**5. Anatomy and physiology of the heart**

The mammalian heart is a four-chambered muscular organ, which is located in the anterior mediastinum, posterior to the sternum and encapsulated by the pericardium. The pumping action of the heart is central to the functioning of the circulatory system. The CVS composed of the blood, the heart and blood vessels [11]. The heart is a strong muscular organ, which continues to pump blood to different parts of the body throughout our lives. It beats continuously using up a vast amount of energy daily [12]. The structure of the heart is depicted in **Figure 1** and it is composed mainly of three layers of muscles, namely the epicardium or the external layer, the middle layer or myocardium and the inner most layer or endocardium. Damage to these muscles and other conduction tissues in the heart due to diabetes and other diseases is responsible for SCD.

The larger and strong muscular tissue of the myocardium is responsible for ventricular contraction, and it is also divided into left and right sides by a septal wall. Each side of the heart is made up of two chambers consisting of the upper atria and the lower ventricles [12]. The left side of the heart pumps oxygen-rich blood to the different parts of the body via the aortic valve to the aorta (systemic circulation), while the right side delivers blood to lungs via the pulmonary valve and the pulmonary artery for oxygen replenishment in the lungs (pulmonary circulation). The heart has four valves which allow for unidirectional flow of blood thereby

*The mammalian heart. Components are described in the text (image courtesy www.beyondbiology.org).*

#### **Figure 2.**

*Diagram showing the electrical system of the mammalian heart with the conducting tissues including the SA node, AV node and Purkinje fiber network. SCD is usually caused by abnormal heart rhythms called arrhythmias and the most common life-threatening arrhythmia is ventricular fibrillation, which is an erratic, disorganized firing of impulses from the ventricles (see lower chambers with Purkinje network of fibers). When this occurs, the heart is unable to pump blood, and death will occur within minutes, if left untreated (image courtesy www.beyondbiology.org).*

preventing backflow. In turn, the right atrium receives retuning deoxygenated blood from the body through the superior and inferior vena cavae, while the left atrium receives oxygenated blood from the lungs through pulmonary vein. The heart itself needs a good supply of blood via the coronary arteries. These include the left anterior descending coronary artery, the left circumflex artery and the right coronary artery supplying the myocardium with oxygen-rich blood. The apex of the heart is the pointed end and the other end is called the base of the heart [13].

The orderly events that take place during the cardiac cycle are controlled by the electrical conduction system of the heart (**Figure 2**). An impulse is initiated at the sinoatrial node (SA-node) and then passes on to the atrioventricular node (AV-node) via conducting fibers via the atria. From the AV node, the impulse is conducted throughout the ventricles via the Purkinje fibers resulting in depolarization of the heart. Damage to the conducting tissues can result in sudden arrhythmias and possible SCD. Blood is pumped by the right ventricle into the pulmonary circulation at a lower pressure than blood pumped by the left ventricle into the systemic circulation. It follows that the haemodynamic stresses in the right and left side of the heart are very different. Even within the ventricles, the electro-mechanical properties of ventricular cardiac myocytes vary trans-murally [14].
