating the contractions of the chambers without requiring a separate nerve command for every heartbeat.
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, this structure generates electrical impulses spontaneously.
The impulse spreads across the atria, causing them to contract. It then reaches the atrioventricular node, or AV node, where conduction briefly slows. This delay gives the ventricles additional time to fill before they contract.
The signal then travels through specialized conducting tissue, including the bundle of His, its branches, and the Purkinje fibers. These pathways distribute the impulse through the ventricular muscle, producing a coordinated contraction that pushes blood into the arteries.
Electrical activation triggers contraction through a process called excitation-contraction coupling. When heart muscle cells receive the electrical signal, calcium becomes available inside the cells. Calcium enables the proteins responsible for muscle contraction to interact. As calcium levels fall, the muscle cells relax.
The autonomic nervous system modifies this built-in rhythm. The sympathetic branch can increase heart rate and the strength of contraction, while the parasympathetic branch, especially through the vagus nerve, generally slows the heart. Hormones and other chemical signals also influence its activity.
This combination of an internal electrical system and external regulation allows the heart to beat automatically while adjusting to the body’s changing needs.
How the heart changes its pumping rate
The amount of blood the heart pumps each minute is called cardiac output. It depends on two factors: heart rate, the number of beats per minute, and stroke volume, the amount of blood ejected by a ventricle with each beat.
Cardiac output can be expressed as:
Cardiac output = Heart rate × Stroke volume
For example, if the heart beats 70 times per minute and ejects 70 milliliters of blood per beat, its cardiac output is 4,900 milliliters per minute, or 4.9 liters per minute.
This example illustrates the relationship between heart rate, stroke volume, and total blood flow. Actual values vary with body size, physical activity, fitness, health, and other factors.
During exercise, working muscles need more oxygen and nutrients. The nervous system increases heart rate and often strengthens contractions, allowing the heart to pump more blood each minute. Blood vessels supplying active muscles also widen, helping direct more blood where it is needed.
Stroke volume can increase because the ventricles fill more fully, the heart contracts more forcefully, or both. When more blood returns to the heart, the ventricular muscle generally stretches more before contracting. Within normal physiological limits, this greater stretch produces a stronger contraction, a relationship known as the Frank-Starling mechanism.
At rest, the body needs less blood flow than during vigorous activity. Heart rate and cardiac output can decrease accordingly, although the heart continues beating throughout sleep and other periods of reduced activity.
The heart must also maintain circulation when a person changes posture, experiences stress, or encounters changes in temperature. Its rate and pumping strength adjust in coordination with blood vessels, hormones, and the nervous system.
How blood pressure helps drive circulation
Blood flows because of pressure differences. The heart creates these differences by generating pressure in its chambers, especially the ventricles. Arteries carry blood away from the heart, and veins return it.
When the left ventricle contracts, it raises the pressure in the aorta and other arteries. This pressure drives blood through progressively smaller vessels toward the capillaries. Pressure falls as blood moves through the circulation, allowing it to continue toward lower-pressure regions.
The right ventricle creates a lower pressure to move blood through the lungs. This is important because the delicate lung blood vessels must receive enough blood for gas exchange without being exposed to the high pressures needed in the systemic circulation.
Blood pressure depends not only on how forcefully the heart pumps but also on the resistance and flexibility of blood vessels and the amount of blood circulating in the body. Narrowed small arteries increase resistance, while changes in vessel diameter can alter how easily blood flows through tissues.
Arteries are relatively thick and elastic because they experience the pressure generated by ventricular contractions. Their walls stretch as blood enters and recoil between beats, helping maintain blood flow even when the ventricles are relaxing.
Veins operate at much lower pressures and can hold substantial amounts of blood. Valves in many veins, together with contractions of surrounding skeletal muscles and pressure changes during breathing, help return blood to the heart.
The heart is therefore central to circulation, but it does not work alone. Blood vessels, the blood itself, and the muscles involved in breathing all contribute to effective transport.
How the heart muscle receives oxygen
Although the heart pumps oxygen-rich blood to the rest of the body, its own muscle also requires a continuous oxygen supply. Most of the blood inside the heart’s chambers does not directly nourish the thick muscular walls.
Instead, the heart muscle receives blood through the coronary arteries, which branch from the aorta near its base. These arteries carry oxygen and nutrients to the heart tissue. After delivering them, blood drains through cardiac veins into the coronary sinus and then into the right atrium.
Coronary blood flow is particularly important during relaxation of the ventricles. As the left ventricle contracts, its muscle compresses blood vessels within the heart wall, temporarily restricting some of the flow through them. During diastole, that compression decreases, allowing blood to flow more readily through much of the left ventricular muscle.
If a coronary artery becomes severely narrowed or blocked, part of the heart muscle may receive too little oxygen. This condition is called myocardial ischemia. If the lack of blood flow is prolonged, heart muscle cells can die, causing a heart attack, also known as a myocardial infarction.
Damage to heart muscle can reduce its ability to contract and may also interfere with the electrical signals that coordinate the heartbeat. Maintaining the heart’s own blood supply is therefore essential to maintaining its pumping function.
What happens when the heart’s pumping system is disrupted
Effective pumping requires the chambers to fill adequately, the muscle to contract effectively, the valves to control blood flow, and the electrical system to coordinate each beat. A problem in any of these components can interfere with circulation.
A damaged or narrowed valve may obstruct forward flow or allow blood to leak backward. When a valve does not open fully, the heart must generate more pressure to move blood through it. When a valve leaks, some blood travels in the wrong direction, reducing the amount delivered effectively to the circulation.
Heart rhythm disorders, known as arrhythmias, occur when electrical impulses form or travel abnormally. Some make the heart beat too quickly or too slowly, while others produce an irregular rhythm. Very rapid rhythms can shorten the time available for ventricular filling, and poorly coordinated contractions can reduce the amount of blood pumped with each beat.
Heart failure occurs when the heart cannot pump enough blood to meet the body’s needs or can do so only with abnormally high filling pressures. It may result from weakened contraction, impaired relaxation, damaged valves, long-standing high blood pressure, or other conditions. Despite its name, heart failure does not necessarily mean the heart has stopped beating.
When pumping becomes less effective, blood may not reach tissues adequately, and pressure may rise behind the affected side of the heart. Fluid can accumulate in the lungs or other tissues, causing symptoms such as shortness of breath, swelling, and fatigue.
The heart can compensate for some problems by beating faster, increasing its force of contraction, or changing its structure. These responses may help maintain circulation initially, but persistent strain can eventually worsen heart function.
Why the heart can keep beating throughout life
The heart is built from specialized muscle cells that contract repeatedly while maintaining the electrical and chemical conditions needed for continued activity. Its automatic rhythm, coordinated conduction pathways, responsive valves, and adjustable pumping strength allow it to serve the body across a wide range of conditions.
Each heartbeat moves blood through a connected circuit: from the body to the right side of the heart, through the lungs, back to the left side, and out to the body again. Electrical signals coordinate the sequence, pressure differences direct blood through the chambers and valves, and the coronary circulation supplies the heart muscle itself.
The result is a dynamic system that continually balances oxygen delivery, waste removal, blood pressure, and the changing demands of tissues. The heart’s pumping action may feel simple as a pulse in the wrist or chest, but it depends on the precise cooperation of muscle, electricity, valves, blood vessels, and the lungs.
