dilate, constrict
When you exercise, the skeletal muscles are doing a lot of work and need access to a lot of blood to provide oxygen and to carry away metabolic wastes such as carbon dioxide. However, the digestive system is fairly low priority, so restricting the blood flow to the digestive organs makes available more blood to the skeletal muscles. You would also expect during exercise for the blood vessels near the skin to also dilate so make the dissipation of excess heat more efficient since the work being done by the muscles will generate waste heat as well.
