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EKG
Tutorial: Overview of Cardiac Anatomy
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The
Heart - It keeps going, and going, and going...Believe it or not, the heart's main function is not to decorate Valentine's Day cards. We all know that by means of its four chambers (two atria and two ventricles), the heart is responsible for circulating blood throughout the lungs and the various tissues throughout the body. Without this circulation, nutrients cannot be delivered, waste products cannot be removed, and the upshot of it all is... you die.
You certainly remember that the cardiac contractions are the result of a well orchestrated electrical phenomenon called depolarization. You might even recall that the cell membranes move from their negative resting potential to a more positive threshold which ultimately stimulates them to contract.
In the myocardium there are specialized fibers that are very conductive
and allow the rapid transmission of electrical impulses across the muscle, telling
them to contract. In order to maximize the force of the contraction there is
uniformity in the sequence. That is, the atria contract, then the ventricles
contract. This allows both sets to fill properly before ejecting the blood to
its next destination. These two sections are independent, yet linked to a single
impulse, (in a healthy heart,) initiated by the sinoatrial,
(or sinus) node. The tissue around
the valves helps to channel the impulse from the sinus node through another
collection of specialized tissue, the atrioventricular
node, that is situated between the two sets of chambers.
This area allows slightly slower transmission of the impulse to the ventricles,
allowing the atria to empty into the ventricles before they contract and force
the blood to the lungs or body. This area, the A/V
Node, slows the impulse down to about 1/25th of the original
signal then passes it through to the atrioventricular
bundle, or the bundle
of His. This bundle divides itself into two distinct tracts
through the ventricles, the
bundle branches, and on to the Purkinje
fibers, where the muscle of the ventricle is stimulated
to contract from the bottom up, maximizing the force of ejection.
The SA node ticks away at a rate of 60-80 beats every minute. That's a good thing... Since the SA node is the fastest pacemaker in the heart, all the other cells follow it in synchrony (we call this syncitium). This property of beating on their own is called automaticity. The really nifty part is that all cardiac muscle posesses automaticity.
It's important for all cells to be automatic because when/if the SA node fails to depolarize, the next fastest cells will take over. All of the atrial cells have intrinsic rates of approximately 60/minute. The AV node (conveniently located between the Atria and the Ventricles) instrinsically fires 40-60 times every minute. Even the ventricular cells will eventually depolarize on their own, although each cell has a rate between 20 and 40 times per minute.
The important things to remember here are:
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