Chapter 9
Cardiac Physiology
Credit: © L. Bassett/Visuals Unlimited
Human heart.
97935
Credit: © L. Bassett/Visuals Unlimited
Human heart with coronary arteries exposed.
97936
Outline
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•
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Circulatory system overview
Transplant surgery
Anatomy
Electrical activity
Mechanical events
Cardiac output
Coronary circulation
Circulatory System
•
Three basic components
– Heart
• Serves as pump that
establishes the pressure
gradient needed for blood to
flow to tissues
– Blood vessels
• Passageways through which
blood is distributed from heart
to all parts of body and back
to heart
– Blood
• Transport medium within
which materials being
transported are dissolved or
suspended
•
•
Pulmonary circulation
– Closed loop of vessels carrying
blood between heart and lungs
Systemic circulation
– Circuit of vessels carrying blood
between heart and other body
systems
Transplant
• http://www.youtube.com/watch?v=cOBWMITf3co
• www.or-live.com
Circulatory System
• Anatomy
– Heart
– Hollow, muscular organ about the size of a clenched fist
– Positioned between two bony structures – sternum and
vertebrate
• Dual pump
– Right and left sides of heart function as two separate
pumps
– Divided into right and left halves and has four chambers
• Atria
– Upper chambers
– Receive blood returning to heart and transfer it to lower
chambers
• Ventricles
– Lower chambers which pump blood from heart
Circulatory System
• Heart
– Arteries
• Carry blood away from ventricles to tissues
– Veins
• Vessels that return blood from tissues to the atria
– Septum
• Continuous muscular partition that prevents mixture of
blood from the two sides of heart
Outline
• Anatomy
– Thoracic cavity, base, apex
– AV and semilunar valves
– endothelium, myocardium, epicardium
– cardiac cells, intercalated disks
– Comparison of cardiac cells to skeletal and
smooth muscle cells
– pericardium
Blood Flow Through and Pump Action of the Heart
What are the parts and what do they do?
Know the flow of blood in order.
Heart Valves
• Atrioventricular (AV) valves
– Right and left AV valves are positioned between
atrium and ventricle on right and left sides
– Prevent backflow of blood from ventricles into
atria during ventricular emptying
– Right AV valve
• Also called tricuspid valve
– Left AV valve
• Also called bicuspid valve or mitral valve
– Chordae tendinae
• Fibrous cords which prevent valves from being everted
• Papillary muscles
Heart Valves
• Semilunar valves
– Aortic and pulmonary valves
– Lie at juncture where major arteries leave
ventricles
– Prevented from everting by anatomic structure
and positioning of cusps
• No valves between atria and veins
– Reasons
• Atrial pressures usually are not much higher than
venous pressures
• Sites where venae cavae enter atria are partially
compressed during atrial contraction
Heart Valves
•
3 layers
• Consists of three distinct layers
– Endothelium
• Thin inner tissue
• Epithelial tissue which lines entire
circulatory system
– Myocardium
• Middle layer
• Composed of cardiac muscle
• Constitutes bulk of heart wall
– Epicardium
•
• Thin external layer which covers the
heart
Pericardium
– the fluid filled sac that surrounds the
heart
Endocardium
Myocardium
Epicardium
• Heart is enclosed by pericardial sac
• Pericardial sac has two layers
– Tough, fibrous covering
– Secretory lining
• Secretes pericardial fluid
– Provides lubrication to prevent friction between
pericardial layers
• Pericarditis
– Inflammation of pericardial sac
• The next series of slides compares cardiac, skeletal,
smooth muscle cells.
Cardiac Muscle Fibers
• Interconnected by intercalated discs and form
functional syncytia
• Within intercalated discs – two kinds of membrane
junctions
– Desmosomes
– Gap junctions
Credit: © Dr. Donald Fawcett/Visuals Unlimited
Cardiac muscle in longitudinal section showing intercalated discs. TEM.
319540
Skeletal Muscle Fiber
Myofibrils
Basal lamina
Sacrolemma
Longitudinal
system
Mitochondria
Transverse
tubule
Nucleus
Intercalated
disc
Myofibril
Opening of
Transverse
tubule
310127
Credit: © Dr. Richard Kessel/Visuals Unlimited
Myofibrils within skeletal muscle fibers. All bands (A, I, Z, M) are evident as well as elements
of the sarcoplasmic reticulum and small dots of glycogen. TEM X32,000.
