Biomedical Engineering Reference
In-Depth Information
Aorta
Left
Atrium
Sinoatrial
(SA) Node
Left Bundle
Branch
Right
Atrium
Left
Ventricle
Right
Ventricle
Purkinje Fiber
Right Bundle
Branch
Atrioventricular
(AV)
Node
Figure 8.1 The human heart has four chambers, two on the left side and two on the right side. Each side is further divided into a receiv-
ing chamber (atrium) and a pumping chamber (ventricle). One-way valves separate the atria from the ventricles. The right side of the heart
pumps blood to the lungs (via the pulmonary artery), and the left side pumps blood to the rest of the organs (via the aorta).
node acts as a natural pacemaker, setting the rate at which the heart beats. At rest, the sinus
node
fires 60 to 80 times per minute. When metabolic demand increases, for example due
to physical activity or emotional stress, the sinus node accelerates to make the heart beat
faster and increase cardiac output.
The depolarization of the sinus node creates a depolarization wave that spreads rapidly
across the right atrium and the left atrium, causing them to contract. The atria are insulated
electrically from the ventricles by the atrioventricular (AV) groove that runs around the
outside of the heart. Electrical activity from the atria is transferred to the ventricles via a
second electrical structure of the heart called the atrioventricular node or AV node , located
deep in the center of the heart. The AV node is connected to the bundle of His, a bundle of
specialized cells in the heart that can rapidly conduct the electrical signal to the muscle
cells of the ventricles. The bundle of His branches downstream into a right bundle branch
(to the right ventricle) and a left bundle branch (to the left ventricle). The
fi
fibers eventually
branch out to the distant ventricular tissues and are at that point referred to as Purkinje
fi
fi
bers .
When the complete conduction system works properly, the atria contract about 200 to
300 ms ahead of ventricular contraction. This allows extra
filling of the ventricles before
they pump the blood through the lungs and peripheral circulation. Another important func-
tion of this system is that it allows all portions of the ventricles to contract almost simul-
taneously, which is essential for e
fi
ective pressure generation in the ventricular chambers.
As such, the heart must maintain a more or less steady rhythm in order to pump properly.
Excessively slow heart rhythms make the cardiac output insu
ff
cient, causing symptoms of
fatigue, weakness, lightheadedness, and loss of consciousness. On the other hand, if the
heart beats too quickly, it does not get enough time in between beats to
fi
fill up with blood.
 
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