When you lace up your exercise shoes and head out the door for your morning walk — or push off from the wall of your favorite swimming pool — you're responding to the orders of your conscious brain to move your muscles in a more vigorous way. As soon as those movements begin, however, a number of rapid, automatic changes also happen throughout your body.
Your working muscles immediately start to burn more energy to fuel their contractions. They do this by stepping up the conversion of oxygen and nutrients into adenosine triphosphate, or ATP (the fuel that all cells run on), inside each individual muscle cell.
During sustained, aerobic activity, like a brisk walk or steady running, your working muscles might use 15 to 25 times more energy than they do at rest — burning carbohydrates and stored fat in about a 50-50 mix. During an intense, short anaerobic effort, such as running a 100-yard dash or sprinting the length of the swimming pool, your muscles may require up to 120 times more energy than at rest!
Your heart immediately begins to beat faster in order to pump more blood to your muscles and other body tissues. During vigorous exercise, your heartbeat may rise to 150 beats per minute or more (compared with 70 or 80 heartbeats per minute at rest, for the average person).
Why this happens: As soon you start a physical activity, nerve receptors in your blood vessels, muscles and joints signal your sympathetic nervous system to release epinephrine (adrenaline) and norepinephrine (noradrenaline) into your bloodstream. These quickly act to speed up your heartbeat. The brain's cortex also contributes to this speeding up — in fact, scientists have found that people's hearts begin to beat faster even before they start to exercise, as the brain anticipates what's about to happen.
Whereas the average heart pumps about five liters of blood per minute at rest, the amount may increase to 20 liters per minute during vigorous exercise. (The hearts of trained endurance athletes have been measured to pump as much as 40 liters in a minute!)
Your blood vessels also go through rapid changes when you start exercising. Stimulated by nerve and chemical signals, the walls of the arteries leading to your working muscles relax, causing the arteries to widen. At the same time, peripheral veins constrict, forcing more blood into your central circulation. The smaller arterioles leading to your muscle fibers also widen, and millions of dormant capillaries (which feed blood directly to the fibers) open up. (At rest, only about one in every 30 capillaries is open.)
The result of all these changes is a vastly increased flow of blood (along with the all-important oxygen and nutrients it carries) to your exercising muscles — including your heart muscle, which receives several times more blood flow than it does at rest. This blood flow is maximized when each muscle relaxes, and then stops as it contracts, creating a "milking" action that helps pump blood throughout your body as you move.
Increased blood flow to the skin during light and moderate exercise encourages heat loss through the skin, which provides a cooling effect (in addition to that provided by sweating). Meanwhile, blood flow is temporarily directed away from the kidneys, liver, digestive system and other organs not directly involved in exercise.
Your lungs also begin breathing faster and more deeply, supplying your body with more oxygen. This response results from a wide array of stimuli, including more output of carbon dioxide (the byproduct of using more oxygen), increased body temperature and rising acid production in the body. At rest, about 12 pints of air pass in and out of the average person's lungs every minute. During vigorous exercise, this rate may increase to as much as 200 pints per minute.
Your metabolic rate, which depends on how many calories you're burning, goes up anywhere from four to 20 times your resting metabolic rate, depending on how hard you exercise.