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chapter
5
The Cardiovascular
System and Its
Control
Learning Objectives
• Review the structure and function of the heart, vascular
system, and blood
• Explore the role of the cardiovascular system in
delivering oxygen and nutrients to active body tissues
• Discover how the cardiovascular system removes
metabolic waste from tissues
Major Cardiovascular Functions
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Delivery of oxygen and other nutrients
Removal of carbon dioxide and other metabolic waste
Transport of hormones
Thermoregulation
Maintenance of acid–base balance and overall body
fluid balance
• Immune function
The Anatomy of the Human Heart
Myocardium—The Cardiac Muscle
• Thickness varies according to the stress placed on
chamber walls
• Left ventricle is the most powerful of chambers and the
largest chamber
• With exercise training the size of the left ventricle
increases
• Intercalated disks in the myocardium allow impulses
travel quickly in cardiac muscle and allow it to act as
one large muscle fiber; all fibers in the heart contract
together as one unit
Mechanism of Cardiac Muscle
Contraction
Courtesy of Dr. Donna Korzick, Pennsylvania State University.
Structural and Functional Characteristics
of Skeletal and Cardiac Muscle
Adapted, by permission, from K.L. Moore and A.F. Dalley, 1999, Clinically oriented anatomy, 4th ed. (Baltimore,
MD: Lippincott, Williams, and Wilkins), 27.
Coronary Circulation
Cardiac Conduction System
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Sinoatrial (SA) node
Atrioventricular (AV) node
AV bundle (bundle of His)
Purkinje fibers
Intrinsic Conduction System
of the Heart
Extrinsic Control of the Heart
• PNS acts through the vagus nerve, releasing ACh to
decrease heart rate and force of cardiac contraction
– Hyperpolarization of the conduction cells
– Absence of vagal tone, HR = 100 beats/min
– Maximal vagal tone, HR = 20-30 beats/min
• SNS increases rate of impulse generation and conduction
speed, increasing heart rate and force of cardiac
contraction
– Maximal sympathetic stimulation, HR = 250 beats/min
• Epinephrine and norepinephrine—released from the
adrenal medulla as a result of sympathetic stimulation—
increase heart rate
Relative Contribution of Sympathetic and
Parasympathetic Nervous Systems to the
Rise in Heart Rate During Exercise
Adapted from L.B. Rowell, Human Cardiovascular Control. Oxford University Press, 1993.
Heart Rate and
Endurance Training
Resting heart rates in adults tend to be between 60
and 85 beats per min. However, extended endurance
training can lower resting heart rate to 35 beats or
lower. This lower heart rate is thought to be due to
increased parasympathetic stimulation.
Electrocardiogram (ECG)
An ECG provides a graphical record of the
electrical activity of the heart and can be used to
aid clinical diagnoses
Cardiac Anatomy, Conduction,
and Control
Key Points
• The atria receive blood from the veins; the ventricles
eject blood from the heart
• The left ventricular myocardium is larger because it
must produce more force than the other ventricles to
pump blood to the systemic circulation
• Cardiac tissue is capable of spontaneous rhythmicity
and has its own conduction system
• The SA node normally establishes heart rate
• Heart rate and contractility can be altered by the PNS,
SNS, and the endocrine system
• The ECG is a recording of the heart’s electrical activity
Cardiac Arrhythmias
Bradycardia: resting heart rate below 60 bpm
Tachycardia: resting heart rate above 100 bpm
Premature ventricular contractions (PVCs): skipped
or extra beats from impulses originating outside the SA
node
Ventricular tachycardia: three or more consecutive
PVCs
Ventricular fibrillation: contraction of the ventricular
tissue is uncoordinated and can result in cardiac death
Endurance Training vs.
Pathological Bradycardia
The decrease in resting heart rate that occurs as an
adaptation to endurance training is different from
pathological bradycardia, an abnormal disturbance in
the resting heart rate.
Cardiac Cycle
• Defined as the mechanical and electrical events that
occur during one heart beat (systole to systole)
• Diastole is the relaxation phase during which the
chambers fill with blood (T wave to QRS)
• Systole is the contraction phase during which the
chambers expel blood (QRS to T wave)
Wiggers Diagram
Figure 14.27, p. 433 from Human Physiology, 2nd ed. By Dee Unglaub Silverthorn. Copyright © 2001 PrenticeHall, Inc. Reprinted by permission of Pearson Education, Inc.
