Key takeaways

  • The heart has four chambers that contract and relax in a specific rhythm to oxygenate blood and pump it throughout the body.
  • The heart’s electrical system coordinates muscle contractions to maintain a typical resting heart rate of 60 to 100 beats per minute. A person’s heart rate will be faster during physical activity.
  • If someone’s heart stops beating, they can die within minutes. Anyone close by should call 911 and perform CPR, which can greatly increase the chances of survival.

The heart is a muscular organ about the size of a closed fist, which sits in the chest, slightly to the left of center. It beats around 100,000 times per day to deliver oxygen- and nutrient-rich blood to tissues and organs and carry away waste.

The heart sends deoxygenated blood to the lungs, where the blood loads up with oxygen and unloads carbon dioxide, a waste product of metabolism.

Together, the heart, the blood, and the blood vessels (arteries, capillaries, and veins) make up the circulatory system.

In this article, we explore the structure of the heart, how it pumps blood around the body, and the electrical system that controls it.

The heart consists of four chambers:

  • The atria: the two upper chambers, which receive blood
  • The ventricles: the two lower chambers, which discharge blood

A wall of tissue called the septum separates the left and right sides of the heart. Valves separate the atria from the ventricles.

The heart’s walls consist of three layers of tissue:

  • Endocardium: This tissue lines the inside of the heart and protects the valves and chambers.
  • Myocardium: This is the muscular tissue of the heart.
  • Epicardium: This is an outer layer consisting mostly of connective tissue. It is part of the pericardium, a thin, protective sac that surrounds the heart.

The rate at which the heart contracts depends on many factors, including:

  • a person’s activity level
  • emotional factors
  • some medical conditions
  • whether a person has a fever
  • some medications
  • whether a person is adequately hydrated

When a person is at rest, the heart typically beats 60 to 100 times each minute. But during exercise, the heart rate may increase to 130 to 150 beats per minute (bpm) or more, depending on the intensity of the activity.

Left and right sides

The left and right sides of the heart work in unison. The atria and ventricles contract and relax in turn, producing a rhythmic heartbeat.

Right side

The right side of the heart receives deoxygenated blood and sends it to the lungs in the following sequence:

  1. The right atrium receives deoxygenated blood from the body through veins called the superior and inferior vena cava. These are the largest veins in the body.
  2. The right atrium contracts, and blood passes into the right ventricle.
  3. Once the right ventricle is full, it contracts and pumps the blood to the lungs via the pulmonary artery.
  4. In the lungs, the blood picks up oxygen and offloads carbon dioxide.

Left side

The left side of the heart receives blood from the lungs and pumps it to the rest of the body:

  1. Newly oxygenated blood returns to the left atrium via the pulmonary veins.
  2. The left atrium contracts, pushing the blood into the left ventricle.
  3. Once the left ventricle is full, it contracts and pushes the blood back out to the body via the aorta.

Diastole, systole, and blood pressure

Each beat of the heart has two parts:

  • Diastole: The ventricles relax and fill with blood as the atria contract, emptying all blood into the ventricles.
  • Systole: The ventricles contract and pump blood out of the heart as the atria relax, filling with blood again.

When a healthcare professional takes someone’s blood pressure, they will record it as a high and a low number.

The high number is the systolic blood pressure, which shows how much pressure the blood creates against the artery walls during systole.

The lower number is the diastolic blood pressure, which shows how much pressure is in the arteries during diastole.

Gas exchange

When blood travels through the pulmonary artery to the lungs, it passes through tiny capillaries that connect on the surface of the alveoli (the lungs’ air sacs).

The body’s cells need oxygen to function, and they produce carbon dioxide as a waste product. The heart enables the body to eliminate the unwanted carbon dioxide.

Oxygen enters the blood and carbon dioxide leaves the blood through the capillaries of the alveoli.

The coronary arteries on the surface of the heart supply oxygenated blood to the heart muscle.

Pulse

A person can feel their pulse at points where arteries pass close to the skin’s surface, such as on the wrist or neck. The pulse is the same as the heart rate. When a person feels their pulse, they are feeling the rush of blood as the heart pumps it throughout the body.

A typical pulse is usually 60 to 100 bpm, but what is normal can vary from person to person.

A very active person may have a resting pulse as low as 40 bpm. People with larger bodies tend to have a faster pulse, but it is not usually over 100 bpm.

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The heart has four valves, which ensure that blood flows in only one direction:

  • Aortic valve: between the left ventricle and the aorta
  • Mitral valve: between the left atrium and the left ventricle
  • Pulmonary valve: between the right ventricle and the pulmonary artery
  • Tricuspid valve: between the right atrium and the right ventricle

Most people are familiar with the sound of the heart. But the heart actually makes many types of sounds, and doctors can distinguish these to monitor heart health.

The opening and closing of the valves are key contributors to the sound of the heartbeat. Leakage or blockage of the heart valves can create sounds called murmurs.

To pump blood throughout the body, the heart’s muscles must work together to squeeze the blood in the right direction, at the right time, and with the right force. Electrical impulses coordinate this activity.

The electrical signal begins at the sino-atrial node (sometimes called the SA node or sinus node). This is the heart’s pacemaker, and it sits at the top of the right atrium. The signal causes the atria to contract, pushing blood down into the ventricles.

The electrical impulse then travels to an area of cells at the bottom of the right atrium, between the atria and ventricles, called the atrioventricular, or AV, node.

These cells act as a gatekeeper. They coordinate the signal so that the atria and ventricles do not contract at the same time. There needs to be a slight delay.

From there, the signal travels along the Purkinje fibers, which are found within the ventricle walls. The fibers pass the impulse to the heart muscle, causing the ventricles to contract.

There are three types of blood vessels:

  • Arteries: These carry oxygenated blood from the heart to the rest of the body. They are strong, muscular, and stretchy to help push blood through the circulatory system, and they help regulate blood pressure. They branch into smaller vessels called arterioles.
  • Veins: These carry deoxygenated blood back to the heart. Veins increase in size as they get closer to the heart, and they have thinner walls than arteries.
  • Capillaries: These connect the smallest arteries to the smallest veins. They have very thin walls, which allow them to exchange substances such as carbon dioxide, water, oxygen, waste, and nutrients with surrounding tissues.

The heart, blood, and blood vessels make up the circulatory, or cardiovascular, system.

The heart is essential to life — if it stops beating, blood will not reach the brain and other organs, and the person can die within minutes. This is called cardiac arrest.

If a person experiences cardiac arrest, they will be unable to speak or breathe, and they will have no heartbeat.

Anyone nearby should call 911 immediately and start cardiopulmonary resuscitation (CPR), pressing hard and fast with locked hands on the center of the person’s chest.

According to the American Heart Association, CPR can double a person’s chances of survival after their heart stops.

The heart is a powerful, essential organ that constantly pumps oxygen and nutrients around the body.

If a person is born with congenital heart disease or sustains heart damage due to illness or other factors, the heart’s function may diminish, potentially leading to life threatening complications such as heart failure.

If the heart stops, a person cannot survive for long. Staying active and maintaining a nutritious diet are two ways to protect the heart.