Question

Summarize the material covered in today’s lab. Name and describe each of the phases of the cardiac cycle. Be sure to include the events that occur during each phase. Focus on the mechanical events (contraction and relaxation) of the chambers, the pressure changes in the heart, the position of the valves, and the heart sounds. Be sure to explain what causes each of the events you describe.

Answer 1

CARDIAC CYCLE- It is the sequence of events that occurs when the heart beats.

Phases of cardiac cycle include:-

• Phase 1- ATRIAL CONTRACTION

Atrial depolarisation cause contraction of the atria. As atria contract, pressure within the atrial chambers increases, which forces more blood flow across the open atrio-ventricular valves(AV valves) , leading to rapid flow of blood into the ventricles.

• Phase 2- ISOVOLUMETRIC CONTRACTION

All valves are closed, ventricular depolarisation occurs and a rapid increase in inter-ventricular pressure. AV valves close as inter-ventricular pressure exceeds atrial pressure, Closure of the AV valves results in first heart sound (S1). During this period, ventricular pressure rises rapidly without a change in ventricular volume.

• Phase 3- RAPID EJECTION

Aortic and pulmonary valves are open. AV valves remain closed. Ejection begins when the inter-ventricular pressure exceed the pressure within the aorta and pulmonary trunk. Blood continues to flow into the atria and atrial pressures begin to rise and continue to rise until the AV valves open at the end of phase 5.

• Phase 4- VENTRICULAR REPOLARIZATION

Ventricular contraction leads to decline in ventricular tension and therefore the rate of ventricular emptying decreases. Ventricular outward flow still occurs. Left and right atrial pressures gradually rise due to continued venous return from the lungs and from the systemic circulation, respectively.

• Phase 5- ISOVOLUMETRIC RELAXATION

All valves are closed. Occurs when the inter- ventricular pressure falls. Aortic and pulmonary valves close causing second heart sound(S2). After valve closure, aortic and pulmonary pressures rise followed by slow decline in pressure. Ventricular volumes remain constant because all valves are closed. Left atrial pressure continues to rise because of venous return from the lungs.

• Phase 6- RAPID FILLING

AV valves open.As the ventricles continue to relax, the interventricular pressure will fall below atrial pressure causing AV valves to open and ventricular filling begins. Ventricular pressures slowly rise as they fill with blood from the atria. Third heart heart(S3) is audible and it represent tnsing of chorda tendinae and ventricular relaxation and filling.

• Phase 7- REDUCED FILLING

AV valves open. As the ventricles continue to fill, the interventricular pressures rise. Rate of filling reduces. Aortic pressure and pulmonary arterial pressure continues to fall during this period.