Cardiac cycle 

Cardiac Cycle

◦ Contraction of the heart chambers is called systole

◦ Relaxation of the heart chambers is called diastole

◦ The atria contract (atrial systole) while the ventricles relax (ventricular diastole).

◦ The ventricles contract (ventricular systole) while the atria relax (atrial diastole)

◦ A cardiac cycle – the contraction and subsequent relaxation of all four heart chambers, constitutes a complete heartbeat.

Cardiac Cycle- Three Phases

◦ Relaxation period

◦ At the end of a cardiac cycle

◦ when the ventricles start to relax, all four chambers are in diastole

◦ the ventricles relax, pressure within the chambers drops

◦ blood starts to flow from the pulmonary trunk and aorta back toward the ventricles

◦ Atrial systole (contraction)

◦ marks the end of the relaxation period

◦ accounts for the remaining 25% of the blood that fills the ventricles

◦ the period of ventricular filling

◦ Ventricular systole (contraction)

◦ pushes blood up against the AV valves, forcing them shut

◦ For a very brief period, all four valves are closed again

◦ When left ventricular pressure rises above the pressure in the arteries, both SL valves open

◦ ejection of blood from the heart begins.

Overview of Cardiac Cycle

The first heart sound, S1

• Likened to the word “lub”

• Atrial contraction lasts ~0.1s

• Rising pressure in the ventricle, forces the atrioventricular valves to close

• Heard through a stethoscope placed over the apex of the heart

The second heart sound, S2

• Likened to the word “dub”

• Heard best over the second right rib

• Ventricular contraction last 0.3s

• Force to open the pulmonary and aortic valves

• Blood is forced into the aorta and pulmonary trunk

Semilunar valves close

Pressure and volume change

◦ Pressure in chambers rises and falls in cardiac cycles.

◦ Changes in pressure open and close the heart valves.

◦ Volume in chambers increases during diastole and decreases during systole.