Making the Pipe Bigger
As you exercise, the blood vessels in your muscles dilate and the blood flow is greater, just as more water flows through a fire hose than through a garden hose. Your body has an interesting way of making those vessels expand. As ATP gets used up in working muscle, the muscle produces several metabolic byproducts (such as adenosine, hydrogen ions and carbon dioxide). These byproducts leave the muscle cells and cause the capillaries (small, thin-walled blood vessels) within the muscle to expand or dilate (vasodilation). The increased blood flow delivers more oxygenated blood to the working muscle.
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As you begin to exercise, blood from organs is diverted to the muscles.
Taking Blood from the Organs
When you begin to exercise, a remarkable diversion happens. Blood that would have gone to the stomach or the kidneys goes instead to the muscles, and the way that happens shows how the body's processes can sometimes override one another. As your muscles begin to work, the sympathetic nervous system, a part of the automatic or autonomic nervous system (that is, the brainstem and spinal cord) stimulates the nerves to the heart and blood vessels. This nervous stimulation causes those blood vessels (arteries and veins) to contract or constrict (vasoconstriction). This vasoconstriction reduces blood flow to tissues. Your muscles also get the command for vasoconstriction, but the metabolic byproducts produced within the muscle override this command and cause vasodilation, as we discussed above. Because the rest of the body gets the message to constrict the blood vessels and the muscles dilate their blood vessels, blood flow from nonessential organs (for example, stomach, intestines and kidney) is diverted to working muscle. This helps increase the delivery of oxygenated blood to working muscle further.