Cardiovascular System:
The Heart
Mary Christenson, PT, PhD
DPT 732: Management Applications
of Physiology II
Spring 2009
Did You Know?
Coronary circulation is the shortest circulation in
the body
“The longest cardiac arrest lasted four hours in
the case of fisherman Jan Egil Refsdahl
(Norway), who fell overboard off Bergen,
Norway, on December 7, 1987. He was rushed
to Haukeland Hospital after his body
temperature fell to 75° F (24° C), and his heart
stopped. He made a full recovery after being
connected to a heart-lung machine.”
Describe the physiologic structure and function of the heart.
Describe the systemic & pulmonary blood flow circuits of the
CV system.
Describe the coronary circulation and compare and contrast to
the pulmonary/systemic circulation.
Describe the function of the different types of cardiac muscle
and the microanatomy (striated, intercalated discs, gap junctions)
of each.
Describe the energy requirements of the heart.
Compare/contrast the intrinsic and extrinsic regulation of heart
Objectives (continued)
Describe the cardiac cycle and compare and contrast the
relationship between EKG tracing, heart sounds, atrial and
ventricular pressure changes, atrial and ventricular volume
changes, and valve actions that occur within the left chambers of
the heart during a cardiac cycle.
Describe the specialized excitatory and conductive system of the
Describe the cellular mechanisms of pacemaker potential and
cardiac muscle contraction, and compare with action potentials
and skeletal muscle contraction.
Describe the characteristics and principle features of a normal
EKG (waves, segments, complexes, polarization, depolarization,
Heart Anatomy: A Review
Functional anatomy
Heart Anatomy: A Review
External anatomy view
Membrane coverings
 Heart wall
 Vessels entering/exiting
Blood flow circuits: arteries versus veins
Pulmonary circuit
Systemic circuit
Coronary arteries
Heart Wall
Heart Anatomy: A Review
Internal anatomy
 Structures
Coronary Blood Flow
Systemic & Pulmonary Blood Flow
Cardiac Muscle & Microanatomy
Atrial muscle
 Ventricular muscle
 Specialized excitatory and conductive muscle fibers
Muscle cells
Intercalated discs
Gap junctions
Electron Micrograph
Cell Connections
Compare/Contrast Cardiac and
Skeletal Mechanism of Contraction
Striated – myosin/actin mechanism
T-tubule mechanism – acting on sarcoplasmic reticulum
T-tubule mechanism – direct diffusion of Ca++
Action potential
Cardiac muscle “plateau”
Strength of contraction
Ability of cardiac mm to depolarize and contract
is intrinsic
Intrinsic conduction system
Sinus node = sinoatrial/S-A node
 Internodal pathways
 A-V node
 A-V bundle
 Left and right bundle branches of Purkinje fibers
Intrinsic Conduction System
Features of the S-A Node
Smaller diameter muscle fibers
Almost no contractile muscle fibers
Connect directly with atrial muscle (mm) fibers
Cell membranes naturally “leaky” to Na+ and
Ca++ ions – therefore, less negative resting
membrane potential than other cardiac mm cells
Fast Na+ channels, at less negative potential,
Electrical impulses passing through the heart also
spread into adjacent tissues and some to the surface of
the body
Can be captured at surface of the body using electrodes
EKG Tracing
Cardiac Cycle
Events that occur from the beginning of one
heartbeat to the beginning of the next
Chamber and vessel blood volume changes
 Chamber and vessel blood pressures changes
 Electrical activity noted
 Heart sounds occur
 Valves open and close
Describe the relationships between the events
Cardiac cycle
Consists of:
Diastole: period of relaxation; heart filling with
 Systole: contraction period, heart ejects blood
What would be:
the definition of end-diastolic volume (EDV)?
 the definition of end-systolic volume (ESV)?
Ejection fraction: fraction of EDV ejected
Guyton Cardiac Cycle
Relationship of Cardiac Cycle to ECG
P wave: spread of depolarization through atrial
tissue followed by contraction - atrial pressure
QRS complex: spread of depolarization through
ventricular tissue followed by contraction ventricular pressure
T wave: repolarization of the ventricles which
represents ventricular relaxation
Atria as “Pumps”
Majority of returning venous blood flows
directly from atrium to ventricle
Atrial contraction usually causes an additional
20% ventricle filling; “primer pump”
Atrial function “unnecessary” except during
vigorous exercise
Atrial pressure changes
Ventricles as “Pumps”
Ventricular filling: after systole, A-V valves open
due to build up of pressure in atria during
systole: period of rapid filling of ventricles
followed by 2 additional phases
Period of Isovolumic Contraction
Period of Ejection
Period of Isovolumic Relaxation
Preload and Afterload
Preload – End-diastolic pressure when the
ventricle is filled; amount of tension on the
muscle when it begins to contract
Afterload – pressure in the artery leading from
the ventricle; load against which the muscle
exerts its contractile force
Heart and/or circulation pathology can severely
alter preload and/or afterload
Chemical Energy Requirements
for Cardiac Contraction
Great dependency/almost exclusive reliance on
O2 for energy metabolism (oxidative) compared
to skeletal muscle which can utilize anaerobic
metabolic sources as well
Energy derived primarily from oxidative
metabolism of fatty acids (food of choice:
primary oxidative nutrient source), some lactate,
Cardiac muscle can also use lactic acid generated
by skeletal muscle activity
Intrinsic Regulation of the
Cardiac Pump
Heart pumps 4-6 liters of blood/minute @ rest
Frank-Starling Mechanism
Heart automatically pumps incoming blood; i.e.,
amount of blood pumped determined primarily by
rate of blood flow into heart
 As cardiac muscle is stretched with returning blood
volume, approach optimal length of actin and
myosin fibers for contraction
Stretch of R atrial wall
Increase HR by 10-20%
Extrinsic Regulation of the
Cardiac Pump (ANS)
Sympathetic Nervous System (SNS)
Norepinephrine released by sympathetic nerve
fibers in response to stressors such as fright,
anxiety, or exercise; threshold reached more
Increase cardiac output (CO)
Pacemaker fires more rapidly
 Enhanced mm contractility
Effects of inhibiting SNS
Extrinsic Regulation of the
Cardiac Pump (ANS)
Parasympathetic Nervous System
Reduces HR when stressors removed
Acetylcholine hyperpolarizes membranes of cells
– opens K+ channels
PNS fibers in Vagus nerves to heart can decrease
Primarily affects HR rather than contractility
Autonomic Innervation of the Heart
Resting Conditions
S-A node receives impulses from both
autonomic divisions continuously
Dominant influence is inhibitory – heart said to
exhibit “vagal tone”
“Disconnect” vagal nerves = HR increases ~25
bpm almost immediately