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The Heart

Chapter 18

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Introduction

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

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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

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Introduction

The blood vessels

that carry bloodfrom the lungs

form the

pulmonary circuit

The vessels that

carry blood to all

the body tissues

form the systemiccircuit

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Heart Size, Location and Position 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 2ndrib to 5th intercostal space

Rests on the superior

surface of diaphram

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Heart Size, Location and Position

The lungs flank the heart laterally and partially

obscure it

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Heart Size, Location and Position The heart lies anterior to

the vertebral column andposterior to the sternum

Two thirds of the heart

lies to the left of the mid-

sternal line; the balance

projects to the right

Its broad flat base, or

posterior surface, pointsto right shoulder

The apex points toward

the left hip

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Location - 4 Corners The heart is has four

corners projected ontothe anterior thoracic

wall

Superior right - where

the costal cartilage joins the 3rd rib

Superior left - costal

cartilage of 2nd rib afingers breadth lateral

to the sternum

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Location - 4 Corners The inferior right - lies

at the costal cartilageof 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

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Coverings of the Heart

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

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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

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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

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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

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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

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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 fluidseep into the pericardial cavity, compressing

the heart and limiting its ability to pump

blood

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Layers of the Heart Wall

The heart wall is composed of three layers

–

Superficial layer of epicardium– Middle layer of myocardium

– Deep layer of endocardium

All three layers are richly supplied with blood vessels

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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

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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

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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

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The connective tissue

forms a dense networkcalled the internal

skeleton of the heart

It reinforces the

myocardium internally

and anchors the cardiac

muscle

This network of fibers isthicker in some areas

than in others to rein-

force valves and where

the major vessels exit

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The internal skeleton

prevents overdilation ofvessels due to the

continual stress of blood

pressure

Additionally, since

connective tissue is not

electrically excitable, it

limits action potentialsacross the heart to

specific pathways

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The endocardium is a glistening white sheet of

endothelium (squamous epithelium) resting on a thinlayer of connective tissue

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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

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Heart Chambers

The heart has four

chambers– Two superior atria

– Two inferior ventricles

The longitudinal wallseparating the

chambers is called the

– Interartial septum

•Between atria

– Interventricular

septum

• Between ventricles

Atria

Septum

Ventricles

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Heart Chambers

The right ventricle

forms most of theanterior surface of the

heart

The left ventricledominates the inferio-

posterior aspect of the

heart and forms the

heart apex

Right Ventricle

Left

Ventricle

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Heart Chambers

Two grooves visible

on the surface of theheart indicate the

boundaries of its four

chambers and carry

the blood vessels that

supply myocardium

The Atrioventricular

groove or coronarysulcus encircles the

junction of the atria

and ventricles Coronary

Sulcus

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Heart Chambers

The anterior inter-

ventricular sulcus,separates the right

and left ventricles

It continues as theposterior inter-

ventricular sulcus

which provides a

similar landmark onthe heart’s posterio-

inferior surface

Anterior

InterventricularSulcus

Posterior

Interventricular

Sulcus

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Heart Chambers

Except for the small,

wrinkled, protrudingappendages called

auricles, the atria are

free of distinguishing

surface features

The auricles increase

the atrial volume

slightly

Auricles

Atria

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Heart Chambers

Internally, the

posterior walls aresmooth, but the

anterior walls are

ridged by bundles of

muscle tissue

These muscle bundles

are called pectinate

muscles

PectinateMuscle

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Heart Chambers

The interatrial

septum bears ashallow depression,

the fovea ovalis

This landmark marks

the spot where an

opening, the foramen

ovale, existed in the

fetal heart

Fovea

Ovalis

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Heart Chambers

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 theventricles, the atria are relatively small,

thin walled chambers

As a rule they contribute little to thepropulsive pumping of the heart

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Atria: The Receiving Chambers

Blood enters the right

atrium via three veins– Superior vena cava

• Returns blood from

body regions superior

to diaphragm– Inferiorn vena cava

• Returns blood from

body areas below the

diaphragm

– Coronary sinus

• Collects blood draining

from the myocardium

itself Inferior

vena cava

Superiorvena

cava

Coronary

sinus

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Atria: The Receiving Chambers

Blood enters the left

atrium via four veins– Right and left

pulmonary veins

The pulmonary veins

transport blood from

the lungs back to the

heart

Posterior

view

Left

pulmonary

veins

Right

Pulmonary

veins

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Ventricles: Discharging Chambers

