This volume is known as the end diastolic volume (EDV) or preload. At the end of atrial systole and just prior to atrial contraction, the ventricles contain approximately 130 mL blood in a resting adult in a standing position. ![]() It may be conveniently divided into two phases, lasting a total of 270 ms. Ventricular systole (see image below) follows the depolarization of the ventricles and is represented by the QRS complex in the ECG. Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole. Atrial contraction, also referred to as the “atrial kick,” contributes the remaining 20–30 percent of filling (see the image below). At the start of atrial systole, the ventricles are normally filled with approximately 70–80 percent of their capacity due to inflow during diastole. As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves. Atrial Systole and DiastoleĬontraction of the atria follows depolarization, represented by the P wave of the ECG. The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left. Approximately 70–80 percent of ventricular filling occurs by this method. The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles. Blood flows into the left atrium from the four pulmonary veins. Blood is flowing into the right atrium from the superior and inferior venae cavae and the coronary sinus. Phases of the Cardiac CycleĪt the beginning of the cardiac cycle, both the atria and ventricles are relaxed (diastole). Again, as you consider this flow and relate it to the conduction pathway, the elegance of the system should become apparent. During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle. When the action potential triggers the muscles in the atria to contract (atrial systole), the pressure within the atria rises further, pumping blood into the ventricles. ![]() As blood flows into the atria, the pressure will rise, so the blood will initially move passively from the atria into the ventricles. Accordingly, when the heart chambers are relaxed (diastole), blood will flow into the atria from the veins, which are higher in pressure. Pressures and Flowįluids, whether gases or liquids, are materials that flow according to pressure gradients-that is, they move from regions that are higher in pressure to regions that are lower in pressure. The cardiac cycle begins with atrial systole and progresses to ventricular systole, atrial diastole, and ventricular diastole, when the cycle begins again.
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