Final answer:
Breathing is regulated by the respiratory center in the brain, which responds to signals about the carbon dioxide content of the blood. The diaphragm is the main muscle responsible for the process of breathing.
Step-by-step explanation:
Breathing is controlled by the respiratory center in the brain, which receives signals about the carbon dioxide content of the blood. It is both a voluntary and involuntary event. The rate and depth of breathing is regulated by the brain stem, specifically the medulla, which detects the level of carbon dioxide in the blood and adjusts breathing accordingly. The main muscle responsible for the process of breathing is called the diaphragm. When it contracts and relaxes, it causes air to be drawn into and expelled from the lungs.