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Chapter 9
Circulatory Responses to Exercise
EXERCISE PHYSIOLOGYTheory and Application to Fitness and Performance,
6th edition
Scott K. Powers & Edward T. Howley
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The Circulatory System
• Works with the pulmonary system– Cardiopulmonary or cardiorespiratory system
• Purposes of the cardiorespiratory system– Transport O2 and nutrients to tissues
– Removal of CO2 wastes from tissues
– Regulation of body temperature
• Two major adjustments of blood flow during exercise– Increased cardiac output– Redistribution of blood flow
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The Circulatory System
• Heart– Creates pressure to pump blood
• Arteries and arterioles– Carry blood away from the heart
• Capillaries– Exchange of O2, CO2, and nutrients with tissues
• Veins and venules– Carry blood toward the heart
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Structure of the Heart
Figure 9.1
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Pulmonary and Systemic Circuits
• Pulmonary circuit– Right side of the heart– Pumps deoxygenated blood to the lungs via
pulmonary arteries– Returns oxygenated blood to the left heart via
pulmonary veins
• Systemic circuit– Left side of the heart– Pumps oxygenated blood to the whole body via
arteries– Returns deoxygenated blood to the right heart via
veins
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Pulmonary and Systemic
Circulations
Figure 9.2
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Cardiac Muscle
• The heart wall– Epicardium– Myocardium– Endocardium
• Receives blood supply via coronary arteries– High demand for oxygen and nutrients
• Myocardial infarction (MI)– Blockage in coronary blood flow results cell damage– Exercise training protects against heart damage
during MI
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The Heart Wall
Figure 9.3
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Endurance Exercise Protects Against Cardiac Injury During Heart Attack
Figure 9.4
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Comparison of Cardiac and Skeletal Muscle
Table 9.1
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The Cardiac Cycle
• Systole– Contraction phase– Ejection of blood
• ~2/3 blood is ejected from ventricles per beat
• Diastole– Relaxation phase– Filling with blood
• At rest, diastole longer than systole• During exercise, both systole and diastole are
shorter
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The Cardiac Cycle at Rest and During Exercise
Figure 9.5
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Pressure Changes During the Cardiac Cycle
• Diastole– Pressure in ventricles is low– Filling with blood from atria
• AV valves open when ventricular P < atrial P
• Systole– Pressure in ventricles rises– Blood ejected in pulmonary and systemic circulation
• Semilunar valves open when ventricular P > aortic P
• Heart sounds– First: closing of AV valves– Second: closing of aortic and pulmonary valves
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Pressure, Volume, and Heart Sounds During the Cardiac Cycle
Figure 9.6
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Arterial Blood Pressure
• Expressed as systolic/diastolic– Normal is 120/80 mmHg
• Systolic pressure– Pressure generated during ventricular contraction
• Diastolic pressure– Pressure in the arteries during cardiac relaxation
• Pulse pressure– Difference between systolic and diastolic
• Mean arterial pressure (MAP)– Average pressure in the arteries
MAP = DBP + 0.33(SBP-DBP)
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Hypertension
• Blood pressure above 140/90 mmHg• Primary (essential) hypertension
– Cause unknown– 90% cases of hypertension
• Secondary hypertension– result of some other disease process
• Risk factor for:– Left ventricular hypertrophy– Atherosclerosis and heart attack– Kidney damage– Stroke
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Measurement of Blood Pressure
Figure 9.7
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Factors that Influence Arterial Blood Pressure
• Determinants of mean arterial pressure– Cardiac output– Total vascular resistance
– Short-term regulation– Sympathetic nervous system– Baroreceptors in aorta and carotid arteries
• Increase in BP = decreased SNS activity• Decrease in BP = increased SNS activity
• Long-term regulation– Kidneys
• Via control of blood volume
MAP = cardiac output x total vascular resistance
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Factors That Influence Arterial Blood Pressure
Figure 9.8
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Electrical Activity of the Heart
• Contraction of the heart depends on electrical stimulation of the myocardium
• Conduction system– Sinoatrial node (SA node)
• Pacemaker, initiates depolarization– Atrioventricular node (AV node)
