How does breathing work?

Breathing requires adequately functioning lungs and respiratory muscles. The thoracic muscles (primarily the diaphragm and intercostal muscles) expand the chest and create a vacuum, causing air to enter the lungs. Once in the lungs, the oxygen in the air diffuses into the blood through thousands of tiny sacs called alveoli. The majority of oxygen then binds to hemoglobin in red blood cells so that it can be delivered to tissues throughout the body. Simultaneously, carbon dioxide that is produced by cells reenters the bloodstream and diffuses out of the alveoli, where it is exhaled.