Cardiac Cycle and Conduction System of the...
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Cardiac Cycle and Conduction System of the Heart
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The Cardiac Cycle
The mechanical events that take place in one heartbeat.
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The Cardiac Cycle ! Contraction of a chamber - systole ! Relaxation of a chamber - diastole ! 1 full contraction and relaxation - cardiac
cycle
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Physiology of Contraction (Atrial diastole)
! AV valves are open, Semilunar closed ! Blood draining into atria ! Blood draining into ventricles (gravity/
pressure gradient) ! SA node fires ending atrial diastole ! AV valves open further. Atria push
remaining blood to ventricles then go into diastole
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Cont...... ! Increased pressure in ventricles
! Pressure depends on volume and chamber size
! Isometric contraction of ventricles (why?) ! Semi lunar valves open AV valves close
due to back pressure ! Ventricles contract and force blood out
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Cont...... ! Ventricles relax - decreased pressure causes
semilunar valves to close ! cycle repeats ! intra-atrial pressures ! intra-ventricular pressure
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Systole and Diastole ! Peak pressure of a chamber or vessel ! Stroke volume
! amount of blood ejected by the heart (LV) in one contraction
• aprox. 70 cc (60-100 cc)
! Relaxation of a chamber ! Pressure at its lowest
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Cardiac Output ! Cardiac Output
! amount of blood ejected every minute • aprox. 5-6L • SV X HR = CO • The strength of contraction is directly related to
venous return
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Starling’s Law ! The more a muscle is stretched the harder it
will contract (up to a certain point) ! Similar to a spring ! Muscle fibers will stretch and contract
harder when there is a increased blood return
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Blood Pressure ! The pressure exerted by blood against the
walls of a given artery.
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Factors Affecting Blood Pressure 1. Blood Volume 2. Strength of heart contractions 3. Heart rate 4. Blood viscosity – thickness of blood
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Electrophysiology ! Before the heart can contract it must be
stimulated. ! This is accomplished through a specialized
network of cells called the conduction system.
! There are numerous “pacemaker” cells to stimulate the heart to contract.
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Sinoatrial Node (SA) ! Located in high RA ! Main pacemaker of heart ! Sends out regular impulses at 60- 100 times
per minute ! Regulated by the Autonomic Nervous
System (vagus nerve)
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Atrioventricular Node (AV) ! Located on floor of TA next to septum and
behind CS os ! Has 2 main jobs
! Delay the impulse from the SA node long enough so the ventricles can fill with blood
! Protect the ventricles from rapid atrial rates ! Inherent rate of 40-70 bpm
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Bundle of His (HB) ! A fibrous ring around the 4 valves ! Only muscular connection between the atria
and ventricles ! AV node and HB collectively are called the
AV junction ! The AV junction can pace the heart at rate
of 40-60 bpm
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Bundle Branches ! HB branches into rt.... And lt.. Branches ! Lt.. Bundle branch (LBB) then divides into
anterior and posterior sections
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Purkinje Fibers ! Terminal portion of the conduction system ! Located in ventricular muscle ! Can pace the heart at 20-40 bpm
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Electrical Properties of Cardiac Muscle
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! Automaticity ! Property by which a cell can reach threshold potential
(depolarize) and send an impulse without help from an external source
! Excitability ! Ability of cell to reach threshold potential in response
to an outside stimulus ! Conductivity
! The propagation of an impulse form cell to cell through intercalated disks
! Contractility ! Ability of a muscle to shorten (contract) when
stimulated
Electrophysiology cont.....
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Electrophysiology Cont..... ! The end result of contraction of the heart is
electrochemical in nature. ! The impulse is sent along the conduction
system because of changes in cellular polarity.
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Cardiac Emergencies ! A thrombosis or
embolism may plug the arteries causing a myocardial infarction or “heart attack”
! Angina pectoris is used to describe the severe chest pain
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infarct