A pump with two sides
The heart is a muscular organ roughly the size of its owner’s clenched fist, located in the chest slightly to the left of center, between the lungs. Its job is to keep blood moving. Because it is divided into a right side and a left side, it works as two pumps joined together. The right side sends blood to the lungs to pick up oxygen. The left side sends oxygen-rich blood to the rest of the body.
The wall of the heart is made of cardiac muscle, which contracts rhythmically without conscious effort. Surrounding the heart is a thin sac called the pericardium, which holds a small amount of lubricating fluid.
Four chambers
Each side has two chambers.
- The atria (singular atrium) are the upper chambers. They receive blood coming into the heart.
- The ventricles are the lower chambers. They pump blood out. The left ventricle has the thickest wall, because it must push blood around the whole body, while the right ventricle only needs to reach the nearby lungs.
A wall of muscle called the septum separates the left and right sides, so oxygen-poor and oxygen-rich blood normally do not mix.
Four valves
Valves keep blood flowing in one direction. They open and close in response to pressure changes, and the familiar “lub-dub” of a heartbeat is largely the sound of valves closing.
- The tricuspid valve lies between the right atrium and right ventricle.
- The pulmonary valve lies between the right ventricle and the artery leading to the lungs.
- The mitral valve lies between the left atrium and left ventricle.
- The aortic valve lies between the left ventricle and the aorta, the body’s largest artery.
The path of blood
Following one trip through the system makes the design clear.
- Oxygen-poor blood returns from the body through large veins called the venae cavae and enters the right atrium.
- It passes through the tricuspid valve into the right ventricle.
- The right ventricle pumps it through the pulmonary valve and along the pulmonary arteries to the lungs.
- In the lungs, blood picks up oxygen and releases carbon dioxide.
- Oxygen-rich blood returns through the pulmonary veins to the left atrium.
- It passes through the mitral valve into the left ventricle.
- The left ventricle pumps it through the aortic valve into the aorta, from which arteries branch to the whole body.
Notice that the pulmonary arteries carry oxygen-poor blood and the pulmonary veins carry oxygen-rich blood. Arteries are defined by carrying blood away from the heart, and veins by carrying it toward the heart, not by oxygen content. Blood travels around the body in tiny vessels called capillaries, where oxygen and nutrients pass to tissues and waste products are collected.
Feeding the heart itself
The heart muscle needs its own supply of oxygen, and it cannot take it directly from the blood passing through its chambers. Instead, coronary arteries branch from the base of the aorta and wrap over the heart’s surface. If one of these arteries becomes blocked, part of the muscle may be starved of oxygen, which is the basis of a heart attack. That is a medical emergency, and symptoms such as chest pain should be treated seriously and assessed by professionals.
What keeps the beat
The heart does not need signals from the brain to beat. A small cluster of specialized cells in the right atrium, the sinoatrial node, acts as a natural pacemaker. It generates a small electrical impulse that spreads across the atria, causing them to contract. The impulse is then briefly delayed at another relay point before traveling through special fibers to the ventricles, which contract a moment later. This delay lets the ventricles fill before they squeeze. Electrical activity of this kind is a form of electric current carried by charged particles, and it is what an electrocardiogram records.
The nervous system and hormones adjust the rate. During exercise or fear, the heart speeds up. The stress response, for example, involves hormones such as adrenaline that raise heart rate and blood pressure. A resting adult heart typically beats somewhere around 60 to 100 times a minute, with fitter people often at the lower end, but normal ranges differ between individuals.
Some scale and context
Over a lifetime, the heart beats billions of times, without a rest. At each beat the ventricles eject a moderate amount of blood, and over a day the heart moves thousands of liters. Exact figures depend on the person and activity, so they are best treated as rough.
Heart valve problems, rhythm disturbances and narrowed arteries are among the conditions doctors look for. Tools such as ultrasound, which uses sound waves to build moving pictures of the heart, make it possible to see the chambers and valves in action. Advances like this are part of the wider story of modern medicine.
Important note
This article is general education about how a healthy heart is organized, not medical advice. Heart health depends on many factors, including age, family history and lifestyle, and individuals who are worried about symptoms or risk should speak with a qualified health professional.