![]() ![]() The pacemaker of the heart, which sets the overall heart rhythm in normal individuals, is the SA node. The most important autorhythmic cells of the heart are found in the sinoatrial (SA) node and the atrioventricular (AV) node. These cells, which lead to the autorhythmic nature of the heart, control the timing of contraction in the atria and ventricles. Some of the conducting cells in the heart normally generate action potentials on their own (without nervous system stimulation). ![]() These specialized cardiomyocytes are called conducting fibers (Figure 2). ![]() Some cardiomyocytes are non-contractile: that is, they only conduct action potentials to the contractile cells. Most cardiomyocytes are contractile, and when they are stimulated by an action potential, they contract. The heart is composed of two basic types of cells – contractile and non-contractile. Electrical conduction system of the heart Figure from Open Stax Anatomy and Physiology CC BY 4.0. Also indicated are the PR, QT, QRS, and ST intervals, plus the P-R and S-T segments. A normal tracing shows the P wave, QRS complex, and T wave. The standard ECG waveforms, intervals, and segments. These sounds can be correlated with the basic waveforms seen on an ECG (Figure 1). When the ventricles relax, the blood pressure drops below what is in the arteries and the semilunar valves close, producing the “dub” sound. The “lub” sound occurs during the early phase of ventricular contraction and is produced by closing of the atrioventricular valves, which prevents blood backflow into the atria. In this lab you will record and electrocardiogram (ECG) from a subject and listen to the characteristic “lub-dub” heart sounds. Blood entering the arterial system from the aorta inflates the arteries and increases blood pressure to a maximum, which is the systolic pressure. Ventricular contraction increases the ventricular pressure until it exceeds the arterial pressure, forcing the semilunar valves open, causing blood to flow into the pulmonary arteries and the aorta. During this time the arterial pressure is at its lowest point during the cardiac cycle this is called diastolic pressure. The atria contract and force the remaining blood into the ventricles, which are relaxed during filling.Īs the pressure of the blood inside the ventricles increases, the atrioventricular valves close. The increased pressure in the atria due to atrial filling eventually forces the atrioventricular (AV) valves open, and blood flows into the ventricles. The physical contraction and relaxation of atria and ventricles is induced by the proper functioning of the heart’s electrical conduction system.Īt the beginning of the cardiac cycle, the atria are relaxed and fill with blood coming from the vena cavae and cardiac venus sinus or the pulmonary veins. In the simplest terms, the cardiac cycle traces the flow of blood through the heart. Several physical and electrical events occur during the cardiac cycle, resulting in the flow of blood through the heart and peripheral tissues. Karri Haen Whitmer Introduction to the cardiac cycle ![]()
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