The heart is an organ of humans and many animal species. It consists mostly of muscles and is hollow inside. It pumps the blood through the body. This creates a blood circulation that supplies all parts of the body with the necessary oxygen and the important parts from food.

The heart is not the same in all animal species. It may have only one cavity, as in spiders and crabs, or up to four, as in mammals and birds. Such a heart is described here.

When the heart muscles contract and relax again, one says: "The heart beats". You can hear this heartbeat if, for example, you put your ear on someone else's chest. But you can also feel it on certain veins, for example on your wrist. There, the doctor feels the pulse, i.e. the beat of the heart.

In humans, the heart is located in the left half of the chest, where the lowest lung is on the right. It is about as big as the fist of its wearer. The heart of a blue whale weighs as much as a small car. An elephant's heart weighs about as much as a child in primary school.

If a person's heart is sick, it can often be helped by an operation. For example, veins that are too narrow are squeezed open again. Sometimes the doctor replaces a defective heart valve. Sometimes the whole heart is replaced by the heart of another person who has died. However, this is only done in an extreme emergency. Before an operation, they usually try medication.