The inner layer of cells of blood vessels are called endothelial cells. This layer of cells plays a vital role in the normal function of blood vessels, including blood clotting and inflammation. Certain conditions, such as aging, smoking, high cholesterol, and some chemotherapies cause these cells to not function properly. Researchers can test whether these cells function properly by doing two simple tests - one is a blood test and the other uses ultrasound and temperature probes on the finger to look at the ability of arm blood vessels to dilate (expand).