The heart
beats when the
cardiac muscles around the outside of the organ
contract. The heart contracts in two phases, first squeasing the
blood that is in the two
atria into the
ventricles, and then squeasing the blood from the ventricles out of the heart, either to the
lungs or to the rest of the body. When the cardiac muscles are
contracting the heart is described as being in
systole, and when it is relaxed it is described as being at
diastole.
The heart muscle is made up of many
cells, bound to each other my "intercolated discs". These intercolated discs spread
electrical signals over the whole
muscle very quickly, so when a message is sent to the muscle, it all acts as once, which is very important when trying to move large volumes of
blood around. When the
atria contract, the "sino-atrial node" sends a signal out, which spreads across all of the
muscle. This signal is stopped by a
non-conductive layer between the muscle surrounding the
atria and the
muscle surrounding the
ventricles, so the whole of the heart doesn't
contract at once. The atrial muscles contract, squeasing blood into the
ventricles of the heart.
The signal from the sino-atrial node
activates another node, the "atrioventricular node". Once the
atrial muscles have
contracted, the atrioventricular node sends another
electrical signal down the "bundle of Hes", a clump of
nerve cells bridging the non-conductive material between the atrial and the ventricular
muscles. The signal travels down this, which half way down splits into two. At the bottom, or "
apex" of the heart, the
electrical signal is released from the
nerve cells out into the muscles, and then spreads throughout the ventricular muscles making the ventricles
contract.
The reason for the
electrical signal travelling to the bottom of the heart before the ventricular muscles contract is two fold:
- To provide a short delay between atrial and ventricular contractions
- To make the ventricles contract from the bottom upwards if there is any delay at all in the signal spreading, ensuring that all the blood is squeased out of the heart
Systole lasts around 0.6 seconds when at rest (this will be shorter when the heart is
beating fast). The heart then goes through
diastole, a period of
resting, which lasts around 0.3 seconds when at rest. During this time all the changes which have occured during
systole are reset, and the heart prepares to
contract again. Although the two sides of the heart pump to different places in the body (the right side
pumps blood to the
lungs for
oxygenation, and the left side
pumps oxygenated
blood to the rest of the body), both sides contract at the same time as it is simply easier to make all the muscle
contract at once.
All this makes the
cardiac muscles surrounding the heart very active, so they use up a lot of
oxygen and
glucose respiring in order to keep up enough
energy. As a result, 5% of the oxygenated
blood pumped out by the heart is fed straight back in to the heart muscles to power them.
An interesting note is that the heart muscles are "Myogenic". This means that even when taken out of the body, the
signals will still be send by the sino-atrial node for the heart to
beat, and, as long as it has sufficient levels of
glucose and
oxygen, the heart will continue to beat. This is what makes
heart transplants possible, as by cooling them the usage of
oxygen and
glucose can be kept to a minimum, and so the heart can be kept beating for up to a day (although it is usually no longer than 4 hours).