The heart is the muscular organ of the circulatory system that constantly
pumps blood throughout the body. Approximately the size of a clenched fist,
the heart is composed of cardiac muscle tissue that is very strong and
able to contract and relax rhythmically throughout a person's lifetime.
The heart has four separate compartments or chambers. The upper
chamber on each side of the heart, which is called an atrium, receives
and collects the blood coming to the heart. The atrium then delivers blood to
the powerful lower chamber, called a ventricle, which pumps blood away
from the heart through powerful, rhythmic contractions.
The human heart is actually two pumps in one. The right side receives
oxygen-poor blood from the various regions of the body and delivers it to
the lungs. In the lungs, oxygen is absorbed in the blood. The left side of
the heartreceives the oxygen-rich blood from the lungs and delivers it to the
rest of the body.
The contraction of the cardiac muscle tissue in the ventricles is called
systole. When the ventricles contract, they force the blood from their
chambers into the arteries leaving the heart. The left ventricle empties into
the aorta and the right ventricle into the pulmonary artery. The increased
pressure due to the contraction of the ventricles is called systolic
The relaxation of the cardiac muscle tissue in the ventricles is called
diastole. When the ventricles relax, they make room to accept the blood from
the atria. The decreased pressure due to the relaxation of the ventricles is
called diastolic pressure.
Electrical Conduction System
The heart is composed primarily of muscle tissue. A network of nerve
fibers coordinates the contraction and relaxation of the cardiac muscle tissue
to obtain an efficient, wave-like pumping action of the heart.
- Sinoatrial node (SA node)
- Atrioventricular node (AV node)
- Common AV Bundle
- Right & Left Bundle Branches
The Sinoatrial Node (often called the SA node or sinus node)
serves as the natural pacemaker for the heart. Nestled in the upper area of
the right atrium, it sends the electrical impulse that triggers each
heartbeat. The impulse spreads through the atria, prompting the cardiac muscle
tissue to contract in a coordinated wave-like manner.
The impulse that originates from the sinoatrial node strikes the
Atrioventricular node (or AV node) which is situated in the lower
portion of the right atrium. The atrioventricular node in turn sends an
impulse through the nerve network to the ventricles, initiating the same
wave-like contraction of the ventricles.
The electrical network serving the ventricles leaves the atrioventricular
node through the Right and Left Bundle Branches. These nerve fibers send
impulses that cause the cardiac muscle tissue to contract.
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