The human body

How does the heart work?

The heart provides the body’s organs and tissues with a constant supply of vital oxygen and nutrients. It is the “engine” of the circulatory system.

When you are at rest, your heart beats about 60 to 90 times per minute, pumping roughly 5 to 6 liters of blood through your body. That is around 2.6 million liters per year, which is more or less the amount of water in a 50-meter swimming pool. When you do strenuous physical activities, your heart can pump up to 20 liters of blood around your body per minute. People who do endurance sports can even pump 35 liters of blood per minute when they are exercising hard.

 Illustration: Position of the heart in the body
Your heart is about the same size as your fist and weighs around 300 g (about 0.7 pounds). In people who do endurance sports, it can weigh up to 500 g (about 1.1 pounds). The heart is located more or less in the middle of the chest, slightly to the left, behind the breastbone (sternum). You can normally feel someone’s heart beat if you put your hand on their chest.

The heart is a hollow muscle that is enclosed in a two-layered sac called the pericardium. The outer layer of this sac is made up of fibrous tissue and fat. The inner layer is attached to the heart muscle. There is a tiny fluid-filled space between these two layers. The fluid functions as a lubricant. This is important because it allows the heart to continuously squeeze and relax without the outer layer of the sac moving.

The coronary blood vessels run along the outer surface of the heart muscle, under the inner layer of the pericardium. These blood vessels supply the heart with oxygen and nutrients. 

Illustration: Flow of blood in the heart

A wall (septum) divides the heart into two halves. Each half has two chambers called the atrium and ventricle, as well as its own pump system. The left ventricle pumps oxygen-rich blood out of the heart and into the body (systemic circulation) through an artery called the aorta. The right ventricle supplies the lungs with blood through the pulmonary artery (pulmonary circulation).

When the blood in the pulmonary circulation reaches the lungs, it gets rid of carbon dioxide and picks up oxygen. Once blood has moved around either the systemic or pulmonary circulatory system, it passes through the atria and ventricles of the heart and on into the other circulatory system.

There are heart valves between the right atrium and right ventricle, the left atrium and left ventricle, and where the blood leaves the heart through the arteries. They ensure that the blood flows in the right direction and does not flow back.

Illustration: Heart from the outside
The heart muscle is made up of special cells – the heart muscle cells – that contract rhythmically. The beating of the heart is made possible by an electrical “wiring system” that enables the heart muscle cells to beat at the same pace, in rhythm. Each heartbeat is triggered by an impulse that starts in the sinus node, which is a group of cells in the wall of the right atrium. Special pathways of nerves carry the electrical impulses to the muscle cells in the atria and ventricles, setting the pace of the heartbeat.

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