1. the arterioles dilate in response to epinephrine and norepinephrine from the adrenal medulla and sympathetic nerves.
2. Precapillary sphincters, which lack innervation, dilate in response to muscle metabolites such as lactic acid, CO2, and adenosine.
3. Blood flow through the muscles can increase more than 20-fold during strenuous exercise, which requires that blood be diverted from other organs such as the digestive tract and kidneys to meet the needs of the working muscles. Muscular contraction compresses the blood vessels and impedes flow.