Vascular endothelial cells are flat cells that form the inner lining of major blood vessels. These cells help regulate blood flow. Their smooth surface allows blood to flow smoothly. They also help regulate blood pressure and prevent coagulation.
The major blood vessels are composed of three layers. The outer layer is called the tunica adventitia. This layer is elastic, and made of collagen and elastin. It also contains nerves and small blood vessels, which supply the vessel wall with oxygen. The elasticity allows the vessels to expand and contract as needed to maintain blood flow.
The middle layer is known as the tunica media, and is composed of smooth muscle. The inner layer is the tunica intima, which is a single layer of vascular endothelial cells, also known as the endothelium. Capillaries consist of only the tunica intima. This allows for the easy exchange of gases, nutrients, and waste products through their walls.
Endothelial nitric oxide synthase (NOS) is a vasodilator. It is produced by the endothelial cells, and causes the muscles of the vessel to relax. One factor that triggers the production of endothelial NOS is physical activity.
Vascular dilation is important during activity to keep blood pressure from rising too much. A second function of edothelial NOS is to keep platelets from sticking together in the vessels. Damage to the vascular endothelial cells causing a reduction of endothelial NOS is a risk factor for atherosclerosis.
Also produced by endothelial cells is the vasoconstrictor endothelin. Endothelin production is stimulated by hormones and causes the vascular muscles to contract. Platelet-stimulating factor is also a vasoconstrictor produced by vascular edothelial cells. In addition to constricting blood vessels, platelet-stimulating factor causes blood platelets to stick together. An overproduction of vasoconstrictors can lead to hypertension and heart failure.
Thrombin is a coagulant, which causes blood to clot. After it has bound with the endothelial-produced thrombomodulin, however, it becomes an anticoagulant. Vascular endothelial cells contain receptors for thromin, where it binds to them.
Too many or too few receptors might be a result of genetic factors and can cause clotting disorders. Injured endothelial cells may produced a procoagulant, called tissue factor (TF). The plaque buildup of atherosclerosis contains tissue factor.
Endothelial cells produce chemicals that regulate the flow of leukocytes and platelets. Injury or infection may cause leukocytes and platelets to bind to vascular endothelial cells and to each other. Too much binding is seem in inflammatory disorders.
Vascular endothelial cells play an important role in homeostasis. An increase or decrease in chemicals produced by these cells can have serious adverse affects on the body. The healing of damaged endothelial cells is partly genetic. Some people are more prone to diseases such as atherosclerosis and hemophilia, which are partly caused by problems with the endothelial cells.
