The endocardium is the innermost layer of heart tissue that lines the cavities and valves of the heart. This layer is composed of loose connective tissue and simple squamous epithelial tissue. The endocardium regulates the contractions of the heart, aids cardiac development, and may regulate the composition of the blood that feeds the tissues of the heart.
The heart sits in a fluid-filled sac called the parietal pericardium. The tough, outer fibrous layer of the parietal pericardium protects the heart and roots it in place. The thin, inner serous layer connects the sac to the heart, which is composed of three layers. On the outside, the epicardium, also called the visceral pericardium, is composed of connective tissue and fat. The visceral pericardium connects loosely with the parietal pericardium and tightly with the myocardium, the middle layer of tissue in the heart.
The myocardium is composed of cardiac muscle and sits between the epicardium and endocardium. The myocardium is responsible for the contractions of the heart, which occur spontaneously, or without stimulation from the nervous system. These contractions allow blood to enter the atria and pump blood out of the ventricles. The endocardium is the inner layer of the heart that connects with the myocardium and lines the atria and ventricles.
Humans have four chambers in their hearts: the right ventricle and the left ventricle in the bottom two quadrants of the heart, and the right atrium and left atrium in the top two quadrants of the heart. The atria receive blood from the body and pass it on to the body through atrioventricular (AV) valves. The ventricles accept blood from the atria and pump it out into the body.
Blood that has already been circulated and “used” by the body is pumped into the right atrium, which then passes it on to the right ventricle. The right ventricle receives de-oxygenated blood from the right atrium and pumps the blood out to the lungs to pick up more oxygen. The left atrium takes back the re-oxygenated blood and passes it on to the left ventricle, which in turn pumps the blood into the body. The muscle in the myocardium executes the contractions that move the blood through the heart, control the valves between chambers, and pump blood out of the heart. The endocardium does not trigger these contractions, but it helps to regulate them.
The endocardium lines the walls of the atria and ventricles and the valves between them. The cellular make-up of the endocardium is close to that of the endothelium, the tissue layer that lines the inside of blood vessels. On its luminal side, or the side closest to the cavities of the heart, it is composed of simple squamous epithelium, a single layer of scaly cells. Underneath lies a layer of loose connective tissue, a tissue with variable, widely spaced fibers.
Usually, heart injuries resulting from heart attacks do not extend as far inward as the endocardium, but if they do, it can be very serious. Damage to the inner lining of the heart can negatively impact the heart’s ability to contract at a quick, regular pace. Diseases such as endocarditis, a bacterial infection of the endocardium, are more typical in people with damaged heart valves.