Youtor
Cardiac Cycle complete explanation Chapter Circulation Video 4
>> YOUR LINK HERE: ___ http://youtube.com/watch?v=WhWTZoygMLU
Cardiac Cycle • The pumping of the heart, or the heartbeat, is caused by alternating contractions and relaxations of the myocardium. These contractions are stimulated by electrical impulses from a natural pacemaker, the sinoatrial, or S-A, node located in the muscle of the right atrium. • The SA (sinus) node represents a cluster of myocytes with pacemaker activity. Under normal circumstances, it generates electrical impulses that set the rhythm and rate of the heart. • An impulse from the S-A node causes the two atria to contract, forcing blood into the ventricles. Contraction of the ventricles is controlled by impulses from the atrioventricular, or A-V, node located at the junction of the two atria. Following contraction, the ventricles relax, and pressure within them falls. Blood again flows into the atria, and an impulse from the S-A starts the cycle over again. This process is called the cardiac cycle. The period of relaxation is called diastole. The period of contraction is called systole. Diastole is the longer of the two phases so that the heart can rest between contractions. • • • The heart has four valves that help ensure that blood only flows in one direction: • Aortic valve: between the left ventricle and the aorta. • Mitral valve: between the left atrium and the left ventricle. • Pulmonary valve: between the right ventricle and the pulmonary artery. • Tricuspid valve: between the right atrium and right ventricle. • • The rhythmic noises accompanying heartbeat are called heart sounds. Normally, two distinct sounds are heard through the stethoscope: a low, slightly prolonged “lub” (first sound) occurring at the beginning of ventricular contraction, or systole, and produced by closure of the mitral and tricuspid valves, and a sharper, higher-pitched “dup” (second sound), caused by closure of aortic and pulmonary valves at the end of systole. • The heart rate can vary quite remarkably depending on various environmental and physiologic factors. At rest, the SA nodal myocytes depolarize at an intrinsic rate between 60 and 100 beats per minute, which is generally considered a normal heart rate. The autonomic nervous system tightly controls input into the sinus node. The autonomic fibers regulate the firing of the sinus node to initiate the start of subsequent cardiac cycles and thus, influence the heart rate. Parasympathetic input slows down the rate of action potential production, thereby decreasing the heart rate; on the other hand, sympathetic input increases the rate of action potential production, thereby increasing the heart rate. This tight, regulated control of the sinus node allows for the heart to adapt to various physiologic stressors placed on the body. For instance, the heart responds to the body’s increased oxygen demand during exercise with an increase in sympathetic input, increasing heart rate. • #HeartCycle • #Pacemaker • #CardiovascularCycle • #HowHeartBeat • #EventsOfCardiacCycle • #PhasesOfHeartBeat
#############################
