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Cell Bio and Physio – Lecture 19: The Cardiac Cycle
5/3/12
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Cardiac Cycle
o The period b/t the start of one heartbeat and the start of the next
 Basically a single heartbeat
o Importance
 The fxn of the heart can be understood by a sequence of electrical and mechanical
events which occur during a single cardiac cycle/heartbeat
Pump Basics
o 4 chambers – 2 atria, 2 ventricles
 Make up two side-by-side pumps
Systole and Diastole
o Background
 The cardiac cycle consists of one complete contraction and relaxation of all the
chambers
o Systole
 Contractile period during which blood is ejected from the heart
o Diastole
 Relaxation period during which the heart fills w/ blood
o Terms apply to any of the chambers if specified, or just the ventricles when alone
Heart Valves
o 4 total
 Right AV/tricuspid valve
 Left AV/bicuspid/mitral valve
 Right/pulmonary semilunar valve
 Left/aortic semilunar valve
o Function
 Enforce one-way flow of blood in the heart
 Prevent backflow of blood as the chambers contract
 Valve closure makes heart sounds which can be auscultated
Blood Flow
o As w/ flow in the circuits, blood flow in the heart is driven by pressure differentials
 High pressure  low pressure
Cardiac Conduction System
o The heart is able to contract w/o neural stimulation – Autorhythmicity
 Accomplished b/c the heart has an intrinsic wiring system of modified muscle
cells organized into clumps and strands
 Cardiac conduction system
o Components
 2 main parts
 Areas that generate electrical impulses
o Sinoatrial (SA) node
o Atrioventricular (AV) node
 Areas that rapidly conduct the impulses throughout the heart
Phases
o Background
 The cardiac cycle can be divided into phases
o Inflow phase (1)
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A-V valves are open; semilunar valves are closed
Begins at the exact moment that ventricular pressure falls below atrial pressure
(b/c the ventricles have just finished ejecting blood)
At this point, the AV valves open and rapidly fill the ventricles
Diastasis
 Ventricular filling begins to slow by the middle of the inflow phase
Atrial Contraction (Atrial systole)
 Occurs at the end of phase 1
 Only ~20% of blood flowing remains in the atria
 The atria subsequently eject this 20% into the ventricles
Importance of Atria
 They act as primer pumps for the ventricles
 Very useful during exercise, as they may contribute up to 40% of the
ventricles’ volume
o Shortness of breath may develop
Electrical Activity
 Each cardiac cycle is initiated by spontaneous generation of an AP in the
SA node
 This AP spreads rapidly w/in the atria, but is slowed by ~0.1 sec before
passing into the ventricles
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o Isovolumetric contraction (2)
 Both valves are closed; no blood flow
 Beginning is defined by the exact moment that ventricular pressure rises above
atrial pressure
 Blood is forced against the AV valve cusps, thus closing them
 First (S1) heart sound*
 Isovolumetric
 For ~0.02 secs (enough time for pressure to build to open the semilunar
valves) the ventricles are contracting but there is no movt of blood (no
volume change) = isovolumetric contraction
 Semilunar valves
 Isovolumetric contraction causes pressure w/in the ventricles to increase
rapidly
 When the pressure in the ventricles exceeds that in the aorta/pulmonary
trunk, the semilunar valves are forced open
o Outflow phase (3)
 Semilunar valves are open; A-V valves are closed
 Begins as the semilunar valves open
 In the initial part of the phase, ventricular pressure is still rising rapidly
 Blood ejection is also rapid; large volume is ejected
 Semilunar Valves Closure
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As blood is ejected into the aorta/pulmonary trunk, pressure in those
great vessels rises
 Once pressure exceeds ventricular pressure, backflow shuts the valves
o Second (S2) heart sound*
o Isovolumetric relaxation (4)
 Both valves are closed; no blood flow
 Begins at the closure of the semilunar valves
 Following closure of the aortic semilunar valve, blood again flows “forward” into
the aorta
 Results in a small upward deflection in the aortic pressure trace, creating
the dicrotic (double-beat) notch
 Isovolumetric
 The AV valves have been closed since the beginning of systole
 Both valves are not closed – no blood may enter (isovolumetric) – and the
ventricle is into its period of relaxation
Additional Events
o Background
 Other characteristic changes accompany the basic cyclical pattern of cardiac
pressure and volume changes
 Electrical
 Acoustic
o Electrocardiogram (ECG)
 ECG tracing begins w/ the P wave = atrial depolarization
 QRS complex = ventricular depolarization
 Prelude to the rapid upswing in ventricular pressure
 T wave = ventricular repolarization
 Occurs in the decreased ejection phase
o Heart Sounds
 Opening and closing of the heart valves are accompanied by heart sounds
 Major sounds
 S1 = closure of AV valves
o Usu. stronger, longer, and of lower frequency that S2
 S2 = closure of semilunar valves
 Sounds actually result from post-closure vibrations
 Minor sounds
 S3 – heard in certain normal individuals
o Occurs during rapid filling of the ventricles when the walls recoil
 S4 – coincides w/ atrial contraction
o Often heard in pathological conditions in which there is unusually
strong atrial contraction due to low ventricular compliance
Duration of Events in the Cardiac Cycle
o At Rest
 Assuming an average heart beats 75 times per minute
 Length of cardiac cycle ~ 800 msec
 Atrial systole ~ 100 msec
 Ventricular systole ~ 270 sec
 Period of total relaxation ~ 430 msec
o During Exercise
 During exercise, heartrate can reach 200 bpm
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Length of cycle ~ 300 msec
Atrial systole ~ 60 msec
Ventricular systole ~ 180 msec
Period of total relaxation ~ 60 msec