the human body in health and illness, 4th edition barbara herlihy chapter 17: function of the heart
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The Human Body in Health and
Illness,
4th edition
Barbara Herlihy
Chapter 17:
Function of the Heart
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Lesson 17-1 Objectives
• Define cardiac cycle with respect to systole and diastole.
• Describe the autonomic innervation of the heart.
• Define cardiac output and explain how changes in heart rate and/or stroke volume change cardiac output.
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Lesson 17-1 Objectives (cont’d.)
• Describe the effect of Starling’s law of the heart on myocardial contraction.
• Define preload and explain how it affects cardiac output.
• Define afterload and identify the major factors that determine afterload.
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Lesson 17-1 Objectives (cont’d.)
• Describe the inotropic effect on myocardial contraction.
• Define the special clinical vocabulary used to describe cardiac function.
• Define heart failure and differentiate between right-sided and left-sided heart failure.
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Cardiac Cycle: One Heartbeat
• Systole (contraction) and diastole (relaxation) act in coordination.
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Autonomic Control of the Heart
• Allows heart to respond to changing body needs
• Involves sympathetic and parasympathetic (vagal) nerves
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Autonomic Wiring
• Sympathetics– SA node– AV node– Ventricular
myocardium
• Parasympathetics– SA node– AV node
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Autonomic Neurotransmitters
• Sympathetics– Adrenergics– Norepinephrine (NE)
• Parasympathetics– Cholinergic– Acetylcholine (ACh)
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Firing of the Sympathetic System
↑SA node activity and heart rate
↑Speed of cardiac impulse through conduction system
↑Force of myocardial contraction
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Firing of the Parasympathetic System
↓SA node activity and heart rate
↓Speed of cardiac impulse from SA to AV node
• No effect on strength of contraction
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Cardiac Output (CO)
• CO is amount of blood pumped by each ventricle in 1 minute (mL/min).
• CO = heart rate × stroke volume• CO in the healthy heart can increase four to
five times; called the cardiac reserve
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Heart Rate and Stroke Volume
• Heart rate: Number of times the heart beats each minute caused by SA node’s firing
• Stroke volume: Amount of blood pumped by the ventricle per beat
• CO can be altered by changing heart rate and/or stroke volume
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How to Change Stroke Volume • Starling’s law of the heart
– Changes the force of contraction by stretching the myocardium
– Mechanism: Aligns the sarcomeres for greater interaction between actin and myosin
• Inotropic effect – Changes the force of contraction without
stretching the myocardium– Mechanism: Makes calcium more available to the
contractile proteinsCopyright © 2011, 2007 by Saunders,
an imprint of Elsevier Inc. All rights reserved.
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How to Change Stroke VolumeStarling’s law of the heart: What goes in, comes out
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Stroke Volume: Ejection Fraction
• Ejection fraction is the percentage of blood volume in the ventricle that is pumped by the heart.
• Normal ejection fraction is about 67%.• Exercise can increase ejection fraction.• Heart failure decreases ejection fraction.
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Heart Talk: Clinical Terms
• Preload: EDV– Amount of blood in
ventricle at the end of diastole (EDV)
– Basis of Starling’s law of the heart
• Afterload: Resistance– Caused by blood
pressure Copyright © 2011, 2007 by Saunders,
an imprint of Elsevier Inc. All rights reserved.
16
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Heart Talk: Clinical Terms (cont’d.)
• Inotropic effect: Change in myocardial contraction not caused by stretch
• Chronotropic effect: Change in heart rate• Dromotropic effect: Change in the speed at
which the cardiac impulse travels from the SA node through the AV node and His-Purkinje system
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Heart Talk: Beta1-Adrenergic Receptors
• Locations SA node AV node Myocardium
• Activated by NE
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Effects of Beta1-Adrenergic Receptors
Effect of Activating(+) Chronotropic(+) Dromotropic (+) Inotropic
Effect of Blocking() Chronotropic() Dromotropic () Inotropic
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Heart Talk: Muscarinic Receptors
• Locations SA node AV node
• Activated by ACh
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Effects of Muscarinic Receptors
Effects of Activating() Chronotropic () Dromotropic
Effects of Blocking(+) Chronotropic (+) Dromotropic
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Heart Failure: Left-Sided• Backward
– Poor left ventricular function
– Fluid backs up into lungs
• Forward– Poor left ventricular
function– Decreases blood flow
to systemic circulationCopyright © 2011, 2007 by Saunders,
an imprint of Elsevier Inc. All rights reserved.
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Heart Failure: Right-Sided
• Backward– Blood backs up into
veins that drain blood to the right heart
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Heart Failure: Treatment Goals
• Strengthen myocardial contractile force• Remove excess water• Decrease work of the heart• Protect the heart from excess sympathetic
nerve activity
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