During physical activity there is a demand for increased blood flow to the contracting muscles to meet oxygen delivery requirements and to the skin to meet the heat transfer requirements of the temperature regulatory system. This paper evaluates the control mechanisms that provide for the meeting of these simultaneous demands when possible and the compromises that are made when the meeting of simultaneous demands is not possible, such as in conditions of heavy exercise in the heat. Also, an initial attempt is made to invoke these same control mechanisms in the events that accompany the circulatory and thermal changes observed in conditions of dehydration.