Basic Respiratory Anatomy and Physiology
The human respiratory system consists of the nasal cavity, mouth, throat (pharynx), voice box (larynx), windpipe (trachea), bronchi, and lungs. Air is normally taken in through the mouth and nose. The nose is lined with a membrane (mucosa) that contain tiny hair-like projections called cilia. To keep dust and foreign particles from reaching the lungs, the cilia trap them and sweep them out of the nose. Airstreams which enter through the nose and mouth meet in the back of the throat (the pharynx). Continuing downward, there are two passages, one for food and liquids (the esophagus) and the other for air (the trachea).
Several mechanisms prevent food and liquid from going down the wrong passageway into the trachea (aspiration). The most important protective mechanism is the action of the vocal cords, which close over the trachea during swallowing to temporarily seal off the airway. The larynx also rises up in the neck during swallowing, which directs material towards the esophagus. Finally, a flap of cartilage called the epiglottis closes over the trachea to shield the airway.
The larynx is a box of cartilage, which lies between the pharynx and the trachea. It corresponds to the "Adam's apple" in the neck. The larynx contains the vocal cords, which act like valves to control the flow of air from the mouth and nose to the trachea. As mentioned above, these membranous bands protect against aspiration. By vibrating as air passes, they also produce the voice. The space between the vocal cords is the glottis. When the vocal cords are apart, the glottis is open and air passes through the larynx without vibrating the vocal cords. As muscles pull the vocal cords together, the glottis closes and sound is produced. The pitch of the sound is determined by the tension of the vocal cords.
The trachea is an open tube which extends downward from the base of the larynx. Like the rest of the airway, it is lined by mucosa and is kept open by a series of cartilage arