The tunica media is a component of the walls of blood and lymph vessels that lies between two other layers. Among other things, it contains muscle cells that the body uses to regulate the width of the veins. Damage to the tunica media can lead to narrowing of the blood vessels (arteriosclerosis).
What is the tunica media?
The tunica media is a part of the walls of veins and arteries. To distinguish it from the middle layer of the eye (tunica media bulbi or uvea), physicians sometimes refer to the middle vascular layer as the tunica media vasorum. It is surrounded by the tunica adventitia or tunica externa. The tunica externa forms the outermost wall of the blood vessels. Towards the inside of the arteries and veins lies the tunica intima. The wall of the lymphatic vessels also has a tunica media in the middle. The tissue of the tunica media is not uniform in nature, but is composed of muscle cells, collagen, elastic fibers and connective tissue. The muscle cells are particularly important for the transport of fluid in the vessels. With age, the elasticity of the vessel walls decreases and can lead to normative narrowing.
Anatomy and structure
Some cells in the vessel wall are muscle cells. Because the larger arteries must pump blood through the body, they have thicker tunica media. The extra muscle cells help the blood vessels build up the necessary pressure. Between them are collagen, a special protein molecule, and elastic fibers. The latter give the tissue its flexibility. In addition, the tunica media consists of connective tissue that supports the other cells and keeps them in shape. The connective tissue also plays a supplying role: it passes on nutrients and oxygen to the other cells and distributes the resources. Physicians divide arteries into different types; the differences are also reflected in the tunica media. For example, muscular arteries have stronger muscles, while elastic arteries have more elastic fibers and collagen.
Function and tasks
The tunica media plays a crucial role in ensuring that blood flows evenly throughout the human body. In the arteries, blood flows away from the heart. In the lungs, red blood cells take up and distribute oxygen. The heart serves as a pump in this process. But the arteries themselves must also drive the blood to keep it flowing. People can easily feel the rhythmic pumping of the larger arteries; this is why the blood vessels are also called arteries. When arteries are injured, blood often literally shoots out of the wound, illustrating the high pressure inside the vessel. In order for the arteries to carry out their pumping movements, they need muscles. The muscle layer is located in the tunica media and forms a ring around the arteries. The muscle cells in the tunica media belong to the smooth muscle and thus belong to the same fiber type as the heart muscle. Humans cannot consciously control or suppress these movements. Not only blood vessels have a vessel wall with a tunica media; lymphatic vessels also depend on it. Lymphatic vessels collect fluid from the spaces between cells. They appear in almost every major tissue. Similar to blood vessels, they can vary in size and flow into each other. Eventually, the lymphatic vessels release the collected fluid into the blood vessels. Excess fluid is excreted by the body through urine. Accordingly, the lymphatic system serves to transport fluid and ensure that water does not accumulate in the intercellular spaces. Furthermore, the lymphatic vessels also transport certain macromolecules – for example, proteins and lymphocytes, which are part of the immune system.
Diseases
The tunica media may be involved in the development of arteriosclerosis, among other diseases. This is a blockage of the bloodstream for which various causes may be responsible. For example, blood fats called triglycerides can form clumps in the arteries and veins by depositing the molecules on the walls of the vessels. This leaves less room for the bloodstream to pass through the affected area. The risk for such deposits is particularly high at vascular valves and in finer veins. As a result of a vascular occlusion, the body can no longer supply the tissue behind it with oxygen and nutrients.The removal of carbon dioxide, other waste products and cellular products is also disturbed by arteriosclerosis. In addition, the deposits can break loose and travel with the bloodstream to other parts of the body. They either dissolve or occlude the vessels in which they become trapped. In this way, a blockage of the arteries potentially leads to stroke, heart attack or pulmonary embolism; other tissue can also be affected by arteriosclerosis and, in the worst case, die. Proper muscle movement of the arteries is also required to keep them from closing. The tunica media contains smooth muscle that allows the blood vessels to dilate or constrict as needed. High blood pressure (hypertension) can cause the tunica media to become damaged: The cells in the vessel wall receive little oxygen and die: As a result, the regulation of arterial width is disturbed and the vein can narrow to the point of arteriosclerosis. In Mönckeberg sclerosis, calcium is deposited in the tunica media and also leads to functional limitations of the blood vessels.
