The Heart
Chapter 18
Introduction


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The heart is the pump of our circulatory
system
The cardiovascular system provides the
transport system of the body
Using blood as the transport medium, the
heart continually propels oxygen,
nutrients, wastes, and many other
substances into the interconnecting blood
vessels that move past the body cells
Introduction

The heart is a muscular double pump
with two functions
– Its right side receives oxygen poor blood
from the body tissues and then pumps it to
the lungs
– Its left side receives oxygenated blood from
the lungs and then pumps it to the body
Introduction


The blood vessels
that carry blood
from the lungs
form the
pulmonary circuit
The vessels that
carry blood to all
the body tissues
form the systemic
circuit
Heart Size, Location and Position
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The heart is about the size
of a fist
It weighs between 250 350 grams (less than a
pound)
Located in the medial
cavity of the thorax, the
mediastinum
It extends from the 2nd
rib to 5th intercostal space
Rests on the superior
surface of diaphram
Heart Size, Location and Position

The lungs flank the heart laterally and partially
obscure it
Heart Size, Location and Position

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The heart lies anterior to
the vertebral column and
posterior to the sternum
Two thirds of the heart
lies to the left of the midsternal line; the balance
projects to the right
Its broad flat base, or
posterior surface, points
to right shoulder
The apex points toward
the left hip
Location - 4 Corners
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
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The heart is has four
corners projected onto
the anterior thoracic
wall
Superior right - where
the costal cartilage
joins the 3rd rib
Superior left - costal
cartilage of 2nd rib a
fingers breadth lateral
to the sternum
Location - 4 Corners
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The inferior right - lies
at the costal cartilage
of the sixth rib, a
finger’s breath lateral
to the sternum
The inferior left (apex)
lies in the fifth
intercostal space at the
mid-clavicular line
These points depict the
normal heart size and
placement
Coverings of the Heart
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The heart is enclosed in a triple-walled sac called the
pericardium
The loose fitting outer layer of the sac is the fibrous
pericardium
– This tough, dense connective tissue layer 1) protects the
heart; 2) anchors the heart; and 3) prevents overfilling
Coverings of the Heart
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Deep to the fibrous pericardium is the double-layered
serous pericardium, a closed sac sandwiched between
the fibrous pericardium and the heart
The two layers are…
– Parietal layer
– Visceral layer
Coverings of the Heart
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The outer parietal layer adheres to the internal
surface of the fibrous pericardium
At the superior reflection of the heart, the parietal
layer is continuous with the visceral layer of the
serous pericardium or epicardium
Coverings of the Heart
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The visceral layer, also called the epicardium, is an
integral part of the heart wall
The two-layer membrane conforms around the heart
much like pushing your fist into a double layer
membrane with an air pocket in between
Coverings of the Heart
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Between the two layers of serous pericardium is the
slitlike pericardial cavity
The cavity contain pericardial fluid
The serous membranes, lubricated by fluid, glide
smoothly against one another during heart activity,
creating a relatively friction-free environment
Inflammation

Inflammation of the heart can lead to
serious problems
– Pericarditis / hinders production of serous
fluid production causing the heart to rub
– Cardiac tamponade / inflammatory fluid
seep into the pericardial cavity, compressing
the heart and limiting its ability to pump
blood
Layers of the Heart Wall
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The heart wall is composed of three layers
– Superficial layer of epicardium
– Middle layer of myocardium
– Deep layer of endocardium
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All three layers are richly supplied with blood vessels
Layers of the Heart Wall
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The epicardium is the visceral layer of the serous
pericardium
The epicardium is often infiltrated with fat, especially
in older people
Layers of the Heart Wall
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The myocardium is the layer of cardiac muscle that
forms the bulk of the heart
It is the layer that actually contracts
The myocardium’s elongated circularly spirally
arranged muscle cells squeeze the blood though the
heart
Layers of the Heart Wall
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Within the myocardium, the branching
cardiac muscle cells are tethered to each
other by crisscrossing connective tissue
fibers also arranged in spiral or circular
bundles
These interlacing bundles effectively link
all parts of the heart together
Layers of the Heart Wall
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The connective tissue
forms a dense network
called the internal
skeleton of the heart
It reinforces the
myocardium internally
and anchors the cardiac
muscle
This network of fibers is
thicker in some areas
than in others to reinforce valves and where
the major vessels exit
Layers of the Heart Wall
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The internal skeleton
prevents overdilation of
vessels due to the
continual stress of blood
pressure
Additionally, since
connective tissue is not
electrically excitable, it
limits action potentials
across the heart to
specific pathways
Layers of the Heart Wall
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The endocardium is a glistening white sheet of
endothelium (squamous epithelium) resting on a thin
layer of connective tissue
Layers of the Heart Wall
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Located on the inner myocardial surface, it
lines the heart chambers and covers the
connective tissue skeleton of the valves
The endocardium is continuous with the
endothelial linings of the blood vessels
leaving and entering the heart
Heart Chambers