Marking the internal

walls of the ventriclechambers are irregular

ridges of muscle called

trabeculae carneae

The papillary muscles

project into the cavity

and play a role in valve

function

Trabeculae

carneae

Papillary

muscles

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Ventricles: Discharging Chambers The ventricles are

the dischargingchambers of the

heart

Note the difference

in thickness of the

wall

When the ventricles

contract blood ispropelled out of the

heart and into

circulation

Atrial

Wall

Ventricular

Wall

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Ventricles: Discharging Chambers The right ventricle

pumps blood intothe pulmonary

trunk, which routes

blood to the lungs

for gas exchange

The left ventricle

pumps blood into

the aorta, thelargest artery in the

systemic circulation

Aorta

Left

ventricle

Right

ventricle

Pulmonary

trunk

f

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Pathway of Blood: Heart

The heart is actually

two pumps, eachserving 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

P h f Bl d H

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The right side of the

heart forms thepulmonary circuit

Blood returning from

the body enters the

right atrium and

passes into the right

ventricle

The ventricle pumpsthe blood to the lungs

via the pulmonary

trunk

P h f Bl d H

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Blood in the

pulmonary circuit isoxygen poor and

carbon dioxide rich

Once in the lungs the

blood unloads carbon

dioxide and picks up

oxygen

Freshly oxygenated iscarried back to the

heart by the

pulmonary veins

P h f Bl d H

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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 lungsand the pulmonary veins carrying oxygen

rich blood back to the heart

P th f Bl d H t

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The left side of the

heart is the systemicsystem pump

Freshly oxygenated

blood leaving the

lungs enters the left

atrium and passes

into the left ventricle

The left ventriclepumps blood into the

aorta and from there

into many

distributing arteries

P th f Bl d H t

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Smaller distributing

arteries carry theblood to all parts of

the body

Gases, wastes and

nutrients are

exchanged across

capillary walls

Blood then returns tothe right atrium of

the heart via systemic

veins and the cycle

continues

P th f Bl d H t

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Although equal volumes of blood are

flowing in the pulmonary and systemiccircuits at any one moment the two

ventricles have very unequal work loads

The pulmonary circuit, served by the rightventricle, is a low pressure circulation

The systemic circuit, served by the left

ventricle, circulates through the entirebody and encounters about five times as

much resistance to blood flow

P th f Bl d H t

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The fact that blood passes through heart

chambers sequentially does not meanthat the four chambers contract in that

order

Rather the two atria contract together,followed by the simultaneous contraction

of the two venticles

P th f Bl d H t

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A single sequence of atrial contraction

followed by the ventricular contraction isa called a heartbeat

The heart of the average adult person at

rest beats 70-80 times a minute

P th f Bl d H t

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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 ventricleswhich are the dominant heart chambers

V i l Di h i Ch b

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Ventricles: Discharging Chambers The difference in

system work load isrevealed in the

comparative

anatomy of the two

ventricles

The walls of the left

ventricle are three

times as thick asthose of the right

ventricle

Left

ventricle

V i l Di h i Ch b

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Ventricles: Discharging Chambers 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 theright and is a far

more powerful

pump

Left

ventricle

P th f Bl d S t

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Pathway of Blood: System

Blood flows through the heart and other

parts of the circulatory system in onedirection

– Right atrium right ventricle pulmonary

arteries lungs– Lungs pulmonary veins left atrium left

ventricle body

This one way flow of blood is controlled byfour heart valves

H t V l

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Heart Valves

Heart valves are

positioned betweenthe atria and the

ventricles and

between the

ventricles and the

large arteries that

leave the heart

Valves open andclose in response to

differences in blood

pressure

Bicuspid

(mitral)

valve

Aortic

valve

Pulmonaryvalve

Tricuspid

valve

Heart Val es

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Heart Valves

The valves of the

heart allow for theblood to flow in

only one direction

Note: View of the

heart with the

superior atria

removed

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Atrioventricular (AV) Valves

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The right AV valve,

the tricuspid, hasthree flexible cusps

The left AV valve,

the bicuspid, has two

flexible cusps

The cusps are flaps

of endocardium

reinforced byconnective tissue

Bicuspid

(mitral)

valve

Tricuspid

valve


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Attached to each of

the AV valve flapsare tiny collagen

cords called chordae

tendoneae

The cords anchor the

cusps to the papillary

muscles protruding

from the ventricularwalls

Chordae

tendoneae

Papillary

muscles


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When the heart is

completed relaxed, theAV 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


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When the ventricles

begin to contract,compressing the blood

in the chambers, intra-

ventricular pressure

rises forcing bloodsuperiorly against the

valve flaps

The chordae tendoneaeand the papillary

muscles anchor the

flaps in their closed

position

Semilunar (SL) Valves

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The aortic and

pulmonarysemilunar valves are

located at the bases

of the large arteries

exiting the ventricles

The valves prevent

backflow of blood

from the aorta andpulmonary trunk

into the associated

ventricles

Aortic

valve

Pulmonaryvalve


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Each semilunar valve

is made up of threepocketlike cusps

Their mechanism of

closure differs from

that of the AV valves

When the ventricles

contract intra-

ventricular pressureexceeds the blood

pressure in the aorta

and pulmonary trunk


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Blood pressure from

the ventricle forcesthe semilunar valves

open and blood is

forced past the valve

and into the artery

When the ventricles

relax, and the blood

flows backwardtoward the heart it

fills the cusps which

closes the valves

Heart Sounds

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Heart Sounds

The closing of the heart valves causes

vibrations in the adjacent blood andheart walls that account for the familiar

“lub-dup” sounds of the heartbeat

The “lub” is produced by the closing ofthe AV valves at the start of ventricular

systole

The “dup” is produced by the closing ofthe semilunar valves at the end of

ventricular systole

Fibrous Skeleton

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Fibrous Skeleton

The fibrous

skeleton of theheart lies in the

plane between the

atria and the

ventricles

It surrounds the

four valves

It is composed ofdense connective

tissue

Fibrous Skeleton

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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 ofcardiac 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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Conducting System

