cardiac physiology cardiac physiology cardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; myocardium structure; the electrical conduction system of the heart; the cardiac cycle and cardiac output and how these interact and depend on one another. blood flow main articles: cardiac cycle and heart valve blood flow through the valves 3d echocardiogram viewed from the top, with the upper part of the ventricles removed and the mitral valve clearly visible (cusps are not clear and pulmonary valve not visible). on the left are two, two-dimensional views showing tricuspid and mitral valves (above) and aortic valve (below). blood flow diagram of the human heart. blue components indicate de-oxygenated blood pathways and red components indicate oxygenated pathways. the heart functions as a pump and acts as a double pump in the cardiovascular system to provide a continuous circulation of blood throughout the body. this circulation includes the systemic circulation and the pulmonary circulation. both circuits transport blood but they can also be seen in terms of the gases they carry. the pulmonary circulation collects oxygen from the lungs and delivers carbon dioxide for exhalation. the systemic circuit transports oxygen to the body and returns relatively de-oxygenated blood and carbon dioxide to the pulmonary circuit. blood flows through the heart in one direction, from the atria to the ventricles, and out through the pulmonary artery into the pulmonary circulation, and the aorta into the systemic circulation. the pulmonary artery (also trunk) branches into the left and right pulmonary arteries to supply each lung. blood is prevented from flowing backwards (regurgitation) by the tricuspid, bicuspid, aortic, and pulmonary valves. the function of the right heart, is to collect de-oxygenated blood, in the right atrium, from the body via the superior vena cava, inferior vena cava and from the coronary sinus and pump it, through the tricuspid valve, via the right ventricle, through the semilunar pulmonary valve and into the pulmonary artery in the pulmonary circulation where carbon dioxide can be exchanged for oxygen in the lungs. this happens through the passive process of diffusion. in the left heart oxygenated blood is returned to the left atrium via the pulmonary vein. it is then pumped into the left ventricle through the bicuspid valve and into the aorta for systemic circulation. eventually in the systemic capillaries exchange with the tissue fluid and cells of the body occurs; oxygen and nutrients are supplied to the cells for their metabolism and exchanged for carbon dioxide and waste products in this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood. the ventricles are stronger and thicker than the atria, and the muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation. atria facilitate circulation primarily by allowing uninterrupted venous flow to the heart, preventing the inertia of interrupted venous flow that would otherwise occur at each ventricular systole. cardiac muscle main article: cardiac muscle cardiac muscle tissue has autorhythmicity, the unique ability to initiate a cardiac action potential at a fixed rate – spreading the impulse rapidly from cell to cell to trigger the contraction of the entire heart. this autorhythmicity is still modulated by the endocrine and nervous systems. there are two types of cardiac muscle cell: cardiomyocytes which have the ability to contract easily, and modified cardiomyocytes the pacemaker cells of the conducting system. the cardiomyocytes make up the bulk (99%) of cells in the atria and ventricles. these contractile cells respond to impulses of action potential from the pacemaker cells and are responsible for the contractions that pump blood through the body. the pacemaker cells make up just (1% of cells) and form the conduction system of the heart. they are generally much smaller than the contractile cells and have few of the myofibrils or myofilaments which means that they have limited contractibility. their function is similar in many respects to neurons. the bundle of his and purkinje fibres are specialised cardiomyocytes that function in the conduction system. structure of cardiac muscle details of intercalated discs cardiomyocytes, are considerably shorter and have smaller diameters than skeletal myocytes. cardiac muscle (like skeletal muscle) is characterized by striations – the stripes of dark and light bands resulting from the organised arrangement of myofilaments and myofibrils in the sarcomere along the length of the cell. t (transverse) tubules are deep invaginations from the sarcolemma (cell membrane) that penetrate the cell, allowing the electrical impulses to reach the interior. in cardiac muscle the t-tubules are only found at the z-lines. when an action potential causes cells to contract, calcium is released from the sarcoplasmic reticulum of the cells as well as the t tubules. the calcium release triggers sliding of the actin and myosin fibrils leading to contraction. a plentiful supply of mitochondria provide the energy for the contractions. typically, cardiomyocytes have a single, central nucleus, but can also have two or more. cardiac muscle cells branch freely and are connected by junctions known as intercalated discs which help the synchronized contraction of the muscle. the sarcolemma (membrane) from adjacent cells bind together at the intercalated discs. they consist of desmosomes, specialized linking proteoglycans, tight junctions, and large numbers of gap junctions that allow the passage of ions between the cells and help to synchronize the contraction. intercellular connective tissue also helps to strongly bind the cells together, in order to withstand the forces of contraction. cardiac muscle undergoes aerobic respiration patterns, primarily metabolizing lipids and carbohydrates. oxygen from the lungs attaches to haemoglobin and is also stored in the myoglobin, so that a plentiful supply of oxygen is available. lipids, and glycogen are also stored within the sarcoplasm and these are broken down by mitochondria to release atp. the cells undergo twitch-type contractions with long refractory periods followed by brief relaxation periods when the heart fills with blood for the next cycle. electrical conduction main articles: electrical conduction system of the heart and cardiac action potential transmission of a cardiac action potential