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Cell Bio and Physio – Lecture 19: The Cardiac Cycle 5/3/12 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) 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 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 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 Length of cycle ~ 300 msec Atrial systole ~ 60 msec Ventricular systole ~ 180 msec Period of total relaxation ~ 60 msec