Chapter 20
The circulatory system II
The Heart
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Circulatory systems
Pulmonary circuit:
Blood flows to the lungs
to exchange CO2 for
O2 in the alveoli.
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Systemic circuit:
Provides oxygenated
blood to all organs and
tissues in the body,
including the heart and
lungs.
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The Heart
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Located: in the thoracic cavity in the mediastinum
between the lungs and deep to the sternum.
Function: to pump blood to all tissues and cells in
the body
Structure: the base of the heart is superior, and is
where the great vessels are attached. The heart tapers
down to the apex (tip) situated immediately above
the diaphragm (left upper quadrant).
Size/Orientation – weighs ~300 gm, ~9 cm
diameter at base,~13 cm from base to apex and 6
cm diameter at the apex.
Location and Orientation
The pericardium
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Two coverings that surround the heart, enclosing it in
a sac with a tough outer layer, the fibrous pericardium
and a thin serous parietal pericardium, with a serous
visceral pericardium (epicardium) attached to the
myocardium. The pericardial space between these
layers is filled with pericardial fluid.
Tissues of the Wall of the Heart
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The outer layer is epicardium and is a serous
membrane (the visceral pericardium).
The myocardium is cardiac muscle and consists of
layers circularly and spirally wrapped around each
other.
The endocardium is a layer of specialized epithelial
cells called endothelium. Endothelial cells cover the
heart valves and line the entire cardiovascular and
lymphatic system.
Pattern of cardiac muscle wrap
Heart Chambers
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The heart contains four chambers: 2 upper atria and 2
lower ventricles
The right side of the heart (right atrium and right
ventricle) receives oxygen poor blood (high in CO2)
from the body and pumps it to the lungs where
exchange of CO2 for O2 takes place.
The left side of the heart (left atrium and left
ventricle) receives oxygenated blood (high in O2)
from the lungs and pumps it to every cell in the body.
Heart chambers
Heart Structure
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The atria (right and left) are separated from each other
by the thin interatrial septum.
The ventricles (left and right) are separated from each
other by a thick muscular interventricular septum.
The atria and ventricles are separated from each other
by valves that regulate blood flow in a one way
direction and by dense fibrous connective tissue.
The ventricles are separated from their outflow
trunks, the pulmonary trunk and the aorta (great
vessels), by semilunar valves.
Right Atrium
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Oxygen poor blood from the entire body and the
heart flows into the right atrium via the superior and
inferior vena cavae and the coronary sinus.
Occupies most of upper right side of heart
Right auricle is conspicuous, looks like a serrated dogs
ear.
Externally right atrium is separated from right
ventricle by right coronary sulcus containing the right
coronary artery.
Internally, the right atrioventricular (tricuspid) valve
separates the right atrium and right ventricle.
Right Atrium
Right ventricle
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Receives blood from RA via tricuspid valve.
Externally it occupies most of anterior right side of
heart.
Internally the RV exhibits muscular ridges (trabeculae
carneae) and papillary muscles with chordae tendonae
that anchor the tricuspid valve. The chordae tendonae
prevent the valve from flopping (prolapsing) back into
the right atrium when RV pumps and assures a one
way flow of the blood.
Pumps blood through pulmonary semilunar valve into
pulmonary trunk for delivery to the lungs.
Right ventricle
Left atrium
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Receives oxygen rich blood from the lungs via 2 to 5
pulmonary veins (right and left).
Separated from right atrium by the interatrial septum,
with the fossa ovale, a remnant of fetal foramen ovale.
Left auricle is on the upper left side of heart and is the
only part of LA visible from front of heart.
Separated externally from the left ventricle by the left
coronary sulcus and internally by the left
atrioventricular (bicuspid or mitral) valve.
Pumps oxygen rich blood into left ventricle.
Left auricle of left atrium
Left ventricle
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Large thick walled pumping chamber of the heart.
Receives oxygen rich blood from LA via bicuspid
valve (left atrioventricular valve).
Contains trabeculae carneae, papillary muscles and
chordae tendonae similar to right ventricle.
Occupies anterior inferior wall of heart down to apex.
Pumps oxygen rich blood through Aortic valve (left
semiliunar valve) into the aorta.
Externally anterior interventricular sulcus separates
RV from LV and contains left anterior descendens
(LAD) coronary (anterior interventricular) artery.
Left ventricle
Ventricular wall thicknesses
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The left ventricular wall is much thicker than the right,
as it must pump blood to the entire body and back
again, whereas the right ventricle pumps blood at
lower pressure through the lungs and back to the left
atrium.
The right side is a lower pressure side as compared to
the left side of the heart (~20mm Hg vs 120 mm Hg).
Comparison of wall thickness
Heart Valves
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The four heart valves assure one way flow of blood
through the heart.
Atrioventricular valves are tricuspid (right side) and
bicuspid or mitral (left side). Attached to ventricular
walls by chordae tendonae to prevent prolapse.
The pulmonary (right side) and aortic (left side)
semilunar valves contain three half moon-shaped
cusps each.
All valves are made of flaps of endocardium and are
reinforced by dense connective tissue.
The heart valves open and close depending on the
pressure exerted on them within each chamber.
Papillary muscle connected to chordae tendonae
Heart Valves
Heart Valves/Open-Closed
Papillary muscle connected to chordae tendonae
Heart Valves/Open-Closed
Blood flow in heart
Blood flow to the heart
Left and right coronary
arteries come off of
aorta behind cusps.
Venous return from
heart drains into great
cardiac vein and into
coronary sinus and
into right atrium
Coronary arteries
Coronary veins
Coronary stenosis
Cardiac conduction system
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For the heart to pump it must be activated by an
electrical impulse generated from its own pacemaker
tissue. This site is the sinoatrial (SA) node located in
the right atrium. The impulse spreads through the
atrial muscle to the atrioventricular (AV) node → AV
bundle → the right and left bundle branches →
Purkinje fibers → ventricular depolarization and
contraction.
Cardiac tissue
The myocardium is a syncitium of muscle fibers that
interdigitate at intercalated discs
Cardiac tissue
Cardiac conduction system
Conduction pathways
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Sinoatrial Node (SA Node)-“sinus rhythm” – rate
generated by the sinoatrial node in right atrium
Atrioventricular Node (AV Node)- located near
tricuspid valve; AV node acts as electrical gateway to ventricles
Atrioventricular Bundle/”Bundle of His”- pathway
by which impulses leave AV node. Forks into right and left
bundles branches which enter interventricular septum and
descend to the apex.
Right/Left Bundle Branches- continuation of Bundle of
His which descend to apex and gives rise to the purkinje fibers.
Purkinje Fibers- conducting fibers which turn upward and
extend into ventricular myocardium. They form a more elaborate
network in LV than RV.
Cardiac conduction system
Cardiac innervation
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Although the heart can beat without neural control, it
receives both sympathetic and parasympathetic input
from the autonomic nervous system.
The parasympathetic input is via CN - X (Vagus)
Sympathetic input is from the medullary
cardioacclerator center → sympathetic chain
(paravertebral) ganglia → the sympathetic nerve.
The heart can also be regulated by neuro-humors
circulating in the blood.
Sympathetic and
parasympathetic
input to the heart.