Final answer:
Breathing is the two-step process of inhaling air into the lungs and exhaling it out. The rate of breathing is regulated by the brain stem to maintain the carbon dioxide level in the blood.
Step-by-step explanation:
Breathing, or ventilation, is the two-step process of drawing air into the lungs (inhaling) and letting the air out of the lungs (exhaling). Inhaling is an active process that results mainly from the contraction of a muscle called the diaphragm. Exhaling is typically a passive process that occurs mainly due to the elasticity of the lungs when the diaphragm relaxes.
The rate of breathing is regulated by the brain stem. It monitors the level of carbon dioxide in the blood and triggers faster or slower breathing as needed to keep the level within a narrow range.