EVENTS DURING CARDIAC CYCLE

The cycle is divided into two major phases, : the period of ventricular contraction and blood ejection called systole (0.3 sec), and the alternating period of ventricular relaxation and blood filling, diastole (0.5 sec).

MID-DIASTOLE TO LATE DIASTOLE

• The left atrium and ventricle are both relaxed, but atrial pressure is very slightly higher than ventricular pressure.
• The AV valve is forced open by this pressure difference, and blood entering the atrium from the pulmonary veins continues on into the ventricle.
• Throughout all of diastole- the aortic valve is closed because the aortic pressure is higher than the ventricular pressure at this time.
• The aortic pressure is slowly falling because blood is moving out of the arteries and through the vascular system.
• In contrast, ventricular pressure is rising slightly because blood is entering the relaxed ventricle from the atrium, thereby expanding the ventricular volume.
• Near the end of diastole, the SA node discharges, as signified by the P wave of the ECG (atrial depolarization)
• Contraction of the atrium causes a rise in atrial pressure.
• The elevated atrial pressure forces a small additional volume of blood into the ventricle, sometimes referred as the "atrial kick".
• This brings us to the end of ventricular diastole, so the amount of blood in the ventricle at this time is called the End- diastolic volume (EDV).

SYSTOLE

Thus far, the ventricle has been relaxed as it fills with blood. But immediately following the atrial contraction, the ventricles begin to contract.

• From the AV node, the wave of depolarization passes into and throughout the ventricular tissue- as signified by the QRS complex of the ECG- and this triggers ventricular contraction.
• As the ventricle contracts, ventricular pressure rises very rapidly, and almost immediately this pressure exceeds the atrial pressure.
• This change in pressure gradient forces the AV valve to close, thus preventing the backflow of blood into the atrium.
• Because the aortic pressure still exceeds the ventricular pressure at this time, the aortic valve remains closed, and the ventricle cannot empty despite its contraction. For brief time, then, all valves are closed during this phase of isovolumetric ventricular contraction.
• This brief phase ends when the rapidly rising ventricular pressure exceeds aortic pressure.
• The pressure gradient now forces the aortic valve to open, and ventricular ejection begins,
• The ventricular volume curve shows that ejection is rapid at first and then tapers off.
• The amount of blood remaining after ejection is called the End-Systolic Volume (ESV).
• Stroke volume (70 ml) = End diastolic volume (135 ml) - End systolic volume (65 ml).
• As blood flows into the aorta, the aortic pressure rises along with the ventricular pressure. Throughout ejection, only very small pressure difference exist between ventricle and aorta because the aortic valve opening offers little resistance to flow.
• Note that peak ventricular and aortic pressures are reached before the end of ventricular ejection; that is, these pressures start to fall during the last part of systole despite continued ventricular contraction. This is because the strength of ventricular contraction diminishes during the last part of systole.
• This force reduction is evidenced by the reduced rate of blood ejection during the last part of systole.
• The volume in the aorta, and therefore the pressure falls at the rate of blood ejection from the ventricles becomes slower than the rate at which blood drains out of the arteries into the tissues.

EARLY DIASTOLE

The phase of diastole begins as the ventricular muscle relaxes, and ejection comes to an end. (ventricular repolarization)

• The T-wave of ECG corresponds to the end of the plateau phase of ventricular action potentials- that is, to the onset of ventricular repolarization.
• As the ventricle relax, the ventricular pressure falls below aortic pressure, which remains significantly elevated due to the volume of blood that just entered.
• This change in the pressure gradient forces the aortic valve to close, and the AV valve also remains closed because the ventricular pressure is still higher than atrial pressure. For a brief time, then, all valves are again closed during this phase of isovolumetric ventricular relaxation.
• This phase ends as the rapidly decreasing ventricular pressure falls below atrial pressure.
• This change in pressure gradient results in the opening of the AV valve.
• Venous blood that had accumulated in the atrium since the AV valve closed, flows rapidly into the ventricles.
• The rate of blood flow is enhanced during this initial filling phase by a paid drop of ventricular pressure.

This occurs because the ventricle's previous contraction compressed the elastic elements of the chamber in such a way that the ventricles actually tends to recoil outwards once systole is over.