Cardiac Cycle

Cardiac Cycle The cardiac events that occur from beginning of one heart beat to the beginning of the next are called the cardiac cycle. Initiated by spontaneous generation of AP in SA node. Atria act as PRIMER PUMPS for ventricles & ventricles provide major source of power for moving the blood through the vascular system.

Cardiac cycle – a period of relaxation - Diastole , during which heart fills with blood - period of contraction, called Systole

Atrial Systole Blood normally flows continually from great veins into atria : 80% flows directly through atria into ventricle before the atria contracts. 20% of filling of ventricles – atrial contraction Atrial contraction is complete before the ventricle begins to contract.

Atrial Systole Pressures & Volumes ‘ a ‘ wave – atrial contraction, when atrial pressure rises. Atrial pressure drops when the atria stop contracting.

Beginning of Ven.Systole Isovolumetric Contraction The atrioventricular (AV) valves close at the beginning of this phase Mechanically, ventricular systole is defined as the interval between the closing of the AV valves and the opening of the semi lunar valves (aortic and pulmonary valves).

Isovolumetric Contraction Pressure & Volume Changes The AV valves close when the pressure in the ventricles (red) exceeds the pressure in the atria (yellow). As the ventricles contract isovolumetrically -- their volume does not change (white) -- the pressure inside increases, approaching the pressure in the aorta and pulmonary arteries (green).

Ejection The Semilunar valves ( aortic , pulmonary ) open at the beginning of this phase. This at first 1/3 rapid ejection (70%) & remaining 2/3 slow ejection.(30%)

Pressure & Volume Changes When ventricles continue to contract , pressure in ventricles exceed that of in aorta & pul arteries & then semilunar valves open, blood is pumped out of ventricles & vol decreases rapidly.

Slow Ejection After the peak in ventricular and arterial pressures , blood flow out of the ventricles decreases and ventricular volume decreases more slowly. When the pressure in the ventricles falls below the pressure in the arteries, blood in the arteries begins to flow back toward the ventricles and causes the semilunar valves to close. This marks the end of ventricular systole mechanically.

Slow Ejection At the end of ejection, the semilunar valves close.

Beginning of Diastole Isovolumetric relaxation Throughout this and the previous two phases, the atrium in diastole has been filling with blood on top of the closed AV valve, causing atrial pressure to rise gradually. pressure & volume of ventricle are low in this phase .

Rapid Inflow ( Rapid Ven. Filling) Once AV valves are open the blood that has accumulated in atria flows into the ventricle

Volume changes Ventricular volume increases rapidly as blood flows from the atria into the ventricles.

Diastasis remaining blood which has accumulated in atria slowly flows into the ventricle.

Diastasis Volume changes Ventricular volume increases more slowly now. The ventricles continue to fill with blood until they are nearly full.

End diastolic vol : During diastole, filling of ventricle increases vol of each ventricle to 110 -120 ml Stroke Vol : amount of blood pumped out of ventricle during systole - 70 ml End systolic vol : the remaining amount of blood in ventricle after the systole , 40 -50 ml