Balancing blood pressure
The layer of smooth muscle cells is stimulated with chemicals secreted by the endothelial cells and the tone of the vessels are controlled through the constriction and relaxation of the vessels. Therefore, an important duty in the regulation of blood pressure is given to these cells. During aninfection, bacteria circulate in the blood stream and the blood pressure falls extremely low. Tissue nutrition is upset (septic shock) and an excess of muscle-relaxing substance is released by the endothelial structure. Veins and arteries become too relaxed and there is a considerable drop in blood pressure (hypotension). On the other hand, with problems like atherosclerosis, the endothelial cells cannot fulfill their duty and due to a deficiency in nitrogen oxide, they become immune to the stimulus to relax the muscles. The resulting problem in this situation is hypertension.
Endothelial cells prevent hemorrhage
In order for a hemorrhage to stop the vessels that are bleeding need to narrow down. This is very important in the first stages of blood loss, particularly when there is a problem with blood clotting. When a hemorrhage begins, the endothelial cells are ordered to excrete a substance called endothelin. This starts the narrowing down of the bleeding vessels. Endothelin is not excreted in normal vessels. When the umbilical cord of a newborn is cut, it prevents the baby from losing blood.
Endothelial cells in blood clotting
The duty of endothelial cells can prevent or facilitate blood clotting, depending on the situation. First of all, they prevent the blood cells from adhering to the vessel walls and prevent clotting inside the vessels. Imagine water flowing through a pipe. The speed of the flow is greater in the center and lower at the periphery. Therefore, in the long run, some residue forms inside the pipe. In the veins and arteries, the flow of blood near the walls is also slower. To prevent the formation of any residue, both the endothelial cells and the blood cells are created with negative loaded surfaces and the blood cells are pushed towards the center. In addition, a substance called prostocyclin (PGI2) is excreted and the thrombocytes change their structure. As a result, residue formation and clotting is prevented along the vessel walls. In a case of any long term damage to the endothelium (e.g. due to smoking, diabetes, or hypertension), the relevant protection mechanism fails, and clotting inside the vessels results in thrombosis. Some serious cases can even necessitate the amputation of a limb. The endothelial cells can also facilitate clotting when necessary. In case of bleeding due to a wound, they function contrarily and help the blood to clot to prevent blood loss.
Endothelial cells in the bone marrow, the liver, and spleen
As a divine blessing, the endothelial cells form a looser layer in these organs and vessel permeability is increased. Thanks to this increase, matter Exchange with blood is easily realized; blood reaches these organs, which are responsible for the constant control of the contents of the blood, easily.
Endothelial cells in the brain and eyes
The endothelial cells in organs like the brain and eyes are very closely integrated forming a barrier between the blood and the organs. This is to such an extent that the major nutrients of the brain, like glucose and oxygen, pass without any obstacles, but several chemicals, including medication, are blocked by the selective-permeability of this protective mechanism. Research has proven that various substances injected into the bloodstream reach almost all the tissues except for the brain. Thanks to the efficient protective mechanism that has been given to these minute cells, the brain is saved from a great deal of negative effects. Even a single cell is not left to chance and nothing happens randomly. As can be seen throughoutthe universe, opposites are made to work hand in hand in the human body as well in a splendid harmony for the continuation of life.
Vinay Kumar, Abul K. Abbas, Nelson Fausto, Richard Mitchell, Robbins Basic Pathology, W.B. Saunders; 8th edition, 2007. Hall, John E., Arthur C. Guyton, Textbook of Medical Physiology,