Heart

The heart is a muscular organ found in humans and other animals. It pumps blood through the blood vessels. The heart and blood vessels together make up the circulatory system. The blood carries oxygen and nutrients to the body's tissues. It also carries metabolic waste, such as carbon dioxide, to the lungs. In humans, the heart is about the size of a closed fist. It sits between the lungs, in the middle part of the chest called the mediastinum.

In humans, the heart has four chambers. There are an upper left and right atria and a lower left and right ventricles. The right atrium and right ventricle are often called the right heart. The left ones are called the left heart. In a healthy heart, blood flows in one direction because of heart valves, which stop backflow. The heart sits inside a protective sac called the pericardium, which also holds a small amount of fluid. The heart wall has three layers: epicardium, myocardium, and endocardium.

The heart beats to a rhythm set by a group of pacemaker cells in the sinoatrial node. These cells create an electric current that makes the heart squeeze. The current travels through the atrioventricular node and along the conduction system of the heart. Blood with less oxygen enters the right atrium from the superior and inferior venae cavae. Then it goes to the right ventricle. From there, it is pumped to the lungs through pulmonary circulation. In the lungs, the blood picks up oxygen and gives off carbon dioxide. The oxygen-rich blood returns to the left atrium and moves through the left ventricle. It is then pumped out through the aorta into the systemic circulation. The blood travels through arteries, arterioles, and capillaries. There, it trades oxygen for carbon dioxide and gives nutrients to the body's cells. Then it returns to the heart through venules and veins. An adult heart beats at a resting rate of about 72 beats per minute.

Cardiovascular diseases are the most common cause of death in the world. Each year, millions of people die from them. Most of these deaths are caused by a heart attack or stroke. Some things can raise the risk of heart disease. These include smoking, being overweight, not getting much exercise, high cholesterol, high blood pressure, and diabetes that is not well controlled. Heart disease often has no symptoms. When it does cause symptoms, these may include chest pain or shortness of breath. Doctors often find heart problems by asking about a patient's medical history. They also listen to the heart-sounds with a stethoscope. They may use an ECG and an echocardiogram, which uses ultrasound. Doctors who specialize in heart diseases are called cardiologists, although many other kinds of medical specialists may also help with treatment.

Structure

Development

The heart is the first working organ to develop. It starts to beat and pump blood at about three weeks into embryogenesis, and this early start is important for later prenatal development. The heart forms from splanchnopleuric mesenchyme in the neural plate, in an area called the cardiogenic region. Two endocardial tubes form there and join to make a simple heart tube, known as the tubular heart. Between the third and fourth weeks, this tube grows longer and folds into an S-shape. This shape lines up the chambers and major vessels. By the end of the fifth week, the walls between the chambers are complete. By the ninth week, the heart valves are complete.

Before the fifth week, the fetal heart has an opening called the foramen ovale. It lets blood pass straight from the right atrium to the left atrium, so some blood skips the lungs. Within seconds after birth, a flap of tissue called the septum primum closes this opening and sets up the usual blood flow pattern. A small dip in the right atrium remains where the opening was. It is called the fossa ovalis. The embryonic heart begins beating at around 22 days after conception. At that point, it beats at a rate close to the mother's, about 75–80 beats per minute (bpm). It speeds up to a peak of 165–185 bpm in the early 7th week. After 9 weeks, when the fetal stage begins, it slows down. At birth, it beats at about 145 (±25) bpm. Heart rates are the same for females and males before birth.

Physiology

Main article: Cardiac physiology

Main article: Cardiac cycle, Systole, and Diastole

The heart works as a pump in the circulatory system. It keeps blood moving around the body. Blood travels through the systemic circulation, which goes to and from the body. It also travels through the pulmonary circulation, which goes to and from the lungs. In the lungs, blood gives up carbon dioxide and takes in oxygen through respiration. The blood then carries oxygen around the body and returns to the heart. The right heart collects blood from the body and pumps it to the lungs through the tricuspid valve. In the lungs, blood flows through tiny capillaries next to alveoli. Carbon dioxide is exchanged for oxygen by diffusion. The left heart then pumps the oxygen-rich blood through the mitral valve to the body. There, it passes through arterioles and capillaries before returning through venules.

The cardiac cycle is the set of steps the heart goes through with each heartbeat. The heart squeezes and then relaxes. The time when the ventricles squeeze and push blood out is called systole. The time when they relax and fill with blood is called diastole. The atria and ventricles take turns. The atria fill with blood while the ventricles push it out. Then the atria push blood into the ventricles.

Main article: Cardiac output

Cardiac output measures how much blood the heart pumps in one minute. To find it, multiply the amount pumped with each beat by the number of beats per minute. The heart has its own pacemaker, the sinoatrial node. It starts the normal rhythm called sinus rhythm. The electrical signal then travels through the heart to the atrioventricular node. From there it moves on to the Purkinje fibers, which make the heart muscle squeeze.

Main article: Electrical conduction system of the heart

The resting heart rate is how fast the heart beats when a person is calm. The sinoatrial node controls it. For adults, it is usually between 60 and 100 beats per minute. A newborn baby's resting heart rate is faster. It slowly slows down as the child grows. Heart rate can go up with exercise and emotions. Certain hormones can also raise it. It can change with dehydration and other body conditions.

