The endothelium is a thin, continuous sheet of cells that lines every blood vessel and lymphatic vessel in the body. It acts as a large, active organ that helps keep blood flowing smoothly, limits unwanted clotting, regulates inflammation, and maintains fluid balance. It is made up of an estimated trillion endothelial cells with a total surface area comparable to several hundred square meters, which is why many experts regard it as one of the body’s largest endocrine organs. When the...