CVS Lect 3

8/14/2019 CVS Lect 3

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CARDIAC CYCLECARDIAC CYCLE

The cardiac events that occur from thebeginning of one heartbeat to thebeginning of the next are called thecardiac cycle. Each cycle is initiated by spontaneousgeneration of an action potential inthe sinus node . This node is located inthe superior lateral wall of the rightatrium near the opening of thesuperior vena cava, and the action

potential travels from here rapidly

8/14/2019 CVS Lect 3


CARDIAC CYCLECARDIAC CYCLE Because of this special arrangement of theconducting system from the atria into theventricles, there is a delay of more than 0.1second during passage of the cardiac

impulse from the atria into the ventricles. This allows the atria to contract ahead of ventricular contraction, thereby pumpingblood into the ventricles before the strong

ventricular contraction begins. Thus, the atria act as primer pumps for theventricles, and the ventricles in turn providethe major source of power for moving blood

through the body's vascular system.

8/14/2019 CVS Lect 3


The heart can beviewed as two separate pumpsrepresented by its rightand left halves. Eachpump consists of a

primer pump—the

atrium —and a power

8/14/2019 CVS Lect 3


Both atrial primer pumps

complete the filling of theventricles with blood, andboth ventricular powerpumps produce the majorforce that causes blood toflow through thepulmonary and systemic

arteries.

8/14/2019 CVS Lect 3


The term cardiac cycle refers

to the repetitive pumpingprocess that begins with theonset of cardiac musclecontraction and ends with thebeginning of the nextcontraction.Pressure changes producedwithin the heart chambers as aresult of cardiac musclecontraction are responsible for blood movement because

8/14/2019 CVS Lect 3


The duration of the cardiaccycle varies considerably amonghumans and also during anindividual’s lifetime.It can be as short as 0.25—0.3seconds in a newborn infant oras long as 1 or more seconds in a well-trained athlete.

The normal cardiac cycle of 0.7—0.8 seconds depends onthe capability of cardiac muscleto contract and on the functional

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The term systole means tocontract , and diastole means todilate . Atrial systole is contraction of the atrial myocardium, and atrialdiastole is relaxation of the atrialmyocardium.Similarly, ventricular systole iscontraction of the ventricularmyocardium, and ventricular diastoleis relaxation of the ventricularmyocardium.When the terms systole and diastoleare used without reference tospecific chambers, however, theymean ventricular systole or diastole.

8/14/2019 CVS Lect 3


Just before systole begins, the

atria and ventricles are relaxed,the ventricles are filled withblood and the semilunar valves

are closed, and the A V valvesare open.As systole begins, contraction

of the ventricles increasesventricular pressures, causingblood to flow toward the atriaand close the AV valves.

8/14/2019 CVS Lect 3


As contraction proceeds,ventricular pressures continue torise, but no blood flows from theventricles because all the valvesare closed. This brief interval is called theperiod of isovolumic contraction because the volumeof blood in the ventricles doesnot change, even though the

ventricles are contracting.

8/14/2019 CVS Lect 3


As the ventricles continue

to contract, ventricularpressures become greaterthan the pressures in thepulmonary trunk and aorta.As a result, during the

period of ejection thesemilunar valves are

pushed open and blood

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As diastole begins the ventricles

relax and ventricular pressuresdecrease below the pressures in the pulmonary trunk and aorta.Consequently, blood begins to flowback to ward the ventricles, causingthe semilunar valves to close.With closure of the semilunar valves,all the heart valves are closed and noblood flows into the relaxingventricles during the period of

isovolumic relaxation.

8/14/2019 CVS Lect 3


Throughout ventricularsystole and the period of isovolumic relaxation, theatria relax and blood flows

into them from the veins.As the ventricles continue to

relax, ventricular pressuresbecome lower than atrialpressures, the AV valve opens

and blood flows from the atria

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Most ventricular filling is a passive process resulting fromthe greater pressure of blood inthe veins and atria than in thecompletely relaxed ventricles.Completion of ventricular filling isan active process resulting fromincreased atrial pressure

produced by contraction of atria. During exercise atrial contraction is more important

for ventricular filling

8/14/2019 CVS Lect 3


8/14/2019 CVS Lect 3


8/14/2019 CVS Lect 3


8/14/2019 CVS Lect 3


8/14/2019 CVS Lect 3


8/14/2019 CVS Lect 3


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CVS Lect 3