The heart

Our heart beats 100,000 times a day, pushing 5,000 gallons of blood through our body every 24 hours. It delivers oxygen- and nutrient-rich blood to our tissues and carries away waste. Here, we explain how this amazing organ carries out this vital work.

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.

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:

Right side

• 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.

Left side

• 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.

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. So, please love your heart now!