In a normal and healthy heart, there are only two audible heart murmurs: S1 and S2. S1 is the sound produced by the closure of the atrioventricular valves during ventricular contraction and is usually described as « lub » or first heart tone. The second cardiac tone, S2, is the sound of crescent-shaped valves that close during the ventricular diastole and is described as a « dub » (Figure 3). In both cases, when the valves close, the openings inside the atrioventricular septum protected by the valves are reduced, and blood flow through the opening becomes more turbulent until the valves are completely closed. There is a third heart tone, S3, but it is rarely heard in healthy people. It can be the sound of blood flowing through the atria, or blood sliding back and forth into the ventricle, or even tension in the chords tendineae. S3 can be heard in teenagers, some athletes and pregnant women. If the noise is heard later in life, it may indicate congestive heart failure, which warrants further testing. Some cardiologists refer to the collective sounds S1, S2 and S3 as « Kentucky gallops » because they mimic those produced by a galloping horse. The fourth cardiac tone, S4, results from the contraction of the atria pushing blood into a stiff or hypertrophic ventricle, indicating failure of the left ventricle.
S4 occurs before S1 and the collective sounds S4, S1 and S2 are called by some cardiologists a « Tennessee gallop » because they resemble the sound of a galloping horse with a different gait. Some people may have both S3 and S4, and this combined sound is called S7. The heart has a remarkable ability to absorb an increased volume of blood that enters the heart. In fact, the increase in the final diastolic volume also leads to an increase in cardiac output. This principle was described by two renowned physiologists and was therefore called the Frank-Starling mechanism of the heart. The underlying principle is that the heart pumps all the blood that returns to it through the veins within physiological limits. In a healthy heart, all activities and rest during each individual heart cycle or heartbeat are initiated and orchestrated by signals from the heart`s electrical conduction system, which is the heart`s « wiring » that carries electrical impulses through the body of cardiomyocytes, the heart`s specialized muscle cells. These impulses eventually stimulate the heart muscle to contract, thereby expelling blood from the heart chambers into the arteries and cardiovascular system; And they provide a complex and persistent signal system that controls the rhythmic beat of heart muscle cells, especially the generation of complex impulses and muscle contractions in the ear chambers. The rhythmic sequence (or sinus rhythm) of this signal transmission through the heart is coordinated by two groups of specialized cells, the sinus node (AS), located in the upper wall of the right atrium, and the atrioventricular (AV) node, located in the lower wall of the right heart between the atrium and ventricle. The sinus node, often called a pacemaker, is the starting point for generating a wave of electrical impulses that stimulates atrial contraction by creating an action potential via myocardial cells.   During ventricular contraction, the atria (atrial diastole) relax and receive venous reflux from the body and lungs. Then, in the ventricular diastole, the lower chambers relax, so that the thick-walled ventricles can first be passively filled and the atria emptied.
Later, during the final phase of ventricular relaxation, the atria contract. This atrial systole improves ventricular filling just before the onset of the next ventricular contraction. Increased excitability in places other than the pacemaker site predisposes the heart to the development of ectopic heartbeat. These can lead to uncoordinated contraction of the ventricles and different types of ventricular arrhythmias. Stages 1 and 2 together – « isovolumic relaxation » plus influx (equivalent to « rapid influence », « diastasis » and « atrial systole ») – include the « diastole » ventricular period, including the atrial systole, in which blood returning to the heart flows through the atria into the relaxed ventricles. Stages 3 and 4 together – « isovolumic contraction » plus « sputum » – are the ventricular period « systole », which is the simultaneous pumping of blood supplies separated from the two ventricles, one to the pulmonary artery and the other to the aorta. Remarkably, towards the end of the « diastole », the atria begin to contract, and then pump blood into the ventricles; This pressure delivery during ventricular relaxation (ventricular diastole) is called the atrial systemstole, also known as the atrial kick. [Citation needed] The cardiac cycle consists of four main phases of activity: 1) « Relaxation isovole », 2) Influx, 3) « Contraction isovole », 4) « Sputum ». (See wiggers` chart, which labels levels in order 3,4,1,2 from left to right.) If you move from the left along the Wiggers diagram, activities are displayed in four steps during a single cardiac cycle. (See the successive panels entitled « Diastole » then « Systole »). [Citation needed] The piping system consists of several components. The first part of the piping system is the sinus node.
Without neuronal stimulation, the sinus node rhythmically initiates impulses 70 to 80 times per minute. Because it determines the basic rhythm of the heart rate, it is called the pacemaker of the heart. Other parts of the conduction system include the atrioventricular node, atrioventricular bundle, beam branches, and conduction myofibers. All these components coordinate the contraction and relaxation of the heart chambers. The cardiac cycle is continuous. Filling of the ventricle (diastole) is followed by ventricular contraction (systole) to ensure adequate cardiac output both during rest and during exercise to meet the body`s metabolic needs. Systole and diastole influence each other intimately to achieve this goal. Normal elastic recoil after left ventricular contraction (LV) promotes early ventricle filling, with late diastolic atrial contraction ensuring that myocardial sarcomeres are stretched enough to optimize contractile strength. Exercise tests the health of this integrated system by shortening the time of myocardial filling and infusion, and a normally functioning cardiac electrical system is also needed for optimal performance.
Figure 6.4. Pressure-volume loop of the left ventricle for a single cardiac cycle. Now follows isovolume relaxation, in which the pressure in the ventricles begins to drop significantly, and after that, the atria begin to replenish, as blood flows into the right atrium (from the vena cava) and into the left atrium (pulmonary veins). When the ventricles begin to relax, the mitral and tricuspid valves open again, and the completed cycle returns to the ventricular diastole and a new « beginning » of the cardiac cycle.   The diastolic pressure curve is determined by filling the heart with increasing amounts of blood and then measuring the intraventricular pressure before the onset of contraction. In other words, this curve represents the terminal diastolic pressure for a given volume of blood. The systolic pressure curve is essentially the exact opposite of the diastolic pressure curve, as it is a measure of the intraventricular pressure at the end of the contraction for each filling volume. The heart is a four-chamber organ composed of the right and left halves, called the right heart and the left heart. The two upper chambers, the left and right atria, are entry points into the heart for blood flow back from the circulatory system, while the two lower chambers, the left and right ventricles, perform the contractions that expel blood from the heart to flow through the circulatory system. Circulation is divided into a pulmonary circulation, in which the right ventricle pumps oxygen-poor blood through the pulmonary trunk and arteries into the lungs; or systemic circulation – in which the left ventricle pumps/expels freshly oxygenated blood through the body through the aorta and all other arteries. [Citation needed] The cardiac cycle refers to all the events that occur from the beginning of a heartbeat to the beginning of the next and can be divided into two parts: a period of relaxation known as diastole and a period of contraction known as systole. The changes in pressure and volume that occur during the cardiac cycle for the left ventricle are illustrated in Figure 6.2 and serve as a platform to describe important events.