The heart rate is controlled by nerves and chemicals in the body. These make sure that the heart beats fast enough to meet the body's need for blood. The nerves and chemicals send the heart messages to increase the heart rate when the body needs to exercise, and to reduce the rate as the body prepares to sleep.
The heart beats by muscles contracting together in an organised way to make sure that blood is pumped around the heart in the appropriate way. Each muscle contracts when a small electrical impulse passes through it.
The electrical impulses are produced by a group of cells (known as the sino atrial node) at the top right corner of the right atrium. The electrical impulse then spreads out over both the right atrium and the left atrium, through pathways known as Bachman's bundles, causing them to contract and force blood down into the ventricles.
A ring of fibres insulate the ventricles from the left atrium and the right atrium to make sure that they contract first.
The electrical impulses then gather at a group of specialised cells, known as the atrioventricular node, before passing down through the centre of the heart in pathways known as the bundles of hiss. They then spread up from the bottom of the heart through pathways know as Purkinje fibres. This makes sure that the heart beats from the bottom up and forces the blood out through the aortic and pulmonary valves at the top of the heart.
The muscle cells then relax back to their resting place, ready to accept the next electrical impulse.
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