As endothelium ( lat. Endothelium ) refers to the the vessel lumen directed towards the cells of the innermost wall layer of lymphatic and blood vessels ( tunica intima ). Regardless of their structure, which varies depending on the type of vessel, all vessels in the cardiovascular system are lined with a single-cell layer of endothelial cells.
The total weight of all endothelial cells of an average person is 1 kilogram, the number of endothelial cells in a human body is a good 10 trillion and the surface area that comes into contact with the blood is 4000 to 7000 m². This alone shows the importance of the endothelial system for the entire organism .
The endothelial glycocalyx clothes luminal any healthy vessel and is adjacent to the endothelial cell itself an important part of the vascular barrier. The endothelial glycocalyx plays a decisive role in the development of a colloid osmotic concentration gradient : the inwardly directed gradient, which retains water and protein in the vascular system, is created below the endothelial glycocalyx through selective protein filtering across it. The findings about the endothelial glycocalyx together with the finding that the interstitium, contrary to what has been assumed for a long time, has an almost plasma-equivalent protein concentration, led to a modification of the Starling equation .
The endothelium is more than a mere coating of the vessel wall and is involved in a multitude of different physiological processes:
- As a barrier, it regulates the exchange of substances between tissue and blood .
- It produces substances that are important for regulating blood pressure, e.g. B. Nitric oxide (NO), which regulates the tone of the vascular muscles in the cardiovascular system .
- Furthermore, it influences the flowability of the blood, u. a. by inhibition and activation of coagulation processes .
Endothelial cells produce a factor that is important for the activation of fibrinolysis , the tissue plasminogen activator (tPA). This factor activates the formation of plasmin from plasminogen through hydrolytic cleavage and thus modulates the thrombus dissolution in the organism. It also prevents menstrual blood from clotting.
- The endothelium also plays an important role in inflammatory processes . Various endogenous or microbial substances can activate the endothelium locally. This activation causes certain white blood cells ( leukocytes ), such as B. neutrophils , monocytes , macrophages and T lymphocytes , from the blood bind to the endothelium and are in turn activated. The activated leukocytes then migrate during leukodiapedesis (also known as transmigration) through the endothelium into the underlying tissue, where they help fight infections .
- The sprouting of new, outbound from existing blood vessels vessels is also carried by endothelial cells ( angiogenesis ).
Therefore, a functional disorder of the endothelium ( endothelial dysfunction ) always has serious pathological consequences. In medical research, for example, injury to the endothelial cell layer is suspected to be a possible cause of arteriosclerosis ( response-to-injury hypothesis). An early marker of endothelial dysfunction is the detection of small amounts of albumin in the urine ( microalbuminuria ).
Different types of endothelium
There are basically three different types of endothelium: the continuous , the fenestrated and the discontinuous endothelium. The permeability of the various types of endothelium to the various substances in the blood increases in the order mentioned.
The continuous endothelium is by the formation of tight junctions in a first approximation impermeable to dissolved substances in the blood. (One example is the endothelium of the blood-brain barrier .) The exchange of substances between tissue and blood, which is nevertheless necessary , is strictly controlled by highly selective transport mechanisms.
The fenestrated endothelium is considerably more permeable: Even larger molecules pass through the “window” (lat. Fenestra ) in the endothelium without any problems. These windows are about 70 nm in diameter and - in humans with one exception, the kidneys - always have diaphragms (imaginable as spokes), which again somewhat restrict the permeability of very large molecules and cells . This type of endothelium has a continuous basement membrane and occurs in the kidney glomerulus , in the intestine and in endocrine glands .
The discontinuous endothelium is very permeable; it can even happen to cells. Some of the endothelial cells are not in contact with each other, which results in large "holes" in the endothelial wall. In addition, the basement membrane is partially or completely non-existent. (An example is the endothelium of the liver .)