The heart is a muscular organ roughly the size of a closed fist. It sits in the chest, slightly to the left of center.
As the heart contracts, it pumps blood around the body. It carries deoxygenated blood to the lungs where it loads up with oxygen and unloads carbon dioxide, a waste product of metabolism.
The heart, blood, and blood vessels combined are referred to as the circulatory system. An average human has around 5 liters (8 pints) of blood, which is constantly pumped throughout the body.
In this article, we will explain the structure of the heart, how it pumps blood around the body, and the electrical system that controls it.
Basic anatomy of the heart
A diagram of the human heart.
The heart consists of four chambers:
- Atria: the two upper chambers (they receive blood).
- Ventricles: the two lower chambers (they discharge blood).
The left atria and left ventricle are separated from the right atria and right ventricle by a wall of muscle called the septum.
The wall of the heart consists of three layers of tissue:
- Epicardium — protective layer mostly made of connective tissue.
- Myocardium — the muscles of the heart.
- Endocardium — lines the inside of the heart and protects the valves and chambers.
These layers are covered in a thin protective coating called the pericardium.
How the heart works
The heart contracts at different rates depending on many factors. At rest, it might beat around 60 times a minute, but it can increase to 100 beats a minute or more. Exercise, emotions, fever, diseases, and some medications can influence heart rate. For more information on what is "normal," read this article.
The left and right side of the heart work in unison. The right side of the heart receives deoxygenated blood and sends it to the lungs; the left side of the heart receives blood from the lungs and pumps it to the rest of the body.
The atria and ventricles contract and relax in turn, producing a rhythmical heartbeat:
Image credit: DrJanaOfficial, 2016
- The right atrium receives deoxygenated blood from the body through veins called the superior and inferior vena cava (the largest veins in the body).
- The right atrium contracts and blood passes to the right ventricle.
- Once the right ventricle is full, it contracts and pumps the blood through to the lungs via the pulmonary artery, where it picks up oxygen and offloads carbon dioxide.
- Newly oxygenated blood returns to the left atrium via the pulmonary vein.
- The left atrium contracts, pushing blood into the left ventricle.
- Once the left ventricle is full, it contracts and pushes the blood back out to the body via the aorta.
Each heartbeat can be split into two parts:
Diastole: the atria and ventricles relax and fill with blood.
Systole: the atria contract (atrial systole) and push blood into the ventricles; then, as the atria start to relax, the ventricles contract (ventricular systole) and pump blood out of the heart.
When blood is sent through the pulmonary artery to the lungs, it travels through tiny capillaries on the surface of the lung's alveoli (air sacs). Oxygen travels into the capillaries, and carbon dioxide travels from the capillaries into the air sacs, where it is breathed out into the atmosphere.
The muscles of the heart need to receive oxygenated blood, too. They are fed by the coronary arteries on the surface of the heart.
Where blood passes near to the surface of the body, such as at the wrist or neck, it is possible to feel your pulse; this is the rush of blood as it is pumped through the body by the heart. If you would like to take your own pulse, this article explains how.
A diagram of the heart's valves.
Image credit: OpenStax College, Anatomy & Physiology, 2013
The heart has four valves that help ensure that blood only flows in 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 right ventricle.
Most people are familiar with the sound of a human heartbeat. It is often described as a "lub-DUB" sound. The "lub" sound is produced by the tricuspid and mitral valves closing, and the "DUB" sound is caused by the closing of the pulmonary and aortic valves.
The heart's electrical system
To pump blood throughout the body, the muscles of the heart must be coordinated perfectly — squeezing the blood in the right direction, at the right time, at the right pressure. The heart's activity is coordinated by electrical impulses.
The electrical signal begins at the sino-atrial (or sinus, SA) node — the heart's pacemaker, positioned at the top of the right atrium. This signal causes the atria to contract, pushing blood down into the ventricles.
The electrical impulse travels to an area of cells at the bottom of the right atrium called the atrioventricular (AV) node. These cells act as a gate; they slow the signal down so that the atria and ventricles do not contract at the same time — there needs to be a slight delay.
From here, the signal is carried along special fibers called Purkinje fibers within the ventricle walls; they pass the impulse to the heart muscle, causing the ventricles to contract.
There are three types of blood vessels:
Arteries: carry oxygenated blood from the heart to the rest of the body. Arteries are strong and stretchy, which helps push blood through the circulatory system. Their elastic walls help keep blood pressure consistent. Arteries branch into smaller arterioles.
Veins: these carry deoxygenated blood back to the heart and increase in size as they get closer to the heart. Veins have thinner walls than arteries.
Capillaries: they connect the smallest arteries to the smallest veins. They have very thin walls, which allow them to exchange compounds with surrounding tissues, such as carbon dioxide, water, oxygen, waste, and nutrients.
In a nutshell
Although we rarely consider it, the heart is an essential and powerful organ. It pumps oxygen and nutrients around our body constantly, never pausing. Powered by muscles and perfectly synchronized by electrical signals, it's one of nature's finest feats of engineering.