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Lecture 3
The Cardiovascular System: The
Heart
The Heart
• Heart is transport system; two
side-by-side pumps
• Pulmonary circuit
– Pumps to lungs to get rid of CO2, pick up O2,
• Systemic circuit
– Pumps to body tissues via
Figure 18.1 The systemic and pulmonary circuits.
Capillary beds of
lungs where gas
exchange occurs
Pulmonary Circuit
Pulmonary
arteries
Aorta and branches
Venae
cavae
Right
atrium
Right
ventricle
Oxygen-rich,
CO2-poor blood
Oxygen-poor,
CO2-rich blood
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Pulmonary veins
Left
atrium
Heart
Left
ventricle
Systemic Circuit
Capillary beds of all
body tissues where
gas exchange occurs
Heart Location
• Located in the medial cavity of the thorax –
mediastinum
– Superior surface of diaphragm
– Two-thirds of heart to left of midsternal line
– Anterior to vertebral column, posterior to sternum
Figure 18.2a Location of the heart in the mediastinum.
Midsternal line
2nd rib
Sternum
Diaphragm
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Location of
apical impulse
Figure 18.2b Location of the heart in the mediastinum.
Mediastinum
Heart
Left lung
Body of T7
vertebra
Posterior
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Figure 18.2c Location of the heart in the mediastinum.
Superior
vena cava
Pulmonary
trunk
Aorta
Parietal pleura
(cut)
Left lung
Pericardium (cut)
Apex of heart
Diaphragm
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Protection for the heart
• The heart is enclosed in a double-walled sac
called the pericardium
• Composed of two layers
– fibrous pericardium
– serous pericardium
• parietal layer
• visceral layer
The Pericardium
• – Fibrous pericardium – superficial layer
– protects heart
– anchors sac/heart to surrounding structures
The Pericardium
• Serous Pericardium – thin, slippery, two layer
between fibrous peridcardium and heart
– Parietal layer lines internal surface of fibrous
pericardium
– Visceral layer (epicardium) on external surface of
heart
– Two layers separated by fluid-filled pericardial
cavity (decreases friction)
Figure 18.3 The pericardial layers and layers of the heart wall.
Pulmonary
trunk
Fibrous pericardium
Pericardium
Parietal layer of serous
pericardium
Myocardium
Pericardial cavity
Epicardium (visceral
layer of serous
pericardium)
Myocardium
Endocardium
Heart chamber
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Heart
wall
Layers of the Heart Wall
• Three layers of heart wall:
– Epicardium – serous pericardium
– Myocardium – heart muscle
– Endocardium – inside the heart
Myocardium
• Spiral bundles of contractile cardiac muscle
cells tethered together by cardiac skeleton
• Cardiac skeleton: crisscrossing, interlacing
layer of connective tissue
– Anchors cardiac muscle fibers
– Supports great vessels and valves
– Limits spread of action potentials to specific paths
Figure 18.4 The circular and spiral arrangement of cardiac muscle bundles in the myocardium of the heart.
Cardiac
muscle
bundles
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Endocardium
• Inner layer of the heart
• Continuous with endothelial lining of blood
vessels
• Lines heart chambers; covers cardiac skeleton
of valves
Chambers of the Heart
• Four chambers:
– Two superior atria
– Two inferior ventricles
• Interatrial septum – separates atria
• Interventricular septum – separates ventricles
Atria
• Receiving chambers, thin walled
– only need to push down to ventricles
• Consists of smooth and pectinate muscles
• Contribute little to the pumping activity of the
heart
Right Atrium
• Right atrium receives blood from three veins
– Superior vena cava – blood above diaphragm
– Inferior vena cava – blood below diaphragm
– Coronary sinus – blood from myocardium
• Holds blood coming back from the body
Left Atrium
• Left atrium receives blood from four veins
– two left pulmonary veins
– two right pulmonary veins
• Holds blood coming back from the lungs
The Ventricles
• Makes up most of the heart
• Thicker walls than atria
• Actual pumps of heart
– Right ventricle
– Left ventricle
Right Ventricle
• Forms most of the heart’s anterior surface
• Pumps blood into the pulmonary track
– blood from heart to lungs for gas exchange
Left Ventricle
• Dominates the posterior surface
• Pumps blood into aorta – the largest artery in
the blood
– oxygenated blood from heart to body
Figure 18.5b Gross anatomy of the heart.
