The human heart is a muscular pump that keeps blood moving through the body. With every heartbeat, it sends blood to the lungs to pick up oxygen and then pushes oxygen-rich blood to the brain, muscles, organs, and other tissues. At the same time, blood returning from the body carries carbon dioxide and other waste products away.
The heart does this continuously because its muscle can contract on its own, while an electrical signaling system coordinates when different parts of the heart contract and relax. Valves make sure blood moves in the correct direction, and the coronary arteries supply the heart muscle itself with oxygen and nutrients.
Understanding how those systems work together explains what a heartbeat actually is and how the heart supports circulation.
The heart has four chambers
The heart sits in the chest between the lungs, slightly left of the body’s midline. It is divided into four chambers: two upper chambers called atria and two lower chambers called ventricles.
The right and left sides perform different parts of the circulation:
- The right atrium receives oxygen-poor blood returning from the body.
- The right ventricle pumps that blood to the lungs.
- The left atrium receives oxygen-rich blood returning from the lungs.
- The left ventricle pumps oxygen-rich blood throughout the body.
A muscular wall called the septum separates the right and left sides. This separation prevents oxygen-poor and oxygen-rich blood from normally mixing inside the heart.
The left ventricle has the thickest muscular wall because it must generate enough pressure to push blood through the entire systemic circulation—the network supplying the rest of the body. The right ventricle only has to pump blood through the lungs, a much shorter and lower-pressure circuit.
Blood travels through two connected circuits
The heart works as two pumps operating in sequence.
The first circuit carries blood between the heart and lungs. The second carries blood between the heart and the rest of the body.
Blood returning from the body enters the right atrium through two large veins: the superior vena cava, which drains the upper body, and the inferior vena cava, which drains the lower body. Blood then passes into the right ventricle and is pumped through the pulmonary arteries to the lungs.
In the lungs, blood releases carbon dioxide and takes up oxygen. It then returns to the left atrium through the pulmonary veins.
From there, it enters the left ventricle and is pumped through the aorta, the body’s largest artery. The aorta branches into progressively smaller arteries that deliver blood throughout the body.
An important detail is that arteries and veins are defined by the direction of blood flow relative to the heart, not by whether the blood contains oxygen. Arteries carry blood away from the heart, while veins carry it toward the heart. That is why the pulmonary arteries carry oxygen-poor blood and the pulmonary veins carry oxygen-rich blood.
What happens during one heartbeat
A heartbeat is a coordinated sequence of filling and pumping rather than a single simple squeeze.
When the heart muscle is relaxed, the chambers fill with blood. The atria then contract and help move additional blood into the ventricles. The ventricles subsequently contract, pushing blood out toward the lungs and body. Afterward, the ventricles relax and begin filling again.
This sequence is often described as the cardiac cycle.
During ventricular contraction, called systole, the ventricles generate pressure and eject blood. During ventricular relaxation, called diastole, the ventricles fill with blood.
The familiar two-part heart sound, often described as “lub-dub,” is produced primarily by the closing of the heart valves. The first sound occurs when the valves between the atria and ventricles close as the ventricles begin to contract. The second occurs when the valves leading out of the ventricles close as ventricular contraction ends.
Heart valves keep blood moving forward
Four valves act as one-way gates inside the heart.
The tricuspid valve lies between the right atrium and right ventricle, while the mitral valve lies between the left atrium and left ventricle. These are the atrioventricular valves.
The pulmonary valve sits between the right ventricle and pulmonary artery, and the aortic valve sits between the left ventricle and aorta. These are the semilunar valves.
The valves do not actively pump blood. Instead, they open and close in response to differences in pressure on either side of them. When pressure pushes in the appropriate direction, a valve opens; when pressure reverses, it closes.
This arrangement keeps blood from flowing backward as the heart contracts and relaxes.
The heartbeat is controlled by electrical signals
Heart muscle has a distinctive property: specialized cardiac cells can generate electrical activity without requiring every heartbeat to be initiated by the brain.
The normal rhythm begins in the sinoatrial (SA) node, a group of specialized cells in the right atrium. The SA node generates an electrical impulse that spreads through the atria, causing them to contract.
The signal then reaches the atrioventricular (AV) node. The AV node briefly delays the signal before passing it into specialized conducting tissue that carries the impulse through the ventricles.
This delay is important because it gives the ventricles time to fill after the atria contract.
