Transcript Review of Cardiac Structure and Function

Review of Cardiac
Structure and Function
Location
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Heart lies in the mediastinum behind
the sternum
1/3 of bulk of the heart lies to the
right of the midline; 2/3 lies to the left
Apex lies between the 4th & 5th ribs
when lying down and between the 5th
and 6th ribs when sitting or standing
Pericardium
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The heart lies within the pericardium
Outer layer of the pericardium is fibrous
connective tissue – fibrous pericardium
Anchors the pericardium to the great
vessels, the sternum and diaphragm
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Two inner layers of serous membranesparietal pericardium and visceral
pericardium (epicardium)
Between the serous membranes is the
pericardial space, filled with 15- 50 ml of
fluid to reduce friction
Fibrous pericardium and parietal
pericardium are called the pericardial sac
Visceral pericardium is the epicardium
Heart Wall
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Epicardium
Myocardium – muscular layer
– Thickest layer
– Attached to fibrous skeleton of the heart
– Bands of muscle arranged longitudinally
– Fibers from one side enter other side
– Better integration of contractions
– Pathology of one ventricle can affect the
other
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Endocardium
– Internal lining of squamous epithelium
– Continuous throughout cardiovascular
system
Four chambers (two
pumps)
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Right heart acts as a volume pump –
through low-resistance vessels of the
pulmonary system
Left heart acts as a pressure pump –
through high-resistance vessels of the
systemic circulation
Atria
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Two upper, thin walled chambers that
collect blood returning to the heart
Their contraction aids in filling the
ventricles
Right atrium receives blood from
superior and inferior vena cavae and
coronary sinus
Left atrium receives blood from
pulmonary veins
Ventricles
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Thick walled pumping chambers of the
heart
Make up about 60 % of the mass of the
heart and receive most of coronary blood
flow
Blood from right ventricle enters the
pulmonary trunk
Blood from left ventricle enters the aorta
Path of Blood Flow
Vena cavae → RA→ RV→
Pulmonary trunk→ Lungs→
Pulmonary veins→ LA→ LV→
Aorta → Systemic circulation→
Vena cavae
Valves
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Made of connective tissue covered by
endothelium
Prevent back flow of blood
Open and close passively in response
to ventricular contraction
Anchored to the annuli fibrosi cordis to
prevent dilation during contraction of
the heart
Atrioventricular valves
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Lie between the atria and ventricles
Tricuspid valve on the right
Bicuspid or mitral valve on left
Anchored against high pressure by the
chordae tendineae and papillary
muscles
Semilunar Valves
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Lie between ventricles and great
vessels
Pulmonary on right side
Aortic on left side
Cardiac Cycle
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Sequence of events that compose the
repeating pumping action of the heart
Typically, systole refers to ventricular
contraction and diastole refers to
ventricular relaxation
If referring to atria, specify atrial
systole, etc.
Review
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Coronary arteries and cardiac veins –
note that it is the right coronary artery
that supplies blood to the SA and AV
nodes
Review
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Pressures in chambers: look over the
next figure - note especially the
pressures in the right atrium – central
venous pressure, and the pressures in
the aorta – systemic blood pressure
Cardiac muscle Myocytes
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Contractile fibers
Fibers are branched
Intercalated discs - contain
desmosomes and gap junctions
Gap junctions connect cytoplasm of
adjoining cells so that the action
potentials pass from one cell to the
next
Cells contract as if one cell - syncytium
Muscle contraction
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Striated muscle – contains actin and myosin
Action potential stimulates release of Ca++
Ca++ binds to troponin
Troponin moves the tropomyosin exposing
myosin binding sites on the actin
Myosin pulls on the actin, filaments slide
past each other and muscle shortens
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Remember, that unlike skeletal muscle,
much of the Ca++ used in cardiac muscle
contraction comes from the extracellular
fluid.
Conduction System
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Conduction system cells are specialized
myocardial fibers
Heart has autorhythmicity- beats
spontaneously at about 100 beats/min
Impulse begins at sinoatrial (SA) node or
pacemaker – sinus rhythm
Spreads through atria via conducting
myofibers to atrioventricular (AV) node in
fibrous skeleton of heart
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Signal pauses briefly, for ventricular filling
by atrial contraction.
Signal continues into ventricle through
conduction pathways:
AV bundle ( bundle of His) →right and
left bundle branches→ Purkinje fibers→
ventricular contraction (bottom up)
Regulation of Heart Rate
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Sympathetic N.S. increases heart rate and
force of contraction – secrete
norepinephrine –accelerator nerves
Parasympathetic N.S. decrease heart rate
and force of contraction through the
vagus nerve. Sends continuous impulses.
Secretes acetylcholine
Electrocardiogram ECG
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Measures electrical activity of the
heart
P wave – represents atrial contraction
(depolarization)
QRS complex – represents ventricular
contraction (depolarization)
T wave – represents ventricular
relaxation (repolarization)
Cardiac Output
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Cardiac Output:
SV (ml/beat) X HR (beats/min) =
CO(ml/min.)
70 ml
X 80
= 5600 ml
/min. or
5.6
liters L/min
Factors affecting cardiac
performance
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Preload: pressure generated at the end
of diastole; depends on both heart and
vascular system –the amount of filling of
the ventricle during relaxation
Afterload: resistance to ejection during
systole; depends on both heart and
vascular system - the force that opposes
ejection of blood from the heart; for the
LV, this is the aortic systolic pressure
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Heart rate: a intrinsic characteristic of
the heart tissue that is influenced by
nervous and endocrine systems
Myocardial contractility: the ability of
the heart muscle to contract, the force of
contraction - another cardiac tissue
characteristic that is influenced by nervous
and endocrine systems
(Frank-) Starling Law
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Within limits, the greater the
stretching of the muscle fibers
(preload), the greater the force of
contraction.
The extra force of contraction is
necessary to pump the increased
volume of blood from the ventricle.
Cardiac output increases
Neural reflexes
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Bainbridge reflex – increased heart
rate due to increased right atrial
pressure
Increased pressure in arteries
stimulates a baroreceptor reflex that
decreases heart rate.