Human Heart
Heart has two chambers in each side: atria and ventricle. Inter-atrial septum separates the two thin-walled atria and two thicker-walled ventricles possessing common wall in the inter-ventricular septum. Fibrous A-V ring is used to connect these atria and ventricles (Saini, 2009). Four valves are used to regulate the blood flow in the heart. The right auricle (RA) collects the impure blood and then the blood passes from the right auricle (RA) to right ventricle (RV) via Tricuspid valve. Thus, it regulates the impure blood flow between the right atria and right ventricles. The impure blood is pumped out through the pulmonary valve to the lungs for its purification by the right ventricle (RV). Afterwards, the blood from the left atrium (LA) is passed to the left ventricle (LV) via mitral valve (Renu Madhavi, 2012). Lastly, the aortic valve is situated at the aortic orifice. The AV ring is penetrated on left side and right side by mitral and tricuspid valve as shown in Figure 1.
The conduction system of the heart consists of Sinoatrial (SA) node, Atrioventricular (AV) node, bundle of His, bundle branches and purkinje fibres as shown in Figure 2. SA node placed at the top of the right atrium, initiates an electrical signal to the AV node which is further transmitted to the left and right bundle branches. The impulse then reaches to the myocardium through the purkinje fibres and contracts it. The two physiological events such as heart mechanical activity (pumping of the blood) and heart electrical activity (the transmission of electrical impulses) gives rise to an orderly heartbeat.
The SA node transmits electrical impulses depending upon the physical demands, stress and hormonal factors (Saini, 2009; Renu Madhavi, 2012). The presence of various cell types such as the atrial cells, ventricular cells, and the cells that constitute the conduction system (Purkinje fibers) and the pacemaker cells in the heart are recognized by the cardiac electrophysiology. The pacemaker cells undergo repetitive cyclic activation which further initiates a contraction. This contraction results in cyclic electrical activity i.e., the periodic heartbeat. This cyclic activation propagates to adjoining atrial tissues by means of local circuit (action) currents. The presence of low resistance intercellular structure facilitates the flow of this current from active to inactive neighboring cells.
The current flow from one cell to another cell until the entire left and right atria are activated. The direct propagation from the atria to ventricles cannot occur because both are separated by fibrous tissue. Initially the conduction in the AV junction is slow and cardiac muscles instigate the successive mechanical contraction which results in the completion of atrial contraction and delay in the ventricular activation and contraction. The ventricular activation occurs over a wide region when electrical impulse reaches the bundles of His where the conduction becomes very fast. The impulse then reaches to the myocardium through the purkinje fibers and contracts it which results in QRS complex. Ventricular repolarisation is the last activity of cycle. The outcome of all these activities of the heart generates the Electrocardiogram (ECG) signal (Saini, 2009; Renu Madhavi, 2012).