The heart has four
chambers
Atria
– Two superior atria
– Two inferior ventricles

The longitudinal wall
separating the
chambers is called the
– Interartial septum
• Between atria
– Interventricular
septum
Septum
• Between ventricles
Ventricles
Heart Chambers
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The right ventricle
forms most of the
anterior surface of the
heart
The left ventricle
dominates the inferioposterior aspect of the
heart and forms the
heart apex
Left
Ventricle
Right Ventricle
Heart Chambers
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Two grooves visible on
the surface of the
heart indicate the
boundaries of its four
chambers and carry
the blood vessels that
supply myocardium
The Atrioventricular
groove or coronary
sulcus encircles the
junction of the atria
and ventricles
Coronary
Sulcus
Heart Chambers
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The anterior interventricular sulcus,
separates the right
and left ventricles
It continues as the
posterior interventricular sulcus
which provides a
similar landmark on
the heart’s posterioinferior surface
Posterior
Interventricular
Sulcus
Anterior
Interventricular
Sulcus
Heart Chambers
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Except for the small,
wrinkled, protruding
appendages called
auricles, the atria are
free of distinguishing
surface features
The auricles increase
the atrial volume
slightly
Atria
Auricles
Heart Chambers
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Internally, the
posterior walls are
smooth, but the
anterior walls are
ridged by bundles of
muscle tissue
These muscle bundles
are called pectinate
muscles
Pectinate
Muscle
Heart Chambers
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The interatrial
septum bears a
shallow depression,
the fovea ovalis
This landmark marks
the spot where an
opening, the foramen
ovale, existed in the
fetal heart
Fovea
Ovalis
Heart Chambers
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Functionally, the atria are receiving
chambers for blood returning to the
heart from the circulation
Because they need to contract only
minimally to push blood into the
ventricles, the atria are relatively small,
thin walled chambers
As a rule they contribute little to the
propulsive pumping of the heart
Atria: The Receiving Chambers
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Blood enters the right
atrium via three veins
Superior
vena
cava
– Superior vena cava
• Returns blood from
body regions superior
to diaphragm
– Inferiorn vena cava
• Returns blood from
body areas below the
diaphragm
Coronary
– Coronary sinus
sinus
• Collects blood draining
from the myocardium
itself
Inferior
vena cava
Atria: The Receiving Chambers
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Blood enters the left
atrium via four veins
– Right and left
pulmonary veins
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The pulmonary veins
transport blood from
the lungs back to the
heart
Left
pulmonary
veins
Right
Pulmonary
veins
Posterior
view
Ventricles: Discharging Chambers
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Marking the internal
walls of the ventricle
chambers are irregular
ridges of muscle called
trabeculae carneae
The papillary muscles
project into the cavity
and play a role in valve
function
Papillary
muscles
Trabeculae
carneae
Ventricles: Discharging Chambers
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The ventricles are
the discharging
chambers of the
heart
Note the difference
in thickness of the
wall
When the ventricles
contract blood is
propelled out of the
heart and into
circulation
Atrial
Wall
Ventricular
Wall
Ventricles: Discharging Chambers
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The right ventricle
pumps blood into
the pulmonary
trunk, which routes
blood to the lungs
for gas exchange
The left ventricle
pumps blood into
the aorta, the
largest artery in the
systemic circulation
Aorta
Left
ventricle
Right
