Download Chapter 18, part 2 - Trimble County Schools

Differences in Contraction Mechanisms

Heart has autorhythmicity (approx. 1%)

Heart contracts as a unit or not

Absolute refractory period is longer in
cardiac cells

Action potential lasts longer providing
sustained contraction
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Similarities in contracting mechanisms
depolarization – Na+ moves into cell

1.

2. propagation of depolarization via T
tubules

3. Ca 2+ released from sarcoplasm

4. cross bridges form
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Energy requirements

Heart has more mitochondria

Heart relies on aerobic respiration

Heart readily switches metabolic pathways to use
whatever nutrient supply is available

Biggest danger to heart is lack of oxygen

Lack of blood (ischemia)-can ultimately lead to
heart attack
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The autorhythmic cells (the 1%)

Responsible for the rhythmic contractions of the
heart

Contain special ion channels that allow for Ca2+
influx which produces the action potential and not
the Na+

Cells found in Sinoatrial Node (right atrial wall
just below entry of superior vena cava.) generates
impulses 75 times per minute – called the
pacemaker
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Autorhythmic cells (the 1%)

AV node –located in interatrial septum above the
tricuspid valve

Impulses move from AV node to AV bundle which
conducts the electrical connection between atria
and ventricle

Impulse moves to Purkinje fibers which conduct
impulses to the rest of the bulk of the heart
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Electrocardiography page 696-697)
 Electrical activity is recorded by electrocardiogram
(ECG)

P wave corresponds to depolarization of SA node

QRS complex corresponds to ventricular
depolarization

T wave corresponds to ventricular repolarization

Atrial repolarization record is masked by the larger
QRS complex
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Electrocardiography
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Figure 18.16
Heart Sounds
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Figure 18.19
Heart Sounds

Heart sounds (lub-dup) are associated with closing
of heart valves

First sound occurs as AV valves close and signifies
beginning of systole

Second sound occurs when SL valves close at the
beginning of ventricular diastole
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Cardiac Cycle

Cardiac cycle refers to all events associated with
blood flow through the heart

Systole – contraction of heart muscle

Diastole – relaxation of heart muscle
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Cardiac Output (CO) and Reserve

CO is the amount of blood pumped by each
ventricle in one minute

CO is the product of heart rate (HR) and stroke
volume (SV)

HR is the number of heart beats per minute

SV is the amount of blood pumped out by a
ventricle with each beat

Cardiac reserve is the difference between resting
and maximal CO
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Cardiac Output: Example

CO (ml/min) = HR (75 beats/min) x SV (70
ml/beat)

CO = 5250 ml/min (5.25 L/min)
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Extrinsic Factors Influencing Stroke Volume

Agents/factors that decrease
contractility include:

Acidosis

Increased extracellular K+

Calcium channel blockers
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Regulation of Heart Rate: Autonomic Nervous
System

Sympathetic nervous system (SNS)
stimulation is activated by stress, anxiety,
excitement, or exercise

Parasympathetic nervous system (PNS)
stimulation is mediated by acetylcholine
and opposes the SNS
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Chemical Regulation of the Heart

The hormones epinephrine and thyroxine
increase heart rate

Intra- and extracellular ion concentrations
must be maintained for normal heart
function – specifically Ca2+ and K+
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Congestive Heart Failure (CHF)

Congestive heart failure – when the CO is so low
that blood circulation is inadequate to meet tissue
needs



Coronary atherosclerosis – fatty buildup in
coronary vessels
Persistent high blood pressure – above 90 mm Hg
in the aortic diastole phase
Multiple myocardial infarcts – dead cells replaced
by scar tissue
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Homeostatic Imbalance


Tachycardia – abnormally fast heart rate (110
beats/min.)
Bradycardia – abnormally slow heart rate (60
beats/min.)
~low body temperature
~certain drugs
~endurance exercise
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Examples of Congenital Heart Defects
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Figure 18.25
Age-Related Changes Affecting the Heart

Sclerosis and thickening of valve flaps

Decline in cardiac reserve

Fibrosis of cardiac muscle

Atherosclerosis
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