Brachiocephalic trunk
Superior vena cava
Right pulmonary artery
Ascending aorta
Pulmonary trunk
Right pulmonary veins
Left common carotid artery
Left subclavian artery
Aortic arch
Ligamentum arteriosum
Left pulmonary artery
Left pulmonary veins
Auricle of
left atrium
Right atrium
Right coronary artery
(in coronary sulcus)
Anterior cardiac vein
Right ventricle
Circumflex artery
Right marginal artery
Great cardiac vein
Anterior interventricular
artery (in anterior
interventricular sulcus)
Apex
Small cardiac vein
Inferior vena cava
Anterior view
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Left coronary artery
(in coronary sulcus)
Left ventricle
Figure 18.5e Gross anatomy of the heart.
Aorta
Superior vena cava
Right pulmonary artery
Pulmonary trunk
Right atrium
Right pulmonary veins
Fossa ovalis
Pectinate muscles
Tricuspid valve
Right ventricle
Chordae tendineae
Trabeculae carneae
Inferior vena cava
Frontal section
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Left pulmonary artery
Left atrium
Left pulmonary veins
Mitral (bicuspid) valve
Aortic valve
Pulmonary valve
Left ventricle
Papillary muscle
Interventricular septum
Epicardium
Myocardium
Endocardium
The Heart Valves
• Ensure unidirectional blood flow through heart
• Open and close in response to pressure changes
on both sides
• Two atrioventricular (AV) valves
• Two semilumar (SL) values
Atrioventricular Valves
• Tricuspid valve
– on right atrium
• Mitral valve (biscupal)
– on left atrium
• chordae tendineae – white collagen cords that
attach to each valve
– anchor valves from moving upward during ventricular
pressure
Figure 18.7 The atrioventricular (AV) valves.
1 Blood returning to the heart fills
atria, pressing against the AV valves.
The increased pressure forces AV
valves open.
Direction of
blood flow
Atrium
2 As ventricles fill, AV valve flaps
hang limply into ventricles.
Cusp of
atrioventricular
valve (open)
Chordae
tendineae
3 Atria contract, forcing additional
blood into ventricles.
Ventricle
Papillary
muscle
AV valves open; atrial pressure greater than ventricular pressure
Atrium
1 Ventricles contract, forcing
blood against AV valve cusps.
2 AV valves close.
3 Papillary muscles contract and
chordae tendineae tighten,
preventing valve flaps from everting
into atria.
AV valves closed; atrial pressure less than ventricular pressure
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Cusps of
atrioventricular
valve (closed)
Blood in
ventricle
Figure 18.6c Heart valves.
Pulmonary valve
Aortic valve
Area of cutaway
Mitral valve
Tricuspid valve
Chordae tendineae attached
to tricuspid valve flap
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Papillary
muscle
Semilunar Valves
• Two semilunar (SL) valves
– Prevent backflow into ventricles when ventricles
relax
– Open and close in response to pressure changes
• Named after where they pump blood to:
– Aortic semilunar valve
– Pulmonary semilunar valve
Figure 18.8 The semilunar (SL) valves.
Aorta
Pulmonary
trunk
As ventricles contract
and intraventricular
pressure rises, blood
is pushed up against
semilunar valves,
forcing them open.
Semilunar valves open
As ventricles relax
and intraventricular
pressure falls, blood
flows back from
arteries, filling the
cusps of semilunar
valves and forcing
them to close.
