Endothelium

Single-layered, non-cornifying squamous epithelium lining cardiac chambers, blood and lymph vessels, which acts as a selective barrier between the blood stream and the vessel wall for dissolved substances, macromolecules and cells. The endothelium actively controls the exchange between vascular lumen and interstitium.

The endothelium regulates vascular tone, e.g. by the vasoconstrictor endothelin-1, and plays an important role in the interaction of leukocytes with the vessel wall, e.g. in inflammation.

Changes in the blood flow, mechanical or metabolic, are registered and transmitted by the endothelium. The endothelium synthesizes a large number of mediators and can therefore be considered a large endocrine organ.

In addition to its tasks as a vascular regulator, the endothelium plays a central role in the coagulation process and participates in both coagulation-promoting and anticoagulation processes.

An important function of the endothelium for the optimisation of the flow properties of the blood is based on its slightly negative charge. Since thrombocytes and erythrocytes also have a slightly negative charge, the electromagnetic repulsive forces enable a smooth flow of the cellular particles of the blood.

Soluble substances and macromolecules can diffuse through the endothelium by moving between the cells of the endothelium. Another possibility of substance exchange in the endothelium is the transcellular movement in cytoplasmic vesicles. In case of inflammation or injury, the permeability of the endothelium increases. This process is triggered by histamine, thrombin, serotonin, thrombin and bradykinin. These mediators cause gaps between the endothelial cells, resulting in an unselective extravasation of macromolecules and water and thus tissue oedema, which manifests itself clinically by swelling, redness and pressure pain.

During inflammatory processes, cytokines and leukocytes migrate into the vessel walls and surrounding tissue and are bound to the endothelium with the aid of so-called selectins. In a further step, the leukocytes are additionally bound more strongly to the endothelium by adhesion molecules (ICAM). By secretion of proteases, the endothelia are dissolved and the leukocytes migrate into the surrounding tissue. These processes also play an important role in the development of arteriosclerosis. As soon as tissue injury occurs and subendothelial structures come into contact with the components of the blood, the coagulation process is initiated.

The first response to trauma is nerve endothelin-1 and adrenalin-mediated vasoconstriction with the aim of throttling the blood vessels. Endothelin is released immediately after injury and leads to vascular contraction. The endothelium produces a large number of clot-activating substances.

1. HA Neumann (2014) The coagulation system. ABW-Scientific Publisher GmbH Berlin