Hint: The heart, blood, and blood vessels are part of the circulatory system. The human heart is responsible for transporting blood to the various organs and tissues in the body. The human heart has four chambers- two auricles and two ventricles. The relaxation and contraction of the four chambers enable the blood flow.
The blood enters the right auricle of the heart through the superior and inferior vena cava. A tricuspid valve prevents the backflow of blood once it enters the right ventricle. The blood then goes into the lungs to be oxygenated. The pulmonary valve prevents the backflow of blood once it enters the pulmonary trunk from the right ventricle. The oxygenated blood enters the left auricle and then enters the left ventricles. The bicuspid valves prevent the blood from flowing back to the left auricle. From the left ventricle, it enters the aorta. This blood is then supplied to various organs and tissues.
The cardiac cycle includes all the events that occur in a single heartbeat. The cardiac cycle involves systole (contraction) and diastole (relaxation). An increase in pressure helps blood flow into the next chamber. Contraction of the chamber always increases the pressure.
When the auricles contract the tricuspid and the bicuspid valves open, the semilunar valves remain closed. As the ventricles contract, the semilunar valves i.e. the pulmonary and aortic valves open. The blood from the auricles enters the ventricle. As soon as the ventricles get filled with blood the pressure in the auricle chamber decreases and the tricuspid and the bicuspid valves close.
Now the ventricles begin contraction. When the pressure in the ventricles surpasses the pressure in the aorta and the pulmonary trunk the blood from the ventricles rapidly enters these blood vessels. The semilunar valves open to allow blood flow. Once the blood enters the blood vessels and the pressure in the ventricles reduces, the semilunar valves close. This event marks the end of the systole phase. This is when diastole begins.
Hence, the correct answer is option (D).
Note: An electrocardiogram represents the cardiac cycle events. Due to passive blood flow, almost eighty percent of the ventricles are filled even before the auricles contract. Initially, when the ventricles begin contraction the semilunar valves remain closed. This phase is called isovolumetric contraction because the volume of blood in the ventricles remains the same.