The cells lining blood vessels throughout the body. These cells play a crucial role in maintaining the integrity of the vascular system, and they are far more complex than they appear at first! They are NOT just cells that sit passively on the inside of blood vessels. Endothelial cells respond to an abundance of local and remote stimuli by secreting numerous substances that regulate blood flow by constricting or dilating blood vessels. One of the functions important in transfusion medicine is the release of von Willebrand Factor (vWF) and Factor VIII by endothelial cells in response to DDAVP administration.