Q. Identify the correct sequence of events in a cardiac cycle.
Solution:
CARDIAC CYCLE:
The cardiac cycle is referred to the events which take place in the heart, from the endpoint of one heartbeat to the beginning of the next (one heartbeat accounts for one cycle of contraction and relaxation of the atria and ventricles). The phase of contraction is called the systole, while the relaxed phased is called the diastole. At the beginning of the cycle, both the atria and ventricles are in a relaxed state, called the joint diastole. The following events occur in succession:
a) Atrial Systole: As both the AV valves (bicuspid and tricuspid) are open, blood flows into the left and right ventricles from the pulmonary veins and vena cava through the left and right atria, respectively. The semilunar valves are closed. An action potential is generated by the SA node, which stimulates both the atria to contract simultaneously (atrial systole). An approximate 30% increase is observed in the flow of blood into the ventricles.
b) Ventricular Systole: The action potential generated by the SA node is transmitted to the ventricles by the AV node and bundle, from where the bundle of His conducts it throughout the entire ventricular musculature. This signal causes contraction of the ventricular muscles (ventricular systole). At the same time, the atria undergo relaxation (atrial diastole), coinciding with the ventricular contraction. The ventricular systole increases the ventricular pressure, which causes the AV valves to close, preventing the attempted backflow of blood into the atria. With increasing pressure, the semilunar valves at the base of the pulmonary artery (right side) and the aorta (left side) are opened forcefully, allowing the blood in the ventricles to flow through these vessels into their respective circulatory pathways.
c) Joint Diastole: The ventricles now relax (ventricular diastole), and the ventricular pressure falls, leading to the closure of the semilunar valves; it prevents the backflow of blood into the ventricles. As the ventricular pressure continues to decline, it falls below the pressure of the blood in the atria. At this point, the AV valves open and blood starts flowing again from the relaxed atria to the relaxed ventricles (joint diastole). Soon the SA node generates a new action potential and the events described above are repeated in that sequence and the cycle continues.
The cardiac cycle is referred to the events which take place in the heart, from the endpoint of one heartbeat to the beginning of the next (one heartbeat accounts for one cycle of contraction and relaxation of the atria and ventricles). The phase of contraction is called the systole, while the relaxed phased is called the diastole. At the beginning of the cycle, both the atria and ventricles are in a relaxed state, called the joint diastole. The following events occur in succession:
a) Atrial Systole: As both the AV valves (bicuspid and tricuspid) are open, blood flows into the left and right ventricles from the pulmonary veins and vena cava through the left and right atria, respectively. The semilunar valves are closed. An action potential is generated by the SA node, which stimulates both the atria to contract simultaneously (atrial systole). An approximate 30% increase is observed in the flow of blood into the ventricles.
c) Joint Diastole: The ventricles now relax (ventricular diastole), and the ventricular pressure falls, leading to the closure of the semilunar valves; it prevents the backflow of blood into the ventricles. As the ventricular pressure continues to decline, it falls below the pressure of the blood in the atria. At this point, the AV valves open and blood starts flowing again from the relaxed atria to the relaxed ventricles (joint diastole). Soon the SA node generates a new action potential and the events described above are repeated in that sequence and the cycle continues.