A cardiac cycle is a set of activities --electrical and mechanical-that takes place during the time interval during which the chambers of the heart relax and fill, and then contract and ejects blood. This series of activities is often referred to as a heart beat. This designation may oversimplify and mis-characterize a very complex process. In describing the cardiac cycles, physiologists and physicians start as different points. In any case these descriptions usually divide the cycle into two large phases diastole ( relaxation phase) and systole( contraction phase) It is convenient to start describing the cycle with both atria and ventricles in diastole: both atria and ventricles are relaxed; atria are receiving blood from the great veins( the right atrium form the Vena cavae and the left atrium from the pulmonary veins). The AV valves are open and blood is flows passively from atria to ventricles. When the ventricles are about 80% full a signal from the SA node depolarizes the myocardium of the atria and causes them to contract. This contraction pushes blood into the ventricle and tops them off (fills them up). As the atria relax , the ventricles contract--at first isometrically and then isotonically; increased pressure in the ventricles forces the AV valves closed and the semi-lunar valves ( aortic and pulmonic) open. As the ventricles contract blood is ejected into the aorta( from the left ventricle) and into the pulmonary artery ( from the right ventricle). This is the ventricular systole. After the ejection has taken place the ventricular myocardium relaxes and the ventricles goes into diastole. This set or cycle of events is completed in about eight tenths of a second (0.8 sec.) in a normal healthy adult. It is repeated over and over from before birth (as early as one month after conception)-- until death, in many cases for eighty or more years. The normal healthy resting heart rate (HR) or frequency of heartbeat is ranges fro 60 to 100 beats per minute, vigorous exercise can raise the heart rate to as high as 160-190 beats per minute. A slow heart rate may mean an efficient heart or it may be a sign of bradycardia.
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The cardiac cycle can be described simply as a set of activities by which the chambers of the heart relax and fill with blood ( diastole), then contract and pump out that blood ( systole). The actual process is much more complex. The heart has four chambers two upper ( atria left and right), and two lower ( ventricles, right and left). The atria receive blood and the ventricles eject blood. It is also informative to think of left side of the heart and right side of the heart. The left side of the heart deals with oxygenated arterial blood and the right side of the heart deals with deoxygenated venous blood. The atria relax and contract together and the lower chambers ( ventricles) relax and contract together. A contraction of the chambers is called a systole and a relaxation is called a diastole. The atria and the ventricles are not the only entities that are involved in the cardiac cycle: there are large veins ( vena cavae and pulmonary veins) that bring blood to the atria, and there are large arteries that take blood away from the heart--the aorta and the pulmonary artery. Arteries always take blood towards the heart ; veins always take blood away from the heart.. Arteries may carry impure deoxgenated blood, and veins may carry pure or oxygenated blood. The valves also perform important functions in the cardiac cycle. The valves between the atria and the ventricles are the atrio-ventricular or AV valves ; the tricsupid is on the right, and the mitral or bicuspid is on the left side. There are also so-called semi-lunar (SL) valves between the heart chambers and the great blood vessels. There is the aortic valve between the left ventricle and the aorta and the right SL valve or pulmonic valve between the right ventricle and the pulmonary artery. The heart works automatically like a robot, doing the same processes over and over with minimal extra-cardiac control of interference or direction. This is accomplishcd by the hearts own internal system of innervation. This includes the sino-atrial node( SA node) , the atrio-ventricular node(AV node) , the Bundle Branches of His, and the Purkinje fibers. A cardiac cycle is started when the SA generates an electrical impulse that then passes through and depolarizes the myocardium of the atria causing them to contract and push blood into the relaxed ventricles. The impulse then passes to the AV node which distributes it to the rest of the heart controlling the depolarization and the repolarization of the myocardium. The deplarization and repolarization of the heart chambers is not confined to the heart: the electrical changes involved spread throughout the body and can be dectedt at the surface of the skin. . An istrument aclled an electrocardiograph is used in medical practice to detecedt and display graphically the voltage fluctuations as atria and ventricle depolarize and repolarize through the phases of the cardiac cycle. The graphical record of the cardiac cycle obtained by an electrocardiograph is called an electrocardiogram. It is a very valuable tool for researchers and physicians to diagnose heart abnormalities in a cardiac cycle. The characteristics of an electrocardiagram are common knowledge: They are the "P" wave , the "QRS" complex and the T wave. The P wave marks the instant of the atrial depolarization, The QRS complex, is the sign of ventricular depolarization and the T wave signifies ventricular repolarization. The interval between the end of the QRS complex and the "T" wave is the absolute refractory period of the myocardium, It is worthy of note the the SA node is not absolutely without external influence. The vagus nerve slows down the rate of firing of the SA node which would exceed normal sinus rythm , if left.uninfluenced. There are other phenomena associated with the cardiac cycle : some physiologists divide it into seven or eight phases, There are more sounds than just Lubb and Dupp -- there are four heart sounds-- and the pressure changes are important aspects of cardiac functioning.