Jonathon Armitage, of Kings High School, asks :-
Why does my heart keep beating?
Mark Richards, the National Heart Foundation Professor of Cardiovascular Studies at Otago University's Christchurch School of Medicine, responded.
The function of the heart is to pump oxygenated blood to all the tissues of the body.
It receives de-oxygenated or "used" blood from the great veins which enter the right sided heart chambers and are then pumped through the circulation to the lungs where they receive oxygen and discard carbon dioxide, following which they enter the left sided chambers of the heart (left atrium and then left ventricle) to then be pumped forward into the arterial circulation to all parts of the body. The heart therefore is a pump with muscles which provide motive power, valves which dictate the appropriate direction of blood flow, and an electrical system which provides a pacemaker instructing the heart to beat at the appropriate rate.
The cells of the pacemaker automatically discharge a regular electrical impulse which triggers the heart to beat. The rate at which the pacemaker fires is governed by nerve signals from the brain and is also conditioned by the metabolic demand from the body. This allows the heart to beat slowly when requirements are low such as during sleep, and to speed up to a high rate when demand is high, such as during exercise or situations with high levels of stress. The pacemaker in the upper right chamber of the heart (right atrium) sends its message through the upper chambers to a relay station called the atrioventricular node and then into specialised electrical conducting cells into the major pumping chambers below.
Diseases such as coronary disease can damage or destroy the function of any part of the heart's wiring system and this can lead to extremely slow heart rhythms (heart block) which may require implantation of an artificial pacemaker.