The circulatory or cardiovascular system is made up of three main parts - the heart, the blood vessels and the blood that flows through them.

The heart is a hollow muscle located inside the thoracic cavity, behind the sternum. It is protected by the ribs of the chest. An adult heart weighs about 300g and is the size of its owner's fist.

The function of the heart is to pump blood to different parts of the body. The heart is divided into two halves, the right and the left. The septum separates the right-hand and left-hand sides of the heart. Both halves are divided into atria and ventricles. The atria (plural of atrium) are where the blood collects when it enters the heart. As the heart contracts, the blood leaves the ventricles through the arteries and is take returns to the atria via the veins.

The heart does its work tirelessly. This is made possible by the fact that it rests briefly between its beats. This periodicity of contractions and pauses is called the heart rate. The heart rate can be measured from the blood vessels of the wrist or by using a blood pressure monitor.

The blood pumped by the heart circulates into the body via the blood vessels. They include thick-walled arteries. The main artery is called aorta. It also includes thin-walled and narrow capillaries, as well as veins.

The heart functions independently. However, when necessary, the function of the heart can be influenced by both the nervous system and hormones.