The heart wall consists of three layers: the inner, which is the endocardium, the middle, which is the myocardium, and the outer, which is the epicardium. These three layers are surrounded by a bi-membranous sac, the pericardium.
The inner layer of the heart wall is called the endocardium and is lined with simple squamous epithelium. It covers the heart’s chambers from the inside and covers its valves. The endocardium continues to line the veins and arteries of the heart, and connects it to the heart muscle under which is a thin layer of connective tissue.[9] The endocardium layer that lines the heart may play a role in regulating myocardial contraction through its secretion of endothelin.[9]
The pattern of muscle fiber distribution in the heart helps it pump blood efficiently.
The middle layer of the heart wall is the muscular layer which is the myocardium – involuntary striated muscle tissue surrounded by a framework of collagen. The muscle fibers are distributed in the heart in a complex and elegant pattern, where the muscle cells take a spiral or vortex shape around the chambers of the heart, where the outer layers take the form of the number (8) around the atria and the base of the great vessels, while the inner fibers take the form of the number (8) around the ventricles and head towards the apex the heart. This complex pattern allows the heart to pump blood more efficiently.[9]
There are two types of cells in the heart muscle: muscle cells that can contract easily, and pacemaker cells that belong to the conductive system of the heart. Muscle cells make up about 99% of the mass of the atria and ventricles. These contractile cells are connected to each other by intercalating discs that allow the muscle cells to respond quickly to action potential impulses released by the pacemaker cells, as these discs allow the muscle cells to contract together as if they were a syncytial mass, thereby allowing the heart to contract so that it can eject blood into the main arteries .[9] While pacemaker cells make up about 1% of the heart’s mass, they form the heart’s conductive system. Pacemaker cells are much smaller than contractile cells, and they have fewer myofibrils than contractile cells, which gives them a limited ability to contract. The function of mucous pacemaker cells is in many ways similar to that of neurons.[9] Cardiac tissue has the property of autorhythmia—the ability to initiate a cardiac action potential at a constant rate—and to rapidly propagate an impulse from cell to cell to trigger contraction of the entire heart. [9]
The pericardium surrounds the heart, and it consists of two membranes: a serous inner called epicardium, and a fibrous outer.[9] Between the two membranes is the pericardial cavity, which contains pericardial fluid, which acts as a lubricant on the surface of the heart.[26]
