Endothelial Cells play a key role in angiogenesis, the development of new blood vessels from pre-existing vessels.They line the entire circulatory system, from the heart to the smallest capillaries. These cells have very distinct and unique functions that are paramount to vascular biology. These functions include fluid filtration, such as in the glomeruli of the kidney, blood vessel tone, hemostasis, neutrophil recruitment, and hormone trafficking.