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Chapter 13

SCIE 22273 Chapter Notes - Chapter 13: Electrical Conduction System Of The Heart, Purkinje Fibers, Interventricular Septum

Faculty of Applied Health and Community Studies
Course Code
SCIE 22273
Tara Hayes

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Chapter 13- Cardiovascular System
Cardiac conduction system
Throughout the myocardium are specialized cardiac muscle tissue that initiate
and distribute impulses
They form the cardiac conduction system
A key portion called the SA node (sinoatrial node) is located just beneath the
epicardium in the right atrium near the opening of the superior vena cava
SA node activity is rhythmic with 70-80 bpm in an adult
SA is also known as the pacemaker because it generates the heart’s rhythmic
AV node (atrioventricular node) is located in inferior part of septum that separates
The junctional fibers that conduct impulse into AV node have small diameters
because small fibers conduct impulses slowly. This delays transmission
-Therefore, impulse gets delayed as it moves towards AV node. This
allows time for atria to contract completely emptying all the blood
intro ventricles before they contract
Once cardiac impulse reaches distal ends of AV node, it passes into group of
fibers that make up the AV bundle
These branches give rise to Purkinje fibers
AV bundle and Purkinje fibers conduct impulses rapidly
Purkinje fibers spread from the interventricular septum into papillary muscles
Also known as ECG
ECG: recording of the electrical changes in the myocardium during the cardiac
These changes are detectable on the body surface because fluids can conduct
electrical currents
A normal ECG pattern includes waves during each cardiac cycle
When SA node triggers a cardiac impulse, the atrial fibers depolarize producing
an electric charge
P wave corresponds to depolarization of atrial fibers (1st wave)
When the electrical charge ends, the recording leads to baseline called the QRS
complex, which corresponds to depolarization of ventricular fibers and
repolarization of atria
T wave ends the ECG pattern, corresponds to ventricular repolarization
Period between P wave and beginning of QRS complex indicates time for cardiac
impulse to go from SA node to AV node
Ischemia or other problems can prolong this interval
Regulation of the cardiac cycle

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Volume of the blood pumped changed to accommodate cellular requirements
For instance, strenuous exercise makes skeletal muscles need more blood,
making the heart beat faster
The sympathetic and parasympathetic divisions of the heart innerate the SA
node, AV node and myocardium
When the parasympathetic division gets stimulated, acetylcholine gets released,
decreasing SA node activity, therefore the heart beats slower
Baroreceptor reflexes ( detect changes in blood pressure) arise from the cardiac
control center
Cardiac control center maintains balance between inhibitory effects in
parasympathetic and excitatory in sympathetic
If blood pressure is highs, medulla oblongata sends parasympathetic impulses to
heart. Therefore, decreasing heart rate which lowers blood pressure (making it
go toward the normal range)
Impulses from the cerebellum and hypothalamus also influence the cardiac
control center which can increase (anxiety) or decrease (emotional/upset)
2 other factors that can influence it are: temperature and certain ions
High body temperature increases heart action, this is why heart rate increases
during fever
Ions that influence heart action are Ka+ and Ca 2+
Excess of Ka+ : hyperkalemia, decreases rate and flow of contractions
Less of Ka+: hypokalemia, develop arrhythmia
Excess of Ca 2+: hypercalcemia, dangerously extended heart contraction
Less of Ca2+: hypocalcemia, depresses heart action
Blood Vessels
Blood vessels are closed circuit of tubes that carry blood from heart to cells and
Include: arteries, arterioles, capillaries, venules, veins
Arteries and Arterioles
Strong, elastic vessels adapted to carry blood away from the heart under high
blood pressure
Give rise to finer, branched arterioles
Artery wall consists of 3 dinstint layers:
1. Tunica interna called endothelium
-Rests on connective tissue membrane, rich in fiber and collagen
-Prevents blood clotting
-Regulates blood flow that secretes substance that can
dilate/constrict blood vessels
2. Middle layer: tunica media
-Smooth muscle fibers
-Makes up bulk of artery wall
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