Semilunar
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Inc.
valves closed
Figure 18.6a Heart valves.
Pulmonary valve
Aortic valve
Area of cutaway
Mitral valve
Tricuspid valve
Myocardium
Mitral
(left atrioventricular)
valve
Tricuspid
(right atrioventricular)
valve
Aortic valve
Pulmonary valve
Cardiac
skeleton
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Anterior
Arteries and Veins
• Pulmonary circuit pump
– veins carry oxygen rich blood to the heart
– arteries carry oxygen poor blood from the heart
• Systemic circuit pump
– veins carry oxygen poor blood to the heart
– arteries carry oxygen rich blood from the heart
Figure 18.1 The systemic and pulmonary circuits.
Capillary beds of
lungs where gas
exchange occurs
Pulmonary Circuit
Pulmonary
arteries
Aorta and branches
Venae
cavae
Right
atrium
Right
ventricle
Oxygen-rich,
CO2-poor blood
Oxygen-poor,
CO2-rich blood
© 2013 Pearson Education, Inc.
Pulmonary veins
Left
atrium
Heart
Left
ventricle
Systemic Circuit
Capillary beds of all
body tissues where
gas exchange occurs
Blood volume and blood pressure
• Equal volumes of blood pumped to pulmonary
and systemic circuits
• Pulmonary circuit
– short, low-pressure circulation
• Systemic circuit
– long, high-friction (resistance) circulation
Figure 18.10 Anatomical differences between the right and left ventricles.
Left
ventricle
Right
ventricle
Interventricular
septum
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Figure 18.6d Heart valves.
Pulmonary valve
Aortic valve
Area of cutaway
Mitral valve
Tricuspid valve
Opening of inferior
vena cava
Tricuspid valve
Mitral valve
Chordae
tendineae
Myocardium
of right
ventricle
Interventricular
septum
Papillary
muscles
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Myocardium
of left ventricle
The Heart’s blood supply
• Nourished by the right and left coronary
arteries
• Arise from the base of the aorta
• Supplies blood when the heart is relaxed
– requires 1/20th the blood supply
– left ventricle needs the most
Heart Muscle Histology
• Cardiac muscle is striated and contracts by
sliding filament mechanism
– similar to skeletal muscle
• Cardiac muscle cells, short, branched, fat, and
interconnected
Heart Cells Interlock
• Intercalated discs - junctions between cells
– anchor cardiac cells via demsmosomes and gap
junctions
• Desmosomes prevent cells from separating
during contraction
• Gap junctions allow ions to pass from cell to
cell; electrically couple adjacent cells
Figure 18.12a Microscopic anatomy of cardiac muscle.
Nucleus
Intercalated
discs
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Cardiac
muscle cell
Gap junctions
Desmosomes
Functional Syncytium
• Heart cells are electrically coupled together
becoming a single coordinated unit
• Heart cells have mitochondria that fill 25-35%
of the cellular volume
Figure 18.12b Microscopic anatomy of cardiac muscle.
Cardiac muscle
cell
Intercalated
disc
Mitochondrion Nucleus
Mitochondrion
T tubule
Sarcoplasmic
reticulum
Z disc
Nucleus
Sarcolemma
I band
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A band
I band
Heart Muscle Contraction
• Do not need nervous system stimulation
• All cardiomyocytes contract as unit, or none do
• 1% are autorhythmic that depolarize
spontaneously and pace the heart
• Bulk is contractile muscle fibers whose action
potentials are mediated by Na+, Ca2+ and K+
channels
Polarization-Depolarization
• Positive feedback cycle
• Fast voltage-gated Na+ channels allow
extracellular Na+ to enter cell
• Slow Ca2+ channels allow extracellular Ca2+ to
enter cell
• K+ channels allow intracellular K+ out of cell
Movement of Action Potential
• Action potential (electrical current) passes
down T tubules throughout cells causing Ca2+
release from sarcoplasmic reticulum
Ion movement creates electrical
current
Action
potential
Plateau
20
2
0
Tension
development
(contraction)
–20
–40
3
1
–60
Absolute
refractory
period
–80
0
150
Time (ms)
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300
Tension (g)
Membrane potential (mV)
Figure 18.13 The action potential of contractile cardiac muscle cells.
