The Heart
Although anatomically one organ, the
heart can be functionally divided into right and left pumps, each consisting of
an atrium and a ventricle. The atria serve as both conduits and priming pumps,
whereas the ventricles act as the major pumping chambers. The right ventricle
receives systemic venous (deoxygenated) blood and pumps it into the pulmonary
circulation, whereas the left ventricle receives pulmonary venous (oxygenated)
blood and pumps it into the systemic circulation. Four valves normally ensure
unidirectional flow through each chamber. The normal pumping action of the
heart is the result of a complex series of electrically driven and mechanical
events. Electrical events precede mechanical ones.
The heart consists of specialized
striated muscle in a connective tissue skeleton. Cardiac muscle can be divided
into atrial, ventricular, and specialized pacemaker and conducting cells. The
self-excitatory nature of cardiac muscle cells and their unique organization
allow the heart to function as a highly efficient pump. Serial low-resistance
connections (intercalated disks) between individual myocardial cells allow the
rapid and orderly spread of depolar-ization in each pumping chamber. Electrical
activity readily spreads from one atrium to another and from one ventricle to
another via specialized conduction pathways. The normal absence of direct
connections between the atria and ventricles except through the
atrioventricular (AV) node delays conduction and enables atrial contraction to
prime the ventricle.
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