Not many people don't know how it beats...
When the SA Node fires it initiates a chain of events. Picture one domino, in front of it two dominoes, in front of those 3 dominoes and so on and so forth. The SA Node is pushing over the first 'domino' and the knock on effect causes all the cells to contract in order. The main contraction cells in your heart are all like dominoes lined up next to each other. If you push one then that will star a chain event (Contraction)
The contraction of all the cells in the top part of your heart (Left and Right Atrium) can be seen on an ECG and is known as the P wave.
Picture 2 shows the chain of cell contraction started by the SA Node and Picture 3 is an ECG of a heart beat with the P Wave circled.
This works pretty well for the top of the heart, that order of contraction from the top down to the middle will push the blood into the bottom of the heart (which is what is wanted). However the bottom of the heart we would like to contract from the bottom upwards to push the blood up and out of the heart.
This is demonstrated in Picture 4, it shows only one half of the heart
but is demonstrating the desired blood flow and why we want the blood to
be pushed down from the top into the bottom of the heart (Red arrow)
and then upwards as the blood is pushed out of the heart and towards the
lung or body (Green Arrow).
So the heart uses some clever electrical circuitry to get the bottom of the heart to contract before the rest (knocking the dominoes at the bottom of the heart over first so that the chain reaction moves in the right direction).
Firstly there is a barrier of non conductive tissue that separates the top chambers and the bottom chambers in the heart which is shown in picture 5 by the red line. There is also a bundle of cells that traverses this barrier called the Atrioventricular Node (AV Node) This cluster of cells is part of the chain reaction but passes on the electrical energy very slowly in relation to the other heart cells. Picture Usain Bolt all of a sudden having to run through treacle. This 'Treacle' is shown in the picture by the blue dot.
As Monsieur Bolt is running through treacle there are no heart cells contracting (no dominoes being knocked over) and this lack of anything exciting going on is circled on the ECG in picture 6. This is what is known in cardiology circles as the PR interval. There is good reason why we want a pause between the top and the bottom sections of the heart contracting. That is to allow time for all the blood possible to be squeezed out of the Atrium and pushed into the ventricle.
So perfect! The Ventricles are now really full and ready to forcefully push all the blood required to the lungs and the rest of your body. Time to tell the bottom of the heart to contract first. The electrical energy eventually passes through the sticky AV Node and arrives the other side at something called the Bundle of HIS.
This bundle of cells carries electrical energy incredibly quickly much quicker than the majority of cells in the heart (Usain Bolt just got a lift in a Ferrari with Michael Schumacher) and it runs from the AV Node to the bottom of the heart.It is shown by the quite disgustingly coloured pink lines in Picture 7.
It passes over its electrical energy to the final part of the conduction system the Pirkinje Fibres!
Easier to pronounce than to spell, the Pirkinje fibres also carry the 'its a time to beat' signal very quickly and have a huge surface area, they are abundant in the bottom of the heart. Wherever the purkinje comes into contact with cardiac cells it will start the chain reaction. This causes a rapid, explosive and simultaneous pushing over of the dominoes at the bottom of the heart which creates a very efficient ventricular contraction and maximises the blood ejected up and out around the body by the heart beat.
The pirkinje fibres are shown in orange (Picture 8) but to help you picture a more likely representation of how they are intertwined into the bottom cardiac tissue there is a picture of a skeleton leaf...(9)
They trigger the contraction of the ventricles from the bottom up which is shown in Picture 10. This mass contraction of cells is visible on the ECG as the 'QRS complex' which is circled (Picture 11).
Finally on the ECG is the 'T' Wave (Picture 12) this is all the dominoes in the bottom of the heart standing themselves up again ready to do it all again! Got it? Good!
P.S. I am sorry if my pictures have made you feel a bit nauseous, but I like bright colours.
Time for Dinner