ventricle
Pulmonary
trunk
Pathway of Blood: Heart
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The heart is actually
two pumps, each
serving a separate
blood circuit
Blood vessels that
carry blood to the
lung form the
pulmonary circuit
(gas exchange)
Vessels carrying
blood to the body
form the systemic
circuit
Pathway of Blood: Heart
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The right side of the
heart forms the
pulmonary circuit
Blood returning from
the body enters the
right atrium and
passes into the right
ventricle
The ventricle pumps
the blood to the lungs
via the pulmonary
trunk
Pathway of Blood: Heart
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Blood in the
pulmonary circuit is
oxygen poor and
carbon dioxide rich
Once in the lungs the
blood unloads carbon
dioxide and picks up
oxygen
Freshly oxygenated is
carried back to the
heart by the
pulmonary veins
Pathway of Blood: Heart
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Note that the circulation of the pulmonary
circuit is unique
Typically veins carry oxygen poor blood to
the heart and arteries carry oxygen rich
blood
The pattern is reversed in the pulmonary
circuit with the pulmonary arteries
carrying oxygen poor blood to the lungs
and the pulmonary veins carrying oxygen
rich blood back to the heart
Pathway of Blood: Heart
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The left side of the
heart is the systemic
system pump
Freshly oxygenated
blood leaving the
lungs enters the left
atrium and passes
into the left ventricle
The left ventricle
pumps blood into the
aorta and from there
into many
distributing arteries
Pathway of Blood: Heart
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Smaller distributing
arteries carry the
blood to all parts of
the body
Gases, wastes and
nutrients are
exchanged across
capillary walls
Blood then returns to
the right atrium of
the heart via
systemic veins and
the cycle continues
Pathway of Blood: Heart
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Although equal volumes of blood are
flowing in the pulmonary and systemic
circuits at any one moment the two
ventricles have very unequal work loads
The pulmonary circuit, served by the right
ventricle, is a low pressure circulation
The systemic circuit, served by the left
ventricle, circulates through the entire
body and encounters about five times as
much resistance to blood flow
Pathway of Blood: Heart
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The fact that blood passes through heart
chambers sequentially does not mean
that the four chambers contract in that
order
Rather the two atria contract together,
followed by the simultaneous contraction
of the two venticles
Pathway of Blood: Heart
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A single sequence of atrial contraction
followed by the ventricular contraction is
a called a heartbeat
The heart of the average adult person at
rest beats 70-80 times a minute
Pathway of Blood: Heart
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The contraction of a heart chamber is
called a systole
The time during which a heart chamber
is relaxing and filling with blood is
termed diastole
Although both atrial and ventricular
chambers experience systole and diastole
the terms usually reference the ventricles
which are the dominant heart chambers
Ventricles: Discharging Chambers
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The difference in
system work load is
revealed in the
comparative
anatomy of the two
ventricles
The walls of the left
ventricle are three
times as thick as
those of the right
ventricle
Left
ventricle
Ventricles: Discharging Chambers
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The cavity of the left
ventricle is circular
The right ventricle
wraps around the
left and is crescent
shaped
The left can
generate much more
pressure than the
right and is a far
more powerful
pump
Left
ventricle
Pathway of Blood: System

