The body responds to vigorous exercise both during the exercise to meet the demands put on the body, and after as it incorporates the results of exercises' demands, according to The Huffington Post. How Stuff Works explains that during strenuous exercise every body system either strives to support the muscles' tasks or shuts down.
Every type of exercise uses muscles. As the muscles work, the body requires more oxygen to fuel the muscles, so breathing gets harder and faster. Lungs use up to 15 times more oxygen during exercise than when at rest, states the Huffington Post. The heart beats faster and harder to pump oxygen to the working muscles, and the brain receives an infusion of blood, causing an alert feeling. The Huffington Post explains that, over time, the brain gets accustomed to this surge and releases endorphins, which are responsible for the euphoric "runner's high."
Muscles run on glucose and adenosine triphosphate or ATP, but during vigorous exercise the body quickly uses up the body's stored resources: It requires more oxygen to convert into ATP to keep the muscles going and carry away waste products being produced, such as lactic acid. Lactic acid build up is responsible for the achy feeling right after exercise. During this process, the stomach and intestines shut down, because blood is diverted to other body systems.The face flushes as capillaries in the skin expand to remove heat produced by the body's exertions.
Under strenuous exercise, the body strains to keep up with the demands of movement. After exercising, the muscles ache due to small tears in the fibers. The repairing of these tears builds and strengthens muscle. How Stuff Works summarizes the benefits of exercise on muscles as an increase in strength, power and endurance. According to the Huffington Post, regular exercise increases heart and lung capacity, resulting in a lowered resting heart rate and breaths per minute.
The dangers of strenuous exercise, according the How Stuff Works, include heatstroke: This occurs when the body overheats; and, it can result in passing out or even death. The Huffington Post states that other dangers of strenuous exercise include injury to over-strained muscles or other body structures, such as bones and joints.