through the heart's conduction system it is not very well known how the electric signal moves in the atria. it seems that it moves in a radial way, but bachmann's bundle and coronary sinus muscle play a role in conduction between the two atria, which have a nearly simultaneous systole. while in the ventricles, the signal is carried by specialized tissue called the purkinje fibers which then transmit the electric charge to the myocardium. if embryonic heart cells are separated into a petri dish and kept alive, each is capable of generating its own electrical impulse followed by contraction. when two independently beating embryonic cardiac muscle cells are placed together, the cell with the higher inherent rate sets the pace, and the impulse spreads from the faster to the slower cell to trigger a contraction. as more cells are joined together, the fastest cell continues to assume control of the rate. a fully developed adult heart maintains the capability of generating its own electrical impulse, triggered by the fastest cells, as part of the cardiac conduction system. the components of the cardiac conduction system include the atrial and ventricular syncytium, the sinoatrial node, the atrioventricular node, the bundle of his (atrioventricular bundle), the bundle branches, and the purkinje cells. sinoatrial (sa) node main article: sa node diagrammatic conduction system of the heart normal sinus rhythm is established by the sinoatrial (sa) node, the heart's pacemaker. the sa node is a specialized grouping of cardiomyocytes in the upper and back walls of the right atrium very close to the opening of the superior vena cava. the sa node has the highest rate of depolarization. this impulse spreads from its initiation in the sa node throughout the atria through specialized internodal pathways, to the atrial myocardial contractile cells and the atrioventricular node. the internodal pathways consist of three bands (anterior, middle, and posterior) that lead directly from the sa node to the next node in the conduction system, the atrioventricular node. the impulse takes approximately 50 ms (milliseconds) to travel between these two nodes. the relative importance of this pathway has been debated since the impulse would reach the atrioventricular node simply following the cell-by-cell pathway through the contractile cells of the myocardium in the atria. in addition, there is a specialized pathway called bachmann's bundle or the interatrial band that conducts the impulse directly from the right atrium to the left atrium. regardless of the pathway, as the impulse reaches the atrioventricular septum, the connective tissue of the cardiac skeleton prevents the impulse from spreading into the myocardial cells in the ventricles except at the atrioventricular node. the electrical event, the wave of depolarization, is the trigger for muscular contraction. the wave of depolarization begins in the right atrium, and the impulse spreads across the superior portions of both atria and then down through the contractile cells. the contractile cells then begin contraction from the superior to the inferior portions of the atria, efficiently pumping blood into the ventricles. atrioventricular (av) node main article: av node the atrioventricular (av) node is a second cluster of specialized myocardial conductive cells, located in the inferior portion of the right atrium within the atrioventricular septum. the septum prevents the impulse from spreading directly to the ventricles without passing through the av node. there is a critical pause before the av node depolarizes and transmits the impulse to the atrioventricular bundle. this delay in transmission is partially attributable to the small diameter of the cells of the node, which slow the impulse. also, conduction between nodal cells is less efficient than between conducting cells. these factors mean that it takes the impulse approximately 100 ms to pass through the node. this pause is critical to heart function, as it allows the atrial cardiomyocytes to complete their contraction that pumps blood into the ventricles before the impulse is transmitted to the cells of the ventricle itself. with extreme stimulation by the sa node, the av node can transmit impulses maximally at 220 per minute. this establishes the typical maximum heart rate in a healthy young individual. damaged hearts or those stimulated by drugs can contract at higher rates, but at these rates, the heart can no longer effectively pump blood. bundle of his, bundle branches, and purkinje fibers arising from the av node, the bundle of his, proceeds through the interventricular septum before dividing into two bundle branches, commonly called the left and right bundle branches. the left bundle branch has two fascicles. the left bundle branch supplies the left ventricle, and the right bundle branch the right ventricle. since the left ventricle is much larger than the right, the left bundle branch is also considerably larger than the right. portions of the right bundle branch are found in the moderator band and supply the right papillary muscles. because of this connection, each papillary muscle receives the impulse at approximately the same time, so they begin to contract simultaneously just prior to the remainder of the myocardial contractile cells of the ventricles. this is believed to allow tension to develop on the chordae tendineae prior to right ventricular contraction. there is no corresponding moderator band on the left. both bundle branches descend and reach the apex of the heart where they connect with the purkinje fibers. this passage takes approximately 25 ms. the purkinje fibers are additional myocardial conductive fibers that spread the impulse to the myocardial contractile cells in the ventricles. they extend throughout the myocardium from the apex of the heart toward the atrioventricular septum and the base of the heart. the purkinje fibers have a fast inherent conduction rate, and the electrical impulse reaches all of the ventricular muscle cells in about 75 ms. since the electrical stimulus begins at the apex, the contraction also begins at the apex and travels toward the base of the heart, similar to squeezing a tube of toothpaste from the bottom. this allows the blood to be pumped out of the ventricles and into the aorta and pulmonary trunk. the total time elapsed from the initiation of the impulse in the sa node until