Stroke Volume and Cardiac Output
Stroke Volume (SV)
• Volume of blood pumped per contraction
• End-diastolic volume (EDV)—volume of blood in
ventricle just before contraction
• End-systolic volume (ESV)—volume of blood in
ventricle just after contraction
• SV = EDV – ESV .
Cardiac Output (Q)
• Total volume of blood pumped by the ventricle per
minute
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• Q = HR  SV
Ejection Fraction (EF)
• Proportion of blood pumped out of the left ventricle with
each beat
• EF = SV / EDV
• Averages 60% at rest
Calculation of Stroke Volume, Ejection
Fraction, and Cardiac Output
The Vascular System
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Arteries
Arterioles
Capillaries
Venules
Veins
Blood Pressure
• Systolic blood pressure (SBP) is the highest pressure
within the vascular system generated during cardiac
contraction
• Diastolic blood pressure (DBP) is the lowest pressure
within the vascular system when the heart is relaxed
• Mean arterial pressure is the average pressure
– MAP = 2/3 DBP + 1/3 SBP
– MAP = DPB + [0.33 (SBP - DBP)]
General Hemodynamics
• Blood flows from a region within the vessel of high
pressure to a region within the vessel with lower
pressure (pressure gradient)
• Pressure gradient across the entire cardiovascular
system = 100 mmHg
• Blood vessels and the blood itself provide resistance to
blood flow
• Resistance to blood flow = [ηL / r4],
– η = viscosity of the blood
– L = length of the vessel
– r4 = radius of the vessel to the 4th power
Changing Blood Flow
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Blood Flow = Δpressure / resistance
Blood flow can change by either changing pressure or
resistance or a combination of the two
Changing resistance has a larger effect on blood flow
because of the fourth power mathematical relationship
between vascular resistance and vessel radius
Vasoconstriction: radius of the vessel decreases,
decreasing blood flow
Vasodilation: radius of the vessel increases,
increasing blood flow
Pressure Changes Across the
Systemic Circulation
Distribution of Cardiac Output at Rest
and During Heavy Exercise
Reprinted, by permission, from P.O. Åstrand et al., 2003, Textbook of work physiology: Physiological bases of
exercise, 4th ed. (Champaign, IL: Human Kinetics), 143.
Intrinsic Control of Blood Flow
Stimuli to increase local blood flow
1. Metabolic factors
– increased oxygen demand
– increases in metabolic by-products
– inflammatory chemicals
2. Endothelium released factors
– Nitric oxide
– Prostaglandins
– Endothelium-derived hyperpolarization factors (EDHF)
3. Myogenic responses
Intrinsic Control of Blood Flow
Figure courtesy of Dr. Donna H. Korzick, Pennsylvania State University.
Extrinsic Neural Control
Accomplished by the sympathetic nervous
system through vasoconstriction
Blood Distribution Within the Vasculature
When the Body Is at Rest
Integrative Control of Blood Pressure
Blood pressure is maintained and controlled by
the autonomic nervous system
Receptors that modify blood pressure control
through the cardiovascular control centers:
– Baroreceptors: stretch receptors in the aortic arch
and carotid arteries that are sensitive to changes in
blood pressure
– Chemoreceptors: chemical receptors that relay
information about the chemical environment
– Mechanoreceptors: receptors that sense changes
in muscle length and tension
Return of Blood to the Heart
• Valves in the veins
• The muscle pump
• The respiratory pump
Return of Blood to the Heart: The
Muscle Pump
Control of Blood Flow
Key Points
• Blood is distributed throughout the body based on the
needs of individual tissues
• Redistribution of blood is controlled locally by the
release of chemical substances, which cause
vasodilation
• Extrinsic neural control of blood flow distribution is
accomplished through vasoconstriction through the
sympathetic nervous system
• Blood returns to the heart through veins, assisted by
valves, the muscle pump, and respiratory pump
Functions of the Blood
Important to Exercise
• Transportation
• Temperature regulation
• Acid–base (pH) balance
Composition of Whole Blood
Hematocrit
Ratio of formed elements to the total blood volume
• White blood cells are involved in the immune
response
• Blood platelets are cell fragments that help blood
coagulation
• Red blood cells carry oxygen to tissues with the help
of hemoglobin
Blood Viscosity
• Thickness of the blood
• The more viscous, the more resistant to flow
• Higher hematocrits result in higher blood viscosity
Blood
Key Points
• Blood is about 55% to 60% plasma and 40% to
45% formed elements
• Oxygen is transported primarily by binding to
hemoglobin in red blood cells
• As blood viscosity increases, so does resistance to
blood flow