Endothelial cells are the single layer of cells that line the endothelium — the tissue that lines our blood vessels and is important for regulating blood pressure, blood flow, clotting, inflammation and structure of organs. The reactivity of the endothelium in blood vessels can tell us about the vascular health of an individual and is the basis of a non-invasive technique called flow-mediated dilation. This Endothelial Function project from the Italian Air Force will use flow-mediated dilation to assess the vascular health of astronauts before, during and after spaceflight and the results will be compared to measurements from non-orbital flight personnel. The goal of this project is to help understand how vascular health changes over the course of spaceflight.