The endothelium (Latin endothelium ) is referred to the vessel lumen directed toward cells of the innermost layer of the wall of lymphatic and blood vessels ( tunica intima ). Regardless of their different depending on the type of container construction all vessels are lined from the cardiovascular system with a single-cell layer of endothelial cells.
The total weight of all endothelial cells of the average person is 1 kg, the number of endothelial cells of a human body is around 10,000 billion, and thereby passing the blood-contacting surface of 4,000 to 7,000 sqm. This alone already shows the importance of the endothelial system for the whole organism.
And the rear of the cornea epithelium is often called the corneal endothelium.
The endothelium composed of endothelial cells that form a single-layer squamous epithelium, which is seated a basal lamina.
The endothelial glycocalyx dresses luminal of any healthy vessel and is adjacent to the endothelial cell itself an important part of the vascular barrier. The endothelial glycocalyx plays a crucial role in building a colloid osmotic concentration gradient: the inward gradient, the water and protein in the vascular system retains arises below the endothelial glycocalyx by selective protein filtration across it. The findings on the endothelial glycocalyx, together with the finding that the interstitium differently long time believed almost same plasma protein concentration has resulted in a modification of the Starling equation.
The endothelium is more than a mere coating of the vessel wall and involved in a variety of different physiological processes:
- It regulates a barrier to the exchange of substances between blood and tissue.
- It produces for the regulation of blood pressure important substances such as nitric oxide ( NO), which is used in the regulation of vascular tone of the muscles in the cardiovascular system.
- Furthermore, it affects the blood fluidity, including by inhibition and activation of coagulation processes.
Endothelial cells produce an important for the activation of fibrinolysis factor, tissue plasminogen activator ( tPA). This factor activated by hydrolytic cleavage, the formation of plasmin from plasminogen and thus modulates the Thrombenlösung in the organism. It also prevents the clotting of menstrual blood.
- The endothelium also plays an important role in inflammatory processes. Various endogenous or microbial substances can activate the endothelium locally. This activation leads to the fact that bind certain white blood cells (leukocytes), such as neutrophils, monocytes, macrophages and T-lymphocytes from the blood to the endothelium, and in turn, are activated. The activated leukocytes then migrate in the Leukodiapedese (also called transmigration ) through the endothelium into the underlying tissue and help there to fight infection.
- The sprouting of new, outbound from existing blood vessels vessels is also effected by endothelial cells ( angiogenesis).
Therefore, a dysfunction of the endothelium ( endothelial dysfunction ) has always serious pathological consequences. Thus, a violation of the endothelial cell layer in medical research is suspected as a possible cause for the formation of atherosclerosis (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 the endothelium
There are basically three different types of endothelia: the continuous, the fenestrated and discontinuous endothelium. The permeability of the different Endothelarten for the various substances in the blood is increased in the order named.
The continuous endothelium is impermeable by the formation of tight junctions in a first approximation for dissolved substances in the blood. ( An example is the endothelium of the blood -brain barrier. ) The still necessary exchange of substances between blood and tissue is strictly controlled by highly selective transport mechanisms.
The fenestrated endothelium is already permeable to some: Even larger molecules pass through the " window " (Latin fenestra ) in the endothelium without problems. These windows are large in diameter about 70 nm and possess - in humans, with one exception, the kidney - always Diaphragmata ( conceivable as spokes), which again restrict the transmission of very large molecules and cells a little. This Endothelart has a continuous basement membrane and comes in the renal glomerulus, in the intestine as well as in endocrine glands before.
The discontinuous endothelium is highly permeable; even cells can make it happen. The endothelial cells are not in contact with each other, resulting in large " holes" in the endothelial wall result in part. In addition, the basement membrane is partially or completely non-existent. ( An example is the endothelium of the liver. )
- Histology of the circulatory organs
- Circulatory system