Slide 4
1 Depolarization is due to Na+
influx through fast voltage-gated Na+
channels. A positive feedback cycle
rapidly opens many Na+ channels,
reversing the membrane potential.
Channel inactivation ends this phase.
2 Plateau phase is due to Ca2+
influx through slow Ca2+ channels.
This keeps the cell depolarized
because few K+ channels are open.
3 Repolarization is due to Ca2+
channels inactivating and K+
channels opening. This allows K+
efflux, which brings the membrane
potential back to its resting voltage.
Energy is needed!
• Has many mitochondria
– Great dependence on aerobic respiration
– Little anaerobic respiration ability
• Readily switches fuel source for respiration
– can use lactic acid from skeletal muscles
Intrinsic Cardiac Conduction System
• 1% autorhythmic cells set pace for cardiac
rhythm
– pacemaker cells
Membrane potential (mV)
Figure 18.14 Pacemaker and action potentials of pacemaker cells in the heart.
+10
0
–10
–20
–30
–40
–50
–60
–70
Action
potential
2
1 Pacemaker potential This
slow depolarization is due to both
opening of Na+ channels and
closing of K+ channels. Notice
that the membrane potential is
never a flat line.
Threshold
2 Depolarization The action
potential begins when the
pacemaker potential reaches
threshold. Depolarization is due
to Ca2+ influx through Ca2+
channels.
2
3
3
1
1
Pacemaker
potential
Time (ms)
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Slide 1
3 Repolarization is due to
Ca2+ channels inactivating and
K+ channels opening. This allows
K+ efflux, which brings the
membrane potential back to its
most negative voltage.
Pacemaker cells are found in nodes
• Sinoatrial (SA) node
– sets the pace for the heart – the pacemaker
• Atrioventricular (AV) node
– transmite to atrioventricular (AV) bundles
• Both found in right atrium
Figure 18.15a Intrinsic cardiac conduction system and action potential succession during one heartbeat.
Superior vena cava
Right atrium
1 The sinoatrial (SA)
node (pacemaker)
generates impulses.
Internodal pathway
2 The impulses
pause (0.1 s) at the
atrioventricular
(AV) node.
3 The
atrioventricular
(AV) bundle
connects the atria
to the ventricles.
4 The bundle branches
conduct the impulses
through the
interventricular septum.
Left atrium
Subendocardial
conducting
network
(Purkinje fibers)
Interventricular
septum
5 The subendocardial
conducting network
depolarizes the contractile
cells of both ventricles.
Anatomy of the intrinsic conduction system showing the sequence of
electrical excitation
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Slide 1
Figure 18.15b Intrinsic cardiac conduction system and action potential succession during one heartbeat.
Pacemaker potential
SA node
Atrial muscle
AV node
Ventricular
muscle
Pacemaker
potential
Plateau
0
100
200
300
400
Milliseconds
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Comparison of action potential shape at
various locations
Nervous System Control
• Neurons do lie in heart wall and activate SA
and AV nodes when needed
– can slow down and speed up heartbeat
Electrocardiography
• A graphic record of heart activity
– measurement of deflection waves
• Three waves:
– P wave – depolarization SA node  atria
– QRS complex - ventricular depolarization and
atrial repolarization
– T wave - ventricular repolarization
Figure 18.17 An electrocardiogram (ECG) tracing.
Sinoatrial
node
Atrioventricular
node
QRS complex
R
Ventricular
depolarization
Ventricular
repolarization
Atrial
depolarization
T
P
Q
P-R
Interval
0
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S
0.2
S-T
Segment
Q-T
Interval
0.4
Time (s)
0.6
0.8
Figure 18.18 The sequence of depolarization and repolarization of the heart related to the deflection waves of an ECG
tracing.
