The heart is a four chambered, muscular organ that
functions as a powerful pump. About the size of a fist, the heart is located in
the chest between the lungs, just to the left of center. The heart continuously
pumps blood through the body’s extensive network of arteries and veins. Arteries
transport blood away from the heart, and veins blood back to the heart. The circulation
of blood delivers oxygen and nutrients to the body while removing waste
products.
Blood Supply to the Heart
The coronary arteries supply the myocardium, the
muscular layer of the heart wall, with oxygen and nutrients. These arteries
originate from the aorta and lie within the epicardium, the outermost layer of
the heart wall. Smaller arterial branches penetrate the myocardium. The cardiac
veins collect venous blood from the heart wall and return it to the right
atrium.
Heart Valves
There are two pair of heart valves that regulate
blood flow within the heart, the atrioventricular valves and the semilunar
valves. The atrioventricular (AV) valves lie at the borders of the atria and
the ventricles and prevent blood from flowing back into the atria during
systole. The two semilunar valves are located at the bases of the pulmonary
trunk and aorta. They consist of three half-moon shape cups and prevent
backflow of blood into the ventricles.
Heart Sound
The sound of one heartbeat is created by the two set
of heart valves closing during the cardiac cycle. These heart sounds can be
heard through a stethoscope and are described phonetically as “lub-dub”. The first
sound, or “lub”, is produced by the closing of the AV valves during systole,
the second sound, or “dub” is made by the closing of the semilunar valves
during the diastole.
Cardiac Cycle
The cardiac cycle refers to the alternating
contraction and relaxation of the heart during one heartbeat. One cardiac cycle
takes about four-fifths of a second to complete and repeats continuously. At rest,
the heart beats an average of 60-80 times per minute. The cardiac cycle
consists of two phases, diastole and systole. In diastole, the ventricles relax
and fill with blood. In systole, the ventricles contract forcing blood into the
arteries.
Electrical Pathways
The steady beating of the heart is regulated by
electrical impulses traveling through the heart. The impulses originate in the
sinoatrial node, also known as the body’s pacemaker. The impulses spread across
the atria, causing then to contract. Next the impulses travel to the atrioventricular
node, pause, then spread through the ventricles along special conduction
pathways called bundle branch and Purkinje fibers. This causes the ventricles
to contract.
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