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During respiration, atmospheric gas (air) is pulled into the lungs, utilized for oxygen, and then released in an altered form. This gas exchange is common to all surface-dwelling mammals and many reptiles as well. The basic process is as follows:
Air composed of nitrogen, oxygen, argon, and carbon dioxide is forcibly pulled through the trachea into the lungs via the nose and nasal passages, or through the mouth. While inside the lungs, oxygen from the air is exchanged in the alveoli by the process of diffusion. The alveoli separate the inhaled air from the deoxygenated blood, which has returned to the lungs from the heart, where it was pumped throughout the body, dispersing the blood's oxygen load. The blood flowing under the alveoli contains waste carbon dioxide (CO2) dissolved in blood plasma, and since the air in the alveoli has more oxygen then CO2 diffusion allows the two gasses to exchange places. Oxygen from the inhaled air moves through the thin alveolar wall into capillaries carrying blood, while CO2 from the blood moves into the air. At this point, the lungs contract and the air is exhaled, with the nitrogen and argon mostly intact; the exhaled air contains more CO2 than oxygen, and diffuses into the atmosphere.
The process by which gases are exchanged in the lungs is called BREATHING.
Breathing consists of two phases: Inspiration and expiration.
During the process of inspiration, the diaphragm and the intercostals muscles contract. The volume of the thoracic cavity increases as the diaphragm moves downwards, and the intercostals muscle pull the ribs up expanding the rib cage and further increasing this volume. This increase of volume lowers the air pressure in the alveoli to below atmospheric pressure. Diffusion occurs when molecules move from an area of high concentration to an area of low concentration, so the air rushes in through the respiratory tract and into the alveoli. The air we breathe contains approximately 21% Oxygen and 0.04% Carbon Dioxide.
During the process of expiration, the diaphragm and intercostals muscles relax. Thoracic cavity returns to its original volume, air pressure in the lungs increases, and the air is forced out. When we exhale there is approximately 17% Oxygen and 3% Carbon Dioxide. The level of oxygen is reduces by 3%.
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