P wave atrial depolarization 2 Ventricular systole atrial relaxation a QRS T b

P wave atrial depolarization 2 ventricular systole

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P-wave (atrial depolarization) 2. Ventricular systole (atrial relaxation) a. QRS-T b. Ventricular depolarization, which masks atrial repolarization c. Atria in diastole (relaxed) d. Isovolumetric contraction: ventricles contract; intraventricular pressure increases, atrioventricular valves close to prevent backflow of blood into atria e. Ventricular ejection: semilunar valves open to allow blood to flow from the ventricles 3. Isovolumetric relaxation a. T-wave b. Ventricular repolarization c. Ventricles relax and pressure drops d. Semilunar valves close due to backflow of blood in aorta and pulmonary trunk FROM BOOK The heart is regulated by both neural and endocrine control, yet it is capable of initiating its own action potential followed by muscular contraction. The conductive cells within the heart establish the heart rate and transmit it through the myocardium.
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The contractile cells contract and propel the blood. The normal path of transmission for the conductive cells is the sinoatrial (SA) node, internodal pathways, atrioventricular (AV) node, atrioventricular (AV) bundle of His, bundle branches, and Purkinje fibers. The action potential for the conductive cells consists of a prepotential phase with a slow influx of Na+ followed by a rapid influx of Ca2+ and outflux of K+. Contractile cells have an action potential with an extended plateau phase that results in an extended refractory period to allow complete contraction for the heart to pump blood effectively. Recognizable points on the ECG include the P wave that corresponds to atrial depolarization, the QRS complex that corresponds to ventricular depolarization, and the T wave that corresponds to ventricular repolarization. Cardiac Cycle The cardiac cycle comprises a complete relaxation and contraction of both the atria and ventricles, and lasts approximately 0.8 seconds. Beginning with all chambers in diastole, blood flows passively from the veins into the atria and past the atrioventricular valves into the ventricles. The atria begin to contract (atrial systole), following depolarization of the atria, and pump blood into the ventricles. The ventricles begin to contract (ventricular systole), raising pressure within the ventricles. When ventricular pressure rises above the pressure in the atria, blood flows toward the atria, producing the first heart sound, S1 or lub. As pressure in the ventricles rises above two major arteries, blood pushes open the two semilunar valves and moves into the pulmonary trunk and aorta in the ventricular ejection phase. Following ventricular repolarization, the ventricles begin to relax (ventricular diastole), and pressure within the ventricles drops. As ventricular pressure drops, there is a tendency for blood to flow back into the atria from the major arteries, producing the dicrotic notch in the ECG and closing the two semilunar valves. The second heart sound, S2 or dub, occurs when the semilunar valves close.
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