Transcript Physiology of the Cardiovascular System

Physiology
of the
Cardiovascular System
The Conduction System of the
Heart
Modified cardiac muscle that specializes in
contraction
There are four main structures that compose the
conduction system of the heart
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Sinoatrial (SA) node
Atrioventricular (AV) node
AV bundle
Purkinje system
Sinoatrial (SA) Node
Initiates the mechanical contraction of the heart
(known as the pacemaker)
Located in the right atrium just below the
junction of the superior vena cava
Possesses an intrinsic rhythm. This means that
without any stimulation by nerve impulses from
the brain and cord, they themselves initiate
impulses at regular intervals.
Atrioventricular (AV) Node
The impulse generated in the SA node travels
swiftly throughout the muscle fibers of each
atria.
The stimulated atria begin to contract– they will
completely contract before the impulse reaches
the ventricles
The action potential enters the AV node by way
of three internodal bundles of conducting fibers
AV Bundle and Purkinje Fibers
The impulse slowly passes through the AV
node then speeds up as the impulse is
relayed through the AV bundle (bundle of
His) into the ventricles
It is the right and left bundle branches and
the Purkinje fibers that conduct the impule
through the ventricles, causing them to
contract simultaneously.
Conduction System of the Heart
Artificial Pacemakers
Devices that electrically
stimulate the heart
Electrodes sewn directly
into the epicardium or
directly inserted into the
heart chamber
Inferior to the heart’s
natural pacemaker
Electrocardiogram (ECG)
Impulse conduction generates tiny electrical
currents in the heart that spread through
surrounding tissues to the surface of the body.
An electrocardiogram (ECG OR EKG) is a
graphic record of the hearts electrical activity—
the impules that preced the actual contraction
Electrodes are attached to a voltmeter and to the
limbs and chest of the subject.
Theory of
Electrocardiography
A.
B.
C.
D.
E.
At rest (baseline)
Action potential reaches
the first electrode
(relatively negative)
Action potential reaches
the second electrode (pen
returns to baseline)
End of action potential
passes the first electrode
(relatively positive)
Action potential passes
second electrode (returns
to baseline)
ECG Waves
Represent the
dynamic events that
happen during the
contraction of the
heart
Letters are arbitrary
and do not stand for
any words
P Wave
A. Heart wall is
completely
relaxed—no change
in electrical activity
B. P wave represents
the depolarization of
the atria
QRS Complex
C. Atrial walls are
completely
depolarized—no change
recorded
D. QRS Complex occurs
as atrial walls repolarize
and ventricular walls
depolarize (massive
depolarization of
ventricles overshadows
atrial repolarization)
T Wave
E. Atrial walls are
completely
repolarized and
ventricular walls are
completely
depolarized
F. T wave appears as
the ventricular walls
replarize
Back to Baseline
G. Once the ventricles
are completely
repolarized it is back
to the baseline ECG
Bradycardia
Slow heart rhythm—
below 60 beats per
minute
Slight bradycardia is
normal during sleep and
in conditioned athletes
when awake
Can be caused by
damage to the SA node
Tachycardia
Very rapid heart rhythm—
more than 100 beats per
minute
Normal during and after
exercise and during the
stress response
Can result from improper
autonomic control of the
heart, blood loss, shock
and a host of other
factors
Ventricular Fibrillation
Complete disruption
of the normal heart
rhythm
Death may occur
within minutes if the
heart beat is not
corrected by
defibrillation or other
means