Overview of the cardiovascular system

This section of the heart was taken from an area similar to that outlined by the blue rectangle in the illustration. The left atrium (green X) is the upper chamber; the left ventricle (blue X) is the lower chamber; a flap of the atrioventricular valve (arrow) separates the two chambers.

The innermost tunic of either subdivision faces the blood and is called the endocardium in the heart and the tunica intima in blood vessels. In both subdivisions the innermost tunic is composed of a specialized simple squamous epithelium, called endothelium, and its underlying connective tissue.

The middle tunic of the heart is called the myocardium and is composed of cardiac muscle. In blood vessels the middle tunic is the tunica media and is composed of smooth muscle or smooth muscle plus connective tissue components.

The heart is suspended in an internal body cavity (pericardium), so its outer tunic is a serous membrane (epicardium). Like all serous membranes, epicardium is composed of a CT layer covered by a simple squamous epithelium (mesothelium). In blood vessels the outermost tunic is tunica adventitia, a CT layer, although smooth muscle is present in this tunic in large veins.

The heart and larger blood vessels are so large that they require their own blood supply, which is located in the outermost tunic of both subdivisions. Coronary vessels supply oxygen and nutrients to the heart while vasa vasorum (\”vessels of the vessels\”) are the equivalent blood supply to the larger vessels.