The endothelium is a thin layer of epithelial cells lining the inside of the blood vessels. It lines the entire circulatory system, including the heart and all the veins, arteries, and capillaries. The endothelium lining the heart is also called the endocardium, though it is similar to the tissue the lining blood vessels. The endothelium forms a barrier between the blood and the other structures of the circulatory system, allowing the blood to flow more smoothly, and therefore to circulate more quickly throughout the body.
The endothelium is simple squamous epithelium, meaning that it consists of a single layer of flat, or squamous, epithelial cells, making it the thinnest possible type of membrane. This single layer of cells may also be referred to as a monolayer. An epithelium in general is any type of tissue that serves to line the surface or cavity of any structure in the body. Epithelium is one of the four primary tissue types in the human body, the others being connective tissue, nervous tissue, and muscle tissue.
The endothelium is involved in a number of important functions. It helps control blood pressure through vasodilation and vasoconstriction, the widening and constricting of the blood vessels respectively. It also aids in blood clotting, minimizing damage and blood loss in the case of wounds.
Other endothelial functions include angiogenesis, or the growth of new blood vessels, and serving as a barrier by selectively allowing certain materials to exit or enter the bloodstream. In addition to the above functions, the endocardium also controls the development of the heart muscle in the embryo and into adulthood. It is also responsible for regulating the function of the myocardium, or the heart muscle.
The endothelium can also be involved in a variety of disorders. Both inflammation, an immune response characterized by redness and swelling, and atherosclerosis, in which the blood vessels become clogged with a fatty buildup that impedes blood flow, involve the endothelium. Endothelial dysfunction, in which the endothelium ceases to work properly, is an early sign of atherosclerosis, in addition to other circulatory system disorders. Loss of healthy endothelial function is also associated with smoking, coronary artery disease, diabetes mellitus, hypercholesterolemia, and hypertension or high blood pressure. Endothelial disfunction is typically an indicator of future cardiovascular problems.
The endocardium is sometimes affected in cases of myocardial infarction, or heart attack. In such cases, the tissue can become damaged due to ischemia, or insufficient blood supply. Another disorder affecting the myocardium is infective endocarditis, or bacterial infection of the myocardium. Infective endocarditis can cause symptoms including fever, heart murmur, hemorrhaging, and blood in the urine.