Smooth Muscle FiberRough
Endoplasmic
reticulum
Glycogen
granules
Nucleus
Mitochondria
Thin filament
Thick filament
Dense
bodies
Plasma
membrane
fig 16-9a, pg 479
Dense
bodies
Smooth
muscle
Cells
Fig. 8-27b, p. 288
Outline
• Electrical activity of the heart
– Autorhymicity
– Pacemaker (function, ions)
– Conductive system (SA, AV, bundle of His,
Purkinje fibers)
– Abnormal rhythms
– Spread of cardiac excitation
– Cardiac cell action potentials
• Characteristics vary by location
Electrical Activity of Heart
• Heart beats rhythmically as result of action
potentials it generates by itself (autorhythmicity)
• Two specialized types of cardiac muscle cells
– Contractile cells
• 99% of cardiac muscle cells
• Do mechanical work of pumping
• Normally do not initiate own action potentials
– Autorhythmic cells
• Do not contract but send electrical signals to the
contractile cells
• Specialized for initiating and conducting action
potentials responsible for contraction of working cells
Electrical Activity of Heart
• Locations of noncontractile cells capable of autorhymicity
– Sinoatrial node (SA node)
• Specialized region in right atrial wall near opening of superior
vena cava
• Pacemaker of the heart
– Atrioventricular node (AV node)
• Small bundle of specialized cardiac cells located at base of
right atrium near septum
– Bundle of His (atrioventricular bundle)
• Cells originate at AV node and enters interventricular septum
• Divides to form right and left bundle branches which travel
down septum, curve around tip of ventricular chambers,
travel back toward atria along outer walls
– Purkinje fibers
• Small, terminal fibers that extend from bundle of His and
spread throughout ventricular myocardium
Specialized Conduction System of Heart
Electrical Activity of Heart
• Cardiac impulse originates at SA node
• Action potential spreads throughout right and left atria
• Impulse passes from atria into ventricles through AV node
(only point of electrical contact between chambers)
• Action potential briefly delayed at AV node (ensures atrial
contraction precedes ventricular contraction to allow complete
ventricular filling)
• Impulse travels rapidly down interventricular septum by
means of bundle of His
• Impulse rapidly disperses throughout myocardium by means
of Purkinje fibers
• Rest of ventricular cells activated by cell-to-cell spread of
impulse through gap junctions
Spread of Cardiac Excitation
Open K channels
Open Ca channels
Closure of K channels
Fig. 9-7, p. 305
Relationship of an Action
Potential and the Refractory
Period to the Duration of the
Contractile Response in
Cardiac Muscle
Fig. 9-11, p. 310
SA node
pacemaker
Atrial muscle
Atrioventricular
Bundle branch
Purkinje fibers
Ventricular
muscle
Milliseconds
fig. 18-13; pg: 568
Electrical Activity of Heart
• Atria contract as single unit followed after brief delay
by a synchronized ventricular contraction
• Action potentials of cardiac contractile cells exhibit
prolonged positive phase (plateau) accompanied by
prolonged period of contraction
– Ensures adequate ejection time
– Plateau primarily due to activation of slow L-type
Ca2+ channels
Electrical Activity of Heart
• Ca2+ entry through L-type channels in T tubules
triggers larger release of Ca2+ from sarcoplasmic
reticulum
– Ca2+ induced Ca2+ release leads to cross-bridge
cycling and contraction
ExcitationContraction
Coupling in Cardiac
Contractile Cells
Electrical Activity of Heart
• Because long refractory period occurs in conjunction
with prolonged plateau phase, summation and
tetanus of cardiac muscle is impossible
– Ensures alternate periods of contraction and
relaxation which are essential for pumping blood
Relationship of an Action
Potential and the Refractory
Period to the Duration of the
Contractile Response in
Cardiac Muscle
Electrocardiogram (ECG)
• Record of overall spread of electrical activity through heart
• Represents
– Recording part of electrical activity induced in body fluids
by cardiac impulse that reaches body surface
• Not direct recording of actual electrical activity of heart
– Recording of overall spread of activity throughout heart
during depolarization and repolarization
• Not a recording of a single action potential in a single cell at a
single point in time
– Comparisons in voltage detected by electrodes at two
different points on body surface, not the actual potential
• Does not record potential at all when ventricular muscle is
either completely depolarized or completely repolarized
Electrocardiogram (ECG)
Different parts of ECG record can be correlated to specific cardiac
events
Credit: © Mediscan/Visuals Unlimited
Normal ECG.