The electrical signal then travels through the bundle of His, bundle branches, and Purkinje fibers, causing the ventricular muscle to contract in a coordinated way.
The nervous system does influence the heart. Nerves and hormones can increase or decrease the heart rate and affect how strongly the heart contracts. But the basic rhythm-generating and conducting system is built into the heart itself.
The heart muscle needs its own blood supply
The heart continuously uses energy to contract, so its own muscle requires a steady supply of oxygen and nutrients.
Blood reaches the heart muscle through the coronary arteries, which branch from the beginning of the aorta. Smaller vessels distribute blood through the heart’s muscle, and coronary veins return blood toward the right side of the heart.
This creates an important distinction: blood inside the heart chambers is not simply supplying the heart muscle directly. The myocardium—the muscular tissue of the heart—has its own coronary circulation.
If a coronary artery becomes severely blocked, part of the heart muscle may be deprived of oxygen. Prolonged loss of blood flow can damage or kill heart muscle, producing a heart attack, also called a myocardial infarction.
How the heart adjusts to the body’s demands
The heart does not pump at exactly the same rate or with exactly the same force all the time. Its output changes according to what the body needs.
During exercise, working muscles require more oxygen and nutrients and produce more carbon dioxide and metabolic waste. The heart responds by increasing its rate and, under many circumstances, the amount of blood it pumps with each beat.
Two related measurements help describe this.
Heart rate is the number of heartbeats per minute. Stroke volume is the amount of blood pumped by a ventricle with each contraction. Together they determine cardiac output, the amount of blood the heart pumps per minute:
Cardiac output = heart rate × stroke volume
The heart can increase cardiac output by beating faster, pumping more blood with each beat, or both. The nervous system and circulating hormones help regulate these changes, while the amount of blood returning to the heart also affects how much the ventricles fill and subsequently pump.
Blood pressure is produced by the heart and blood vessels together
The heart creates pressure by contracting, but blood pressure is not determined by the heart alone. The arteries and smaller blood vessels strongly influence how blood flows through the circulation.
When the ventricles contract, pressure rises in the arteries. This is systolic blood pressure. When the ventricles relax, arterial pressure falls but does not normally drop to zero; the elastic properties and resistance of the blood vessels help maintain flow between heartbeats. This lower pressure is diastolic blood pressure.
Blood pressure therefore reflects the interaction between the heart’s pumping activity and the resistance and elasticity of the vascular system.
Why the heart can beat for a lifetime
Cardiac muscle is specialized for repeated, coordinated contractions. Individual heart muscle cells are electrically connected so that signals can spread efficiently through the tissue. The cells also contain abundant structures for producing the energy needed for continual contraction.
The heart alternates contraction with relaxation rather than remaining continuously contracted. That relaxation period allows the chambers to refill and the heart muscle to receive its own blood supply.
The heart’s remarkable endurance comes not from working without rest, but from maintaining a precisely regulated cycle of electrical activation, contraction, relaxation, filling, and coronary blood flow.
What can go wrong with the heart’s normal operation
Because the heart depends on several systems working together, cardiovascular disease can affect different parts of the process.
A coronary artery disease can restrict blood flow to the heart muscle. A damaged or narrowed valve can interfere with normal forward blood flow. An abnormal electrical rhythm, or arrhythmia, can make the heart beat too quickly, too slowly, or irregularly. Conditions that weaken the heart muscle can reduce its ability to pump effectively.
Problems can also develop in the blood vessels that the heart supplies. High blood pressure, for example, increases the workload the heart must overcome when pumping blood through the circulation.
These conditions differ substantially in their causes and effects, but they all illustrate the same underlying principle: effective circulation requires the heart muscle, electrical system, valves, and blood vessels to function as a coordinated whole.
The heart’s essential job
At its core, the heart maintains a continuous circuit of blood flow. The right side sends oxygen-poor blood to the lungs. The left side sends oxygen-rich blood to the body. Four valves keep the flow moving in one direction, while an internal electrical system coordinates the timing of contraction.
Every heartbeat is therefore the result of several processes occurring in a tightly organized sequence: electrical activation, muscle contraction, pressure changes, valve movement, blood ejection, and relaxation. Together, these mechanisms allow the heart to deliver oxygen and nutrients to tissues while helping carry away carbon dioxide and other waste products.
