The inspiratory act begins at the nose. Ambient air is inhaled through the nose and / or mouth and conveyed through the respiratory tract (throat (pharynx), voice box (larynx)) to the windpipe (trachea). The air then continues through the large tubes that lead to the right and left lungs. The two bronchi originating from the trachea further subdivide into smaller conduits, followed by the bronchioles, which wind and narrow into the alveolar ducts. Each lung contains more than 300 million alveoli, small (0,3mm in diameter), elastic, thin-walled sacs that form clusters resembling bunches of grapes. A capillary network on this surface encircles the alveolus, forming the pulmonary functional unit where oxygen is exchanged with carbon dioxide.
When the oxygen (O2) contained in the air reaches the alveoli, a very large (50 to 100 m2) and thin (0.5 to 4.0 nano meter) surface permits a quick exchange of respiratory gasses between the alveolar and the capillary space. More than 98% of oxygen contained in the arterial blood is transported and combined with hemoglobin (the oxygen carrier). Oxygen is transported to the mitochondria by the myoglobin, a molecule similar to hemoglobin but contained in the muscles (Silverthorn 2007).
Carbon dioxide (CO2) is removed during the expiratory phase of the ventilation. During exercise, the majority of carbon dioxide is produced by the active muscles and is removed and transported to the lungs by the blood, mostly as bicarbonate iron (about 60-70%).
But the exchange of carbon dioxide for oxygen isn’t the only function of the lungs. They also serve to humidify (add moisture), warm or cool, and filter the air that is breathed in.
Breathing is divided into two phases: inspiration and expiration. The inspiration process involves the active participation of the diaphragm muscle, supported by the external intercostal muscles. When the diaphragm contracts, it flattens and descends towards the abdominal cavity. At the same time other muscles expand the ribs frontally and laterally, lifting the sternum. When the chest creates more space, the lungs expand their volume and owing to a lower pressure in respect to the external environment, the air is easily conveyed inside the lungs.