• Passed depolarization to ventricles• Brief delay to allow for ventricular filling
– Bundle Branches• To left and right ventricle
– Purkinje fibers• Throughout ventricles
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Conduction System of the Heart
Figure 9.9
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Electrocardiogram (ECG)
• Records the electrical activity of the heart• P-wave
– Atrial depolarization• QRS complex
– Ventricular depolarization and atrial repolarization• T-wave
– Ventricular repolarization• ECG abnormalities may indicate coronary heart disease
– ST-segment depression can indicate myocardial ischemia
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Normal Electrocardiogram
Figure 9.11
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S-T Segment Depression on the Electrocardiogram
Figure 9.10
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Relationship Between Electrical
Events and the ECG
Figure 9.12
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• The amount of blood pumped by the heart each minute
• Product of heart rate and stroke volume– Heart rate
• Number of beats per minute
– Stroke volume • Amount of blood ejected in each beat
• Depends on training state and gender
Q = HR x SV
Cardiac Output
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Regulation of Heart Rate
• Parasympathetic nervous system– Via vagus nerve – Slows HR by inhibiting SA and AV node
• Sympathetic nervous system– Via cardiac accelerator nerves– Increases HR by stimulating SA and AV node
• Low resting HR due to parasympathetic tone• Increase in HR at onset of exercise
– Initial increase due to parasympathetic withdrawal• Up to ~100 beats/min
– Later increase due to increased SNS stimulation
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Nervous System Regulation of Heart Rate
Figure 9.14
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Regulation of Stroke Volume
• End-diastolic volume (EDV)– Volume of blood in the ventricles at the end of diastole
(“preload”)• Average aortic blood pressure
– Pressure the heart must pump against to eject blood (“afterload”)
• Mean arterial pressure• Strength of the ventricular contraction (contractility)
– Enhanced by:• Circulating epinephrine and norepinephrine• Direct sympathetic stimulation of heart
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End-Diastolic Volume
• Frank-Starling mechanism– Greater EDV results in a more forceful contraction
• Due to stretch of ventricles• Dependent on venous return• Venous return increased by:
– Venoconstriction• Via SNS
– Skeletal muscle pump• Rhythmic skeletal muscle contractions force blood in the
extremities toward the heart• One-way valves in veins prevent backflow of blood
– Respiratory pump• Changes in thoracic pressure pulls blood toward heart
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Relationship Between End-Diastolic Volume and Stroke Volume
Figure 9.15
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Skeletal Muscle Pump
Figure 9.16
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Effects of Sympathetic Stimulation on Stroke Volume
Figure 9.17
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Factors that Regulate Cardiac Output
Figure 9.18
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Physical Characteristics of Blood
• Physical characteristics of blood– Plasma
• Liquid portion of blood• Contains ions, proteins, hormones
– Cells• Red blood cells
– Contain hemoglobin to carry oxygen• White blood cells
– Important in preventing infection• Platelets
– Important in blood clotting• Hematocrit
– Percentage of blood composed of cells
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Hematocrit
Figure 9.19
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Hemodynamics
• Based on relationships among pressure, resistance, and flow
• Blood flow– Directly proportional to the pressure difference between
the two ends of the system– Inversely proportional to resistance
• Pressure– Proportional to the difference between MAP and right
atrial pressure ( Pressure)
Blood flow = Pressure
Resistance
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Hemodynamics• Resistance
– Depends upon:• Length of the vessel• Viscosity of the blood• Radius of the vessel
• Sources of vascular resistance– MAP decreases throughout the systemic circulation– Largest drop occurs across the arterioles
• Arterioles are called “resistance vessels”
Resistance = Length x viscosity
Radius4
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Blood Flow Through the Systemic Circuit
Figure 9.20
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Figure 9.21
Pressure Changes Across the Systemic Circulation
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Oxygen Delivery During Exercise
• Oxygen demand by muscles during exercise is 15–25x greater than at rest
