Blood from the superior and inferior vena cava get into the heart in the right atrium. The blood then flows through the open tricuspid valve into the right ventricle. From the right ventricle, it flows to the lungs through the pulmonary artery. Blood leaves the lungs through the pulmonary vein into the left atrium, the left ventricle and finally through the aorta into the systemic circulation. A cardiac cycle involves all the events that occur in a single heartbeat, which include relaxation and contraction of both atria and ventricles. Relaxation constitutes the diastolic phase, while contraction constitutes the systolic phase. Both the atria and the ventricles remain relaxed at the start of the cardiac cycle (diastole). In describing the cardiac cycle, we shall relate it to the EKG patterns. The phases of the cardiac cycle will include atrial systole and diastole, ventricular systole, and ventricular diastole (Pollock & Makaryus, 2018).
Contraction of the atrium represented on the EKG by the P wave occurs after depolarization. Contraction is from the upper part of the atrium inferiorly towards the atrioventricular septum and creates a buildup of pressure in the atrium that allows the pumping of blood into the ventricles. Blood flows through the open atrioventricular valves, the tricuspid, and bicuspid valves into the ventricles. Atrial contraction contributes about 20-30% in filling the ventricles since most of the times they are already filled up to about 70% from a previous phase of diastole. Atrial systole ends before ventricular systole as the muscle relaxes returning to diastole.
Ventricular systole represented by the QRS complex on the EKG occurs after depolarization of the ventricles. At the end of the systolic atrial phase and just before the contraction of the atrium, the ventricle contains typically a total volume of 130ml of blood in a resting standing adult, and this is what forms the end diastolic volume also referred to as the preload. At the start of the ventricular systole, the contraction of the ventricles does not provide enough buildup of pressure to open the semilunar valves and allow the flow of blood out of the ventricles. However, as the contraction continues, there is a buildup of pressure more than that in the atria that have relaxed, which forces blood to flow back towards the atria, causing the closing of the atrioventricular valves. The early stage of ventricular systole is the isovolumic contraction of the ventricles (Marcondes et al., 2015).
In the late stage of ventricular systole, also referred to as the ventricular ejection stage, continued contraction leads to a buildup of pressure in the ventricles higher than that in the pulmonary trunk and aorta. Blood therefore profoundly gets pumped out of the ventricles through the now open pulmonary and aortic valves. The volume of blood pumped out is what constitutes the stroke volume whose normal range is between 70 and 80mls. After ventricular systole, there remains an end systolic volume of between 50 and 60mls of blood.
Ventricular diastole, represented by the T wave on the EKG, follows the repolarization of the ventricles. During the early phase of ventricular relaxation, the pressures inside begin to fall to less than that in the pulmonary trunk and aorta, and thus allows blood to flow back to the heart. There is the closure of the semilunar valves to prevent the backflow of blood into the heart. As a result, there is no change in the volume of blood in the ventricles, and this is the isovolumic ventricular relaxation phase.
In the late phase of ventricular diastole, the blood pressure continues to fall due to ventricular muscle relaxation and falls below that of the atria. Blood, therefore, flows from the atria into the ventricles pushing open the atrioventricular valves. Blood continues to flow from the major veins to the atria and into the ventricles. As a result, there is the completion of the cardiac cycle.
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