**4.2 Layers of the heart wall**

There are three distinct layers that make up the walls of a healthy human heart (as illustrated in **Figure 3**). These are—(i) Superficial Epicardium (ii) Middle Myocardium, and (iii) Inner Endocardium.

i. Superficial Epicardium: This layer is the most external to the heart. It is the visceral layer of the pericardial sac, which constitutes the innermost layer of the serous pericardium. An exterior layer of flat mesothelial cells forms this epicardium, with a layer of adipose and connective tissue lying beneath [25]. This inner layer shields the heart and it directly connects the epicardium to the muscular myocardium. This epicardium houses the blood vessels and nerves that furnish the heart [25]. At the base of the great vessels, the epicardium extends as the pericardial sac, composing an enclosed pericardial cavity.

**Figure 3.** *Physiology of healthy human heart (designed with biorender).*

