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Whether you're pounding out miles on a treadmill, sweating your way through step class or pumping iron in the weight room, all of your body systems work together to respond to the demands of exercise. Your circulatory and respiratory systems supply your muscles with the nutrients and oxygen they need. Coordinating the body's response is the job of your nervous and endocrine systems.
When you work out, your circulatory system increases blood flow to supply your muscles with nutrients and oxygen. The amount of blood your heart pumps with each beat, called the stroke volume, rises, as does your heart rate. Your systolic blood pressure, which reflects the force of your heart's contractions, increases, although the diastolic pressure, when your heart is filling between beats, doesn't change significantly. To maintain your blood pressure, many of the blood vessels in your body constrict, while those that supply your muscles expand.
Your respiratory system also has to work harder to provide your muscles with oxygen. At the beginning of an aerobic workout, the depth of your breathing increases, until it reaches a plateau. Further increases in ventilation come from breathing faster. During very intense exercise, such as during interval training, lactic acid produced by your muscles begins to accumulate in your tissues. As your body buffers lactic acid, it produces excess carbon dioxide, stimulating a further rise in your breathing rate to expel the CO2.
Your nervous system -- specifically the autonomic nervous system -- orchestrates your body's response to exercise. The autonomic nervous system is responsible for controlling unconscious processes in your body. It is divided into two divisions, the sympathetic and the parasympathetic. The sympathetic division, often called the "fight or flight" system, prepares your body for action, while the parasympathetic division is active when you're in "rest and digest" mode. During exercise, parasympathetic activity declines while the sympathetic division goes into overdrive, stimulating your heart and breathing rate and reducing blood flow to your digestive organs.
Your endocrine system also helps to coordinate your body's response to working out. Your adrenal glands secrete stress hormones, including epinephrine, or adrenaline, and cortisol. Epinephrine stimulates your heart to beat more forcefully and your sweat glands to start perspiring. Epinephrine also prompts your fat cells to release fatty acids, while cortisol stimulates your liver to produce more glucose, or blood sugar, providing fuel for your muscles. Your adrenal glands produce aldosterone and your pituitary gland releases antidiuretic hormone, both of which help to maintain blood plasma volume and blood pressure.