One function of endothelial cells is to:

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11ecba2d_0fda_ba3c_a3bf_7b0de21a93c6_MD0008_00 The endothelium is the one-cell-thick layer of specialized epithelial cells (ie, endothelial cells) that line the interior of the cardiovascular system, which includes the heart, arteries, capillaries, and veins. The endothelium, which is in direct contact with the blood in the lumen (inner cavity), is a relatively smooth, friction-reducing surface that enables the efficient movement of blood through the vasculature. The endothelium also acts as a barrier that regulates the exchange of substances between the blood and nearby tissues. For example, in response to inflammatory chemicals released during an infection, endothelial cells promote the movement of white blood cells out of the bloodstream to the affected tissue. (Choice A) In arteries and veins, the interior layer of endothelial cells is surrounded by a layer of smooth muscle. Contraction of the smooth muscle layer results in vasoconstriction and increased blood pressure, whereas relaxation of the smooth muscle layer results in vasodilation and decreased blood pressure. The contraction of smooth muscle is generally regulated by the autonomic nervous system and hormones; however, the endothelium itself does not contract. (Choice B) Epithelial cells are packed together tightly into sheets that make up the outer layer of the skin (the epidermis), the secretory components of glands, and the lining of the inner cavities of the body (eg, the lumen of hollow organs). Endothelial cells are a specialized type of epithelial cell that line the inside of blood vessels and lymph vessels specifically. Endothelial cells do not line the lumen of the digestive tract. (Choice C) When the blood vessel wall is damaged, platelets (non-nucleated cellular fragments derived from megakaryocytes) adhere to the exposed connective tissue and form a plug at the site of injury. This plug is later stabilized as a clot by fibrin, an insoluble protein that polymerizes into long fibers that cross-link like a net over the damaged area. Platelets and fibrin, not endothelial cells, make up the clot. Educational objective: The endothelium is a single-cell layer that makes up the innermost surface of the cardiovascular system (heart and blood vessels). The endothelium promotes blood fluidity and functions as a selective barrier between the blood and surrounding tissues.

One Function of Endothelial Cells Is To