Clinical significance

3D echocardiogram showing the mitral valve (right), tricuspid and mitral valves (top left) and aortic valve (top right).The closure of the heart valves causes the heart sounds.

Doctors listen to the heart with a stethoscope. They can often find heart problems by listening for heart murmurs. Cardiovascular diseases, which include heart diseases, are the leading cause of death in the world. Doctors who specialise in the heart are called cardiologists.

Many heart problems are linked to atherosclerosis, which is a build-up of fatty material inside the arteries. If the coronary arteries become narrow, a person may feel chest pain (angina pectoris) or have a heart attack. In heart failure, the heart cannot pump enough blood for the body. Some people have an irregular heartbeat (arrhythmias). Some people are born with heart defects. Doctors check for heart disease by asking about a person's health, examining the heart, and running tests such as blood tests, echocardiograms, and electrocardiograms. Treatments can include changes in lifestyle, medicines, and surgery.

History

People have known about the heart since ancient times, but for a long time no one fully understood what it did or how it was built. Ancient Greek thinkers were the main source of early ideas about the heart. Aristotle thought the heart made blood. Plato saw it as the source of the blood that moves around the body. Hippocrates noticed blood moving from the body through the heart to the lungs. Erasistratos (304–250 BCE) saw the heart as a pump and found the heart valves. He believed blood vessels were filled with air. The Greek doctor Galen (2nd century CE) knew that blood vessels carried blood. He thought the heart gave heat to the body. His ideas were accepted for almost a thousand years. In 1242, Ibn al-Nafis wrote in the Commentary on Anatomy in Avicenna's Canon about the coronary and pulmonary circulation. He said blood passes through the lungs, which was different from what Galen had believed. His work was later translated into Latin by Andrea Alpago. In Europe, Galen's teachings stayed important for a long time. Andreas Vesalius questioned some of them in De humani corporis fabrica (1543). His magnum opus was seen as a challenge to old authority, and he faced criticism. Michael Servetus wrote in Christianismi Restitutio (1553) that blood flows from one side of the heart to the other through the lungs.

A big step forward came with De Motu Cordis (1628) by the English doctor William Harvey. It described how blood moves around the whole body and how the heart pushes it. This led to a major change from Galen's ideas. Later, Otto Frank and Ernest Starling studied the heart. Their similar findings are recognized in the name "Frank–Starling mechanism." Many scientists studied the electrical system that controls the heartbeat. Purkinje fibers and the bundle of His were found in the 19th century. Their role in the electrical conduction system of the heart was not known until Sunao Tawara published his work in 1906. This led to the discovery of the sinoatrial node by Arthur Keith and Martin Flack. These parts became the basis of the electrocardiogram, which was invented by Willem Einthoven.

The first heart transplant in a human was done by James Hardy in 1964. He used a chimpanzee heart, but the patient died within 2 hours. In 1967, South African surgeon Christiaan Barnard carried out the first human-to-human heart transplant at Groote Schuur Hospital in Cape Town. It was an important step in cardiac surgery. At first, many patients did not survive for long. Louis Washkansky died 18 days after receiving a donated heart. American surgeon Norman Shumway, along with Richard Lower, Vladimir Demikhov and Adrian Kantrowitz, is credited with improving transplant methods. By mid-2018, the International Society for Heart and Lung Transplantation registry had recorded more than 146,000 heart transplants around the world. On 7 January 2022, a heart from a genetically modified pig was successfully transplanted into a human for the first time. The surgery took place in Baltimore and was led by heart surgeon Bartley P. Griffith. The patient, David Bennett, lived for 60 days.

Today, heart disease is the leading cause of death in the world. The Framingham Heart Study, which began in 1948, has shown how diet, exercise, and medicines such as aspirin affect the heart. Newer medicines, including ACE inhibitors and beta blockers, have improved care for people with chronic heart failure. It still remains a major health challenge.

Society and culture

For a long time, people believed the heart was the center of the body. They thought it was the place of life, emotion, reason, and the mind. Many religions use the heart as a symbol. The Georgian script letter ღ is often used as a "heart" symbol. The Chinese character for "heart", 心, comes from a seal script drawing of a heart. In Ancient Egyptian religion, the heart was seen as an important part of the soul. It was needed in a ceremony about the afterlife called the Weighing of the Heart. In this ceremony, Anubis and other deities weighed the heart against the feather of Maat. Catholicism has a long tradition of honoring the heart, connected to the Sacred Heart of Jesus. The Aztec people also saw the heart as the center of a person. Today, many cultures use the idea of a broken heart to show grief. The heart symbol is also linked to Valentine's Day, through pictures of Cupid shooting hearts.

Animal hearts are widely eaten as a type of offal. Because they are almost all muscle, they are high in protein. Chicken hearts are often grilled on skewers, such as Indonesian chicken heart satay. The heart is a hard-working muscle, so it is usually cooked slowly to make it tender. Beef heart is valued for its good meat quality and low price. It can be cut into steaks. Pork heart can be stewed, braised, or made into sausage. In some places, snake heart is considered a delicacy.