How do muscles get increased blood flow from other organs?

When you begin to exercise, blood that would have gone elsewhere, for example to the stomach or the kidneys, instead goes to the muscles. This shows how processes within the body can sometimes override one another. When your muscles begin to work, the nerves to the heart and blood vessels are stimulated by the sympathetic nervous system, which is a part of the automatic or autonomic nervous system (the brainstem and spinal cord). This stimulation causes the blood vessels (arteries and veins) to contract or constrict (called vasoconstriction). Vasoconstriction reduces blood flow to tissues.

Even though your muscles also get the command for vasoconstriction, the command is overridden by the metabolic byproducts produced within the muscle, and instead actually cause vasodilation. The rest of the body might get the message to constrict the blood vessels, but the working muscles dilate their blood vessels instead. Then blood flow from nonessential organs (such as stomach, intestines and kidney) is diverted to the working muscle.