SA node
R
R
T
P
Q
Q
S
S
4 Ventricular depolarization is
complete.
R
R
Q
Q
S
5 Ventricular repolarization begins
at apex, causing the T wave.
S
2 With atrial depolarization
complete, the impulse is delayed at
the AV node.
R
T
P
Q
S
3 Ventricular depolarization begins at
apex, causing the QRS complex. Atrial
repolarization occurs.
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R
T
P
Q
T
P
T
P
T
P
1 Atrial depolarization, initiated by
the SA node, causes the P wave.
AV node
Slide 1
6
Depolarization
S
Ventricular repolarization is complete.
Repolarization
Sitole vs Disitole
• Blood flow through heart during one
complete heartbeat
– atrial systole and diastole followed by ventricular
systole and diastole
Systole—contraction
Diastole—relaxation
Figure 18.21 Summary of events during the cardiac cycle.
Left heart
QRS
P
Electrocardiogram
T
1st
Heart sounds
Dicrotic notch
120
Pressure (mm Hg)
P
2nd
80
Aorta
Left ventricle
40
Atrial systole
Left atrium
0
Ventricular
volume (ml)
120
EDV
SV
50
ESV
Atrioventricular valves
Aortic and pulmonary valves
Phase
Open
Closed
Open
Closed
Open
Closed
1
2a
2b
3
1
Left atrium
Right atrium
Left ventricle
Right ventricle
Atrial
contraction
Ventricular
filling
1
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Ventricular filling
(mid-to-late diastole)
Ventricular
Isovolumetric
contraction phase ejection phase
2a
2b
Ventricular systole
(atria in diastole)
Isovolumetric
relaxation
3
Early diastole
Ventricular
filling
Regulation of Heart Rate
• Autonomic Nervous System
– emotional or physical stressors
– release of norepinephrine
• Hormones
– Epinephrine from adrenal medulla
– Thyroxine from thyroid gland
Cardiac Output
• the amount of blood pumped out of each
ventricle in 1 minute
CO = HR x SV
HR = heart rate (beats/min)
SV = stroke volume (mls/beat)
Figure 18.21 Summary of events during the cardiac cycle.
Left heart
QRS
P
Electrocardiogram
T
1st
Heart sounds
Dicrotic notch
120
Pressure (mm Hg)
P
2nd
80
Aorta
Left ventricle
40
Atrial systole
Left atrium
0
Ventricular
volume (ml)
120
EDV
SV
50
ESV
Atrioventricular valves
Aortic and pulmonary valves
Phase
Open
Closed
Open
Closed
Open
Closed
1
2a
2b
3
1
Left atrium
Right atrium
Left ventricle
Right ventricle
Atrial
contraction
Ventricular
filling
1
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Ventricular filling
(mid-to-late diastole)
Ventricular
Isovolumetric
contraction phase ejection phase
2a
2b
Ventricular systole
(atria in diastole)
Isovolumetric
relaxation
3
Early diastole
Ventricular
filling
Stoke Volume
SV = EDV = ESV
• EDV = end diastolic volume
– volume of blood in ventricles at rest
• ESV = end systolic volume
– volume of blood in ventricles after contraction
Figure 18.22 Factors involved in determining cardiac output.
Exercise (by
sympathetic activity,
skeletal muscle and
respiratory pumps;
see Chapter 19)
Heart rate
(allows more
time for
ventricular
filling)
Venous
return
Physiological response
Result
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Sympathetic
activity
Contractility
EDV
(preload)
Initial stimulus
Exercise,
fright, anxiety
Bloodborne
epinephrine,
thyroxine,
excess Ca2+
Parasympathetic
activity
ESV
Stroke
volume
Heart
rate
Cardiac
output
Lab Exercise 30
• Anatomy of the Heart

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