• Increased O2 delivery accomplished by:
– Increased cardiac output– Redistribution of blood flow
• From inactive organs to working skeletal muscle
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• Cardiac output increases due to:– Increased HR
• Linear increase to max
– Increased SV• Increase, then plateau at ~40% VO2max
• No plateau in highly trained subjects
Changes in Cardiac Output During Exercise
Max HR = 220 - age (years)
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Changes in Cardiovascular
Variables During Exercise
Figure 9.22
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Changes in Arterial-Mixed Venous O2 Content During Exercise
• Higher arteriovenous difference (a-vO2 difference)– Amount of O2 that is taken up from 100 ml blood
– Increase due to higher amount of O2 taken up• Used for oxidative ATP production
• Fick equation– Relationship between cardiac output (Q), a-vO2
difference, and VO2VO2 = Q x a-vO2 difference
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Redistribution of Blood Flow During Exercise
• Increased blood flow to working skeletal muscle– At rest, 15–20% of cardiac output to muscle– Increases to 80–85% during maximal exercise
• Decreased blood flow to less active organs– Liver, kidneys, GI tract
• Redistribution depends of metabolic rate– Exercise intensity
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Changes in Muscle and Splanchnic Blood Flow During Exercise
Figure 9.23
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Redistribution of Blood Flow During Exercise
Figure 9.24
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Regulation of Local Blood Flow During Exercise
• Skeletal muscle vasodilation– Autoregulation
• Blood flow increased to meet metabolic demands of tissue
• Due to changes in O2 tension, CO2 tension, nitric oxide, potassium, adenosine, and pH
• Vasoconstriction to visceral organs and inactive tissues– SNS vasoconstriction
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Circulatory Responses to Exercise
• Changes in heart rate and blood pressure
• Depend on:– Type, intensity, and duration of exercise– Environmental condition– Emotional influence
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Transition From Rest to Exercise and Exercise to Recovery
• At the onset of exercise:– Rapid increase in HR, SV, cardiac output– Plateau in submaximal (below lactate threshold)
exercise
• During recovery– Decrease in HR, SV, and cardiac output toward
resting – Depends on:
• Duration and intensity of exercise• Training state of subject
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Transition From Rest to Exercise to Recovery
Figure 9.25
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Incremental Exercise
• Heart rate and cardiac output– Increases linearly with increasing work rate
– Reaches plateau at 100% VO2max
• Blood pressure– Mean arterial pressure increases linearly
• Systolic BP increases• Diastolic BP remains fairly constant
• Double product– Increases linearly with exercise intensity– Indicates the work of the heart
Double product = HR x systolic BP
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Changes in Double Product During Exercise
Table 9.3
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.
Arm Versus Leg Exercise
• At the same oxygen uptake arm work results in higher:– Heart rate
• Due to higher sympathetic stimulation
– Blood pressure• Due to vasoconstriction of large inactive muscle mass
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Heart Rate and Blood Pressure During Arm and Leg Exercise
Figure 9.26
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.
Intermittent Exercise
• Recovery of heart rate and blood pressure between bouts depend on:– Fitness level– Temperature and humidity– Duration and intensity of exercise
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.
Prolonged Exercise
• Cardiac output is maintained
• Gradual decrease in stroke volume– Due to dehydration and reduced plasma volume
• Gradual increase in heart rate– Cardiovascular drift
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Cardiovascular Changes During Prolonged Exercise
Figure 9.27
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Regulation of Cardiovascular Adjustments to Exercise
Figure 9.28
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Summary of Cardiovascular Control During Exercise
• Initial signal to “drive” cardiovascular system comes from higher brain centers– Central command
• Fine-tuned by feedback from:– Chemoreceptors
• Sensitive to muscle metabolites (K+, lactic acid)– Mechanoreceptors
• Sensitive to force and speed of muscular movement– Baroreceptors
• Sensitive to changes in arterial blood pressure• Redundancy in cardiovascular control
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Summary of Cardiovascular Control During Exercise
Figure 9.29