Cardiac muscle cells have an intrinsic

ability to generate and conduct impulsesthat signal these same cells to contract

rhythmically

These properties are intrinsic to the heartmuscle itself and do not depend on

extrinsic nerve impulses

Even if all nerve connections to the heartare severed, the heart continues to beat

rhythmically

Conducting System

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Conducting System

The conducting system of the heart is a

series of specialized cardiac muscle cellsthat carries impulses throughout the

heart musculature, signaling the heart

chambers to contract in proper sequence

Conducting System

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Conducting System

The components of

the conductingsystem are:

– Sinoatrial node

– Internodal fibers

– Atrioventricular

node

– Atrioventricular

bundle

– Right an left

branches

– Purkinje fibers

Conducting System

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Conducting System

The impulse that

signals eachheartbeat begins at

the sinoatrial (SA)

node

This is a crescent

shaped mass of

muscle cells that

lies in the wall ofthe right atrium,

below the entrance

of the superior

vena cava

Conducting System

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Conducting System

The sinoatrial

node, the heart’sown pacemaker,

sets the basic heart

rate by generating

70-80 impulses perminute

Conducting System

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Conducting System

The sequence that controls each

heartbeat - atrial contraction followed byventricular contraction is specific

Conducting System

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Conducting System

Impulses from the

SA node spread ina wave along the

cardiac muscle

fibers of the atria

signaling the atriato contract

Conducting System

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Conducting System

Some of these

impulses travelalong the intranodal

pathway to the

atrioventricular

(AV) node in theinferior part of the

interatrial septum,

where they are

delayed for a

fraction of a second

Conducting System

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Conducting System

After this delay,

the impulses racethrough the atrio-

ventricular bundle

which enters the

interventricularseptum and

divides into right

and left bundle

branches

Conducting System

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Conducting System

About halfway

down the septum,the Bundle fibers,

(crura), become

bundles of

Purkinje fiberswhich approach

the apex of the

heart, then turn

superiorly into the

ventricular walls

Conducting System

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Conducting System

This arrangement of conducting structures

ensures that the contraction of theventricles begins at the apex of the heart

and travels superiorly, so that the

ventricular blood is ejected superiorly intothe great arteries

The brief delay of the contraction

signaling impulses at the AV node enablesthe ventricles to fill completely before they

start to contract

Conducting System

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Conducting System

Because the fibrous skeleton between the

atria and ventricles is nonconducting, itprevents 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

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Conducting System

Examination of the microscopic anatomy

of the heart’s conducting system revealsthat 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

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Innervation

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Innervation

Although the heart’s

inherent rate ofcontraction is set by the

SA node, this rate can

be altered by extrinsic

neural controls

Innervation

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Innervation

The nerves to the heart

consist of visceralsensory fibers

Parasympathetic fibers

that slow heart rate

Sympathetic fibers that

increase the rate and

force of heart

contractions

Innervation

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Innervation

Parasympathetic

nerve fibers ariseas branches of the

Vagus nerve in the

neck and thorax

Innervation

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Innervation

Sympathetic nerves

travel to the heartfrom the cervical

and upper thoracic

chain ganglia

All nerves serving

the heart pass

through the

cardiac plexus onthe trachea before

entering the heart

Innervation

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Innervation

Although autonomic

fibers project to cardiacmusculature

throughout the heart,

they project most

heavily to the SA andVA nodes and the

coronary arteries

Innervation

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e vat o

The autonomic input to

the heart is controlled bycardiac 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

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These medullary

cardiac centers, in turn,are influenced by such

higher brain regions as

the hypothalamus,

periaqueductal graymatter, amygdala, and

insular cortex

Coronary Circulation

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y

The coronary

circulation, thefunctional blood

supply of the heart,

is the shortest

circulation in thebody

The arterial supply

of the coronary

circulation is

provided by the

right and left

coronary arteries


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y

The left coronary

artery runs towardthe left side of the

heart and then

divides into its major

branches Anterior

interventricular

artery follows the

sulcus and supplies

blood to the inter-

ventricular septum

and walls of ventricle


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y

The right coronary

artery courses to theright side of the heart

where it divides

The marginal artery

serves the myo-cardium of the lateral

part of the right side of

the heart

The posterior inter-

ventricular artery runs

to the apex of the heart


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y

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 entirelyoccluded


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y

The coronary arteries provide an inter-

mittent 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

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End of Material

Chapter 18

chap18 heart 000

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