Breathing, called ventilation consists of two phases, inspiration and expiration. During inspiration the diaphragm and the external intercostal muscles contract. The diaphragm moves downward increasing the volume of the thoracic (chest) cavity, and the external intercostal muscles pull the ribs up and outward, expanding the rib cage, further increasing this chest volume. This increase of volume lowers the air pressure in the lungs as compared to atmospheric air. “Expiration is caused by the relaxation of the Intercostal muscles and the Diaphragm whereby the chest contracts downwards and inwards returning to its normal position. It lasts for about 3 seconds and the next cycle begins.”
Air enters through the nostrils of the nose and is partially filtered by the nasal hair, it then flows into the nasal cavity which helps to warm the inhaled air and filter it further. After passing through the nasal cavity, the air flows down the Pharynx, Larynx and Trachea. Then into bronchi and bronchioles. Lastly to the alveolar sac and passes into the capillaries.
Through the very thin walls of the alveoli, oxygen from the air passes to the surrounding capillaries (blood vessels). A red blood cell protein called hemoglobin helps move oxygen from the air sacs to the blood.
When a person inhales, oxygen moves from the alveoli to the surrounding capillaries and into the bloodstream. At the same time, carbon dioxide moves from the capillaries into the air sacs. The gas has traveled in the