While my first choice would be neither of these options, as the primary reason they exist is as pathways for blood cells to supply oxygen and nutrients to individual cells, Thermoregulations is a close second.
Capillaries which are the tiniest pathways that blood will take and are usually where exchange of gases and nutrients and waste take place between individual cells. They are able to dilate and constrict to limit blood cell speed and amount to both shunt and flush blood to the bodies surface. Shunting is the process of impeding blood flow to an area of the body (usually happens in extremely cold weather), and flushing is process of increasing blood flow and volume to areas (In hotter areas).