3202
Abnormalities in Rate
• Tachycardia
– Rapid heart rate of more than 100 beats per
minute
• Bradycardia
– Slow heart rate of fewer than 60 beats per minute
Abnormalities in Rhythm
• Rhythm
– Regularity or spacing of ECG waves
• Arrhythmia
– Variation from normal rhythm and sequence of
excitation of the heart
– Examples
•
•
•
•
Atrial flutter (200-300 BPM)
Atrial fibrillation
Ventricular fibrillation
Heart block
Cardiac Myopathies
• Damage of the heart muscle
– Myocardial ischemia
• Inadequate delivery of oxygenated blood to heart tissue
– Necrosis
• Death of heart muscle cells
– Acute myocardial infarction (heart attack)
• Occurs when blood vessel supplying area of heart
becomes blocked or ruptured
Representative Heart Conditions Detectable Through ECG
Outline
• Mechanical events
– Systole, diastole
– animation (volumes, pressures, sounds and EKG)
Specialized Conduction System of Heart
•http://library.med.utah.ed
u/kw/pharm/hyper_heart1
.html
Fig. 9-17, p. 316
Cardiac Output
• Volume of blood ejected by each ventricle each
minute
• Determined by
heart rate times
stroke volume
Cardiac Output
• Heart rate is varied by altering balance of
parasympathetic and sympathetic influence on SA
node
– Parasympathetic stimulation slows heart rate
– Sympathetic stimulation speeds it up
Threshold
potential
Threshold
potential
= Inherent SA node pacemaker activity
= SA node pacemaker activity on parasympathetic stimulation
= SA node pacemaker activity on sympathetic stimulation
Fig. 9-20, p. 322
Heart rate
Parasympathetic
activity
Sympathetic
activity
(and epinephrine)
Fig. 9-20b, p. 322
Cardiac Output
• Stroke volume
– Determined by extent of venous return and by
sympathetic activity
– Influenced by two types of controls
• Intrinsic control
• Extrinsic control
– Both factors increase stroke volume by increasing
strength of heart contraction
Frank-Starling Law of the Heart
• States that heart normally pumps out during systole
the volume of blood returned to it during diastole
Fig. 9-22, p. 323
Fig. 9-23, p. 324
Fig. 9-24, p. 324
Outline
• Coronary circulation
Nourishing the Heart Muscle
• Muscle is supplied with oxygen and nutrients by
blood delivered to it by coronary circulation, not from
blood within heart chambers
• Heart receives most of its own blood supply that
occurs during diastole
– During systole, coronary vessels are compressed
by contracting heart muscle
• Coronary blood flow normally varies to keep pace
with cardiac oxygen needs
Coronary Artery Disease (CAD)
• Pathological changes within coronary artery walls
that diminish blood flow through the vessels
• Leading cause of death in United States
• Can cause myocardial ischemia and possibly lead to
acute myocardial infarction
– Three mechanisms
• Profound vascular spasm of coronary arteries
• Formation of atherosclerotic plaques
• Thromboembolism
Normal blood
vessel wall
Collagen-rich
smooth muscle
cap of plaque
Plaque
Lipid-rich core
of plaque
Endothelium
Fig. 9-29, p. 328
Fig. 9-30, p. 330
Area of cardiac
muscle deprived
of blood supply
if coronary vessel
is blocked at
point
Right
coronary
artery
Area of cardiac
muscle deprived
of blood supply
if coronary vessel
is blocked at
point
Left
coronary
artery
Left
ventricle
Right
ventricle
Fig. 9-31, p. 331
Possible Outcomes of Acute Myocardial Infarction (Heart Attack)