Blood flows through the heart and other
parts of the circulatory system in one
direction
– Right atrium  right ventricle  pulmonary
arteries  lungs
– Lungs  pulmonary veins  left atrium  left
ventricle  body

This one way flow of blood is controlled by
four heart valves
Heart Valves
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Heart valves are
positioned between
the atria and the
ventricles and
between the
ventricles and the
large arteries that
leave the heart
Valves open and
close in response to
differences in blood
pressure
Tricuspid
valve
Bicuspid
(mitral)
valve
Aortic
valve
Pulmonary
valve
Heart Valves
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The valves of the
heart allow for the
blood to flow in
only one direction
Note: View of the
heart with the
superior atria
removed
Atrioventricular (AV) Valves
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The AV valves are
located at each
atrial-ventricular
junction
The valves are
positioned to prevent
a backflow of blood
into the atria when
the ventricles are
contracting
The valves are the
– Tricuspid valve
– Bicuspid valve
Tricuspid
valve
Bicuspid
(mitral)
valve
Atrioventricular (AV) Valves
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The right AV valve,
the tricuspid, has
three flexible cusps
The left AV valve, the
bicuspid, has two
flexible cusps
The cusps are flaps
of endocardium
reinforced by
connective tissue
Tricuspid
valve
Bicuspid
(mitral)
valve
Atrioventricular (AV) Valves
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Attached to each of
the AV valve flaps are
tiny collagen cords
called chordae
tendoneae
The cords anchor the
cusps to the papillary
muscles protruding
from the ventricular
walls
Papillary
muscles
Chordae
tendoneae
Atrioventricular (AV) Valves
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When the heart is
completed relaxed, the
AV valve flaps hang
limply into the
ventricular chambers
Blood flows into the
atria and then
through the open AV
valves into the
ventricles
Atria contract, forcing
additional blood into
ventricles
Atrioventricular (AV) Valves
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When the ventricles
begin to contract,
compressing the blood
in the chambers, intraventricular pressure
rises forcing blood
superiorly against the
valve flaps
The chordae tendoneae
and the papillary
muscles anchor the
flaps in their closed
position
Semilunar (SL) Valves
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The aortic and
pulmonary
semilunar valves are
located at the bases
of the large arteries
exiting the ventricles
The valves prevent
backflow of blood
from the aorta and
pulmonary trunk
into the associated
ventricles
Aortic
valve
Pulmonary
valve
Semilunar (SL) Valves
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Each semilunar valve
is made up of three
pocketlike cusps
Their mechanism of
closure differs from
that of the AV valves
When the ventricles
contract intraventricular pressure
exceeds the blood
pressure in the aorta
and pulmonary
trunk
Semilunar (SL) Valves
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Blood pressure from
the ventricle forces
the semilunar valves
open and blood is
forced past the valve
and into the artery
When the ventricles
relax, and the blood
flows backward
toward the heart it
fills the cusps which
closes the valves
Heart Sounds
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The closing of the heart valves causes
vibrations in the adjacent blood and
heart walls that account for the familiar
“lub-dup” sounds of the heartbeat
The “lub” is produced by the closing of
the AV valves at the start of ventricular
systole
The “dup” is produced by the closing of
the semilunar valves at the end of
ventricular systole
Fibrous Skeleton
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The fibrous
skeleton of the
heart lies in the
plane between the
atria and the
ventricles
It surrounds the
four valves
It is composed of
dense connective
tissue
Fibrous Skeleton

The fibrous skeleton has four functions
– It anchors the valve cusps
– It prevents overdilation of the valve openings
as blood pulses through them
– It is the point of insertion for the bundles of
cardiac muscle in the atria and ventricles
– It blocks the direct spread of electrical
impulses from the atria to the ventricles
Conducting System
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Cardiac muscle cells have an intrinsic
ability to generate and conduct impulses
that signal these same cells to contract
rhythmically
These properties are intrinsic to the heart
muscle itself and do not depend on
extrinsic nerve impulses
Even if all nerve connections to the heart
are severed, the heart continues to beat
rhythmically
Conducting System

The conducting system of the heart is a
series of specialized cardiac muscle cells
that carries impulses throughout the
heart musculature, signaling the heart
chambers to contract in proper sequence
Conducting System

The components of
the conducting
system are:
– Sinoatrial node
– Internodal fibers
– Atrioventricular
node
– Atrioventricular
bundle
– Right an left
branches
– Purkinje fibers
Conducting System
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The impulse that
signals each
heartbeat begins
at the sinoatrial
(SA) node
This is a crescent
shaped mass of
muscle cells that
lies in the wall of
the right atrium,
below the entrance
of the superior
vena cava
Conducting System

The sinoatrial
node, the heart’s
own pacemaker,
sets the basic heart
rate by generating
70-80 impulses per
minute
Conducting System

The sequence that controls each
heartbeat - atrial contraction followed by
ventricular contraction is specific
Conducting System

Impulses from the
SA node spread in
a wave along the
cardiac muscle
fibers of the atria
signaling the atria
to contract
Conducting System

Some of these
impulses travel
along the intranodal
pathway to the
atrioventricular
(AV) node in the
inferior part of the
interatrial septum,
where they are
delayed for a
fraction of a second
Conducting System

