In mammals, oxygen first passes through the nasal cavity. The nasal cavity is covered with mucus and cicilia to filter the air. The nasal cavity leads to the pharynx. The pharynx consists of the eustachian tube and the tonsils. The inhaled air then passes to the larynx, trachea, and bronchi. The bronchi lead to the bronchioles in the lungs. In the lungs the pleural membrane facilitates breathing. The bronchioles end in microscopic alveoli lined by a thin, moist epithelium. The alveoli is the primary site of gas exchange. Branches of the pulmonary arteries send oxygen poor blood to the alveoli; branches of the pulmonary veins transport oxygen rich blood from the alveoli back to the heart.
Inspiration (the process of inhaling) begins as the external intercostals and diaphragm contract. When this happens, the lungs expand. After this, negative pressure is used to facilitate respiration. So, air moves from an area of higher pressure, which is the air, to an area of lower pressure in the lungs and aveoli. During inspiration the diaphragm and intercostal muscles contract. The diaphragm moves downwards, while the intercostal muscles make the rib cage move upwards. These two processes increase the volume of the thoracic cavity and also reduces the air pressure to below atmospheric pressure allowing air to rush into the airways then into the alveoli.
With expiration (the process of exhaling) the opposite occurs. Here, the diaphragm and intercostal muscles relax. This allows the diaphragm to move upwards and the intercostal muscles let the rib cage relax to its resting state. This concept is called passive recoil. After passive recoil occurs, the volume within the thoracic cavity now decreases. This decrease in volume causes an increase in pressure above atmospheric pressure which forces air up and out the airway.
In mammals, a...