How we breathe

Most of the time, you breathe without thinking of it. Breathing is mostly an involuntary action that is controlled by a part of your brain that also controls your heart beat. If you swim or sing, by instance, you know you can also control your breathing.
The taking in and expelling out of air is done by two movements:Inspiration and expiration. Inspiration: breathing in (a.k.a., inhalation)
Taking air into the body through the nose and mouth. It’s always an active process
caused by muscular contraction, mainly of the diaphragm with the help of intercostal muscles. During inhalation, the diaphragm contracts and moves downward. The rib muscles contract and cause the ribs to move outward. This causes the chest volume to increase. Because the chest volume is larger, the air pressure inside the lungs is less than the air pressure outside. This difference in air pressures causes air to be sucked into the lungs.
Expiration: breathing out (a.k.a., exhalation)
Pushing air out of the body through the nose or mouth It’s typically a passive process caused by elastic recoil of the lungs and relaxation of diaphragm and intercostal muscles. When the diaphragm and rib muscles relax, the chest volume is smaller, the air pressure inside the lungs is bigger than the air pressure outside. This difference in air pressures causes air to be pushed out of the lungs. Exhalation is similar to letting the air out of a balloon.
The lungs cannot move by themselves. As mentioned above, air moves into and out of the lungs by the movement of muscles. The diaphragm and rib muscles contract and relax to move air in to and out of the lungs.
After ventilation, the second stage of breathing is the gas exchange. It takes place in the alveoli. The walls of the alveoli are very thin and are permeable to gases. The alveoli are lined with capillaries, the walls of which are also thin enough to allow gas exchange.
Oxygen diffuses from the alveoli to the blood in the capillaries that surround the alveoli. At the same time, carbon dioxide diffuses in the opposite direction, from capillary blood to the alveoli. At this point, the pulmonary blood is oxygen-rich, and the lungs are holding carbon dioxide. Exhalation follows, thereby ridding the body of the carbon dioxide and completing the cycle of respiration.