Download The Heart

The Heart
Part
of the Cardiovascular System
The transport system that keeps
blood circulating
Structure and Function
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250-350 grams
Within the thorax, in the mediastinum
2/3 lies to the left of midsternal line
Base – broad flat posterior surface is
directed toward right shoulder
Apex–points inferiorly toward left hip
Homeostatic Imbalance
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Pericarditis – inflammation of the
pericardium
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Hinders production of serous fluid and
roughens serous membrane surfaces
“Creaking sound” when heart rubs
against pericardial sac, pain deep to
sternum
Severe cases – cardiac tamponade
“heart plug” caused by fluid compressing
the heart
Conduction System of Heart
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Rhythmical electrical activity causes
continuous beating
Autorhythmic cells are self-excitable
and repeatedly generate action
potentials that trigger contractions
(heart continues to beat after nerves
cut)
Pacemaker that forms conduction
system
Pacemaker
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Cardiac chambers stimulated in
coordinated manner to pump blood
Autonomic nervous system and
hormones modify but don’t establish
the rhythm
Conduction of Impulse
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Cardiac excitation begins in the
sinoatrial (SA) node in right atrial wall
inferior to opening of sup. vena cava
Action potential propagates
throughout both atria and atria
contract
Action potential reaches
atrioventricular (AV) node → bundle of
His (connection between atria and ventricles)
Conduction
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→ right and left bundle branches toward the
apex → Purkinje fibers → throughout
ventricles
About 200 milliseconds after the atria contract,
the ventricles contract
Homeostatic Imbalance
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Angina pectoris or “choked chest”
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Thoracic pain caused by fleeting
deficiency in blood delivery to
myocardium
Serious, prolonged blockage can lead to
heart attack or coronary (myocardial
infarction MI) because blockage results
in noncontractile scar tissue
Homeostatic Imbalance
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Valve deformities
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Incompetent valve – forces heart to
repump same blood; improper closure
and blood back flows
Valvular stenosis – valve flap becomes
stiff and constricts the opening (scar
tissue after endocarditis or Ca2+ salt
deposit)
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Both increase work load and weaken heart
Faulty valve is usually the mitral (bicuspid)
Homeostatic Imbalance
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Heart region deprived of blood becomes
ischemic
Cells begin to metabolize anaerobically
producing lactic acid
Increased acidity inhibits cardiac cells’
ability to produce ATP (ATP needed to
pump Ca2+ into extracellular space)
Increased H+ and Ca2+ inside cell causes
gap junctions to close
Forces action potentials to find alternate
routes (if area is large leads to heart
attack)
Homeostatic Imbalance
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Defect in intrinsic conduction system
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Irregular heart rhythms (arrhythmias)
Uncoordinated atrial and ventricular contractions
Fibrillation: rapid and irregular or out of phase
contractions
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Heart rhythm taken away from the SA node
(pacemaker) by rapid activity in other heart
regions
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Defibrillation – electrically shocking the heart
which stops the chaotic twitching by
depolarizing the entire myocardium
Homeostatic Imbalance
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Defective SA node (Sinoatrial node)
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Ectopic focus – abnormal pacemaker
takes over pacing of heart
AV node may take over (atrioventricular)
Only route for impulse transmission from
atria to ventricles is through AV node
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Damage to AV node = heart block and
interferes with the ability of ventricles to
receive pacing impulses
Homeostatic Imbalance
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Heart murmurs are abnormal heart
sounds
Healthy blood flow is silent in adults
Sounds may indicate valve problems
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Incompetent valve = swishy sound with
backflow through the partially open valve
after the valve has closed
Stenotic valve = “narrowed” high-pitch
sound or click when valve should be wide
open during systole
Homeostatic Imbalance
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Reduced Ca2+ blood levels (hypocalcemia)
depress the heart
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Hypercalcemia prolongs the plateau phase of the
action potential that can lead to spastic heart
Excess Na+ (hypernatremia) inhibits
transport of Ca2+ into cardiac cells and
blocks heart contractions
Excess K+ (hyperkalemia) interferes with
depolarization by lowering resting potential
–may lead to heart block and cardiac arrest
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Hypokalemia is also life threatening because
heart beats feebly
Homeostatic Imbalance
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Tachycardia “heart hurry” – abnormally
fast heartbeat – more than 100 beats/min
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Elevated body temperature
Stress
Drugs
Heart disease
Promotes fibrillation and is pathological
Bradycardia – less than 60 beats/min
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Low body temperature
Drugs
Parasympathetic nervous activation
Warning of brain edema after head trauma
Homeostatic Imbalance
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Congestive heart failure (CHF)
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Pumping efficiency (CO) is so low that blood
circulation is inadequate
Progressively worsens, reflects weakening of
myocardium caused by
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Coronary artherosclerosis (clogging of vessels
with fatty buildup)
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Increasing hypoxia due to insufficient O2
Persistent high blood pressure
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The myocardium must exert more force to open the
aortic valve and pump out the same amount of
blood (becomes weaker)
Homeostatic Imbalance
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CHF (continued)
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Multiple myocardial infarcts
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MI’s depress pumping efficiency because
dead heart cells are replaced by scar tissue
Dilated cardiomyopathy (DCM)
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Ventricles stretch and become flabby and
myocardium deteriorates
Cause often unknown
Contractility is impaired, CO is poor and
condition progressively worsens
Homeostatic Imbalance
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Pulmonary congestion
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Left side of heart fails
Lungs become engorged with blood
Leads to pulmonary edema
Can cause suffocation
Peripheral congestion
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Right side of heart fails
Blood stagnates in organs
Inadequate oxygen and nutrients
Edema pronounced in extremities
Homeostatic Imbalance
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Seriously weakened heart is
irreparable
Treatment involves:
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Conserving heart energy with digitalis
which reduces heart rate
Removing excess fluids with diuretics
Use drugs to lower blood pressure and
reduce afterload
Transplants