After this delay,
the impulses race
through the atrioventricular bundle
which enters the
interventricular
septum and
divides into right
and left bundle
branches
Conducting System

About halfway
down the septum,
the Bundle fibers,
(crura), become
bundles of
Purkinje fibers
which approach
the apex of the
heart, then turn
superiorly into the
ventricular walls
Conducting System
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This arrangement of conducting structures
ensures that the contraction of the
ventricles begins at the apex of the heart
and travels superiorly, so that the
ventricular blood is ejected superiorly into
the great arteries
The brief delay of the contraction signaling
impulses at the AV node enables the
ventricles to fill completely before they
start to contract
Conducting System

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Because the fibrous skeleton between the
atria and ventricles is nonconducting, it
prevents impulses in the atrial wall from
proceeding directly on to the ventricular
wall
As a result, only those signals that go
through the AV node can continue on
Conducting System


Examination of the microscopic anatomy
of the heart’s conducting system reveals
that the cells of the nodes and AV bundle
are small, but otherwise typical cardiac
muscle cells
Each Purkinje fiber, by contrast, is a long
row of special, large-diameter cells called
Purkinje myocytes
Conducting System
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Purkinje myocytes are cardiac muscle
cells containing relatively few
myofilaments because they are adapted
more for conduction than contraction
Their large diameter maximizes the speed
of impulse conduction
Purkinje fibers are located in the deepest
part of the ventricular endocardium,
between the endocardium and
myocardium layers
Innervation

Although the heart’s
inherent rate of
contraction is set by the
SA node, this rate can
be altered by extrinsic
neural controls
Innervation

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
The nerves to the heart
consist of visceral
sensory fibers
Parasympathetic fibers
that slow heart rate
Sympathetic fibers that
increase the rate and
force of heart
contractions
Innervation

Parasympathetic
nerve fibers arise
as branches of the
Vagus nerve in the
neck and thorax
Innervation


Sympathetic nerves
travel to the heart
from the cervical
and upper thoracic
chain ganglia
All nerves serving
the heart pass
through the
cardiac plexus on
the trachea before
entering the heart
Innervation

Although autonomic
fibers project to cardiac
musculature
throughout the heart,
they project most
heavily to the SA and
VA nodes and the
coronary arteries
Innervation


The autonomic input to
the heart is controlled by
cardiac centers in the
reticular formation of
the medulla of the brain
In the medulla, the
cardio-inhibitory center
influences
parasympathetic
neurons, whereas the
cardioacceleratory
center influences
sympathetic neurons
Innervation

These medullary
cardiac centers, in turn,
are influenced by such
higher brain regions as
the hypothalamus,
periaqueductal gray
matter, amygdala, and
insular cortex
Coronary Circulation


The coronary
circulation, the
functional blood
supply of the heart,
is the shortest
circulation in the
body
The arterial supply
of the coronary
circulation is
provided by the
right and left
coronary arteries
Coronary Circulation


The left coronary
artery runs toward
the left side of the
heart and then
divides into its major
branches
Anterior
interventricular
artery follows the
sulcus and supplies
blood to the interventricular septum
and walls of ventricle
Coronary Circulation



The right coronary
artery courses to the
right side of the heart
where it divides
The marginal artery
serves the myocardium of the lateral
part of the right side
of the heart
The posterior interventricular artery
runs to the apex of the
heart
Coronary Circulation


There are many merging blood vessels
that delivery blood to the heart muscle
This explains how the heart can receive
an adequate supply when one of its
coronary arteries is almost entirely
occluded
Coronary Circulation




The coronary arteries provide an intermittent pulsating flow to the myocardium
These vessels and their main branches lie
in the epicardium and send branches
inward to nourish the myocardium
Although the heart represents only about
1/200 of body weight, it requires 1/20 of
the body’s blood supply
The left ventricle receives the largest
proportion of the blood supply
Coronary Circulation


After passing
through the myocardium, the venous
blood is collected by
the cardiac veins
The veins join
together to form an
enlarged vessel
called the coronary
sinus which empties
into the right
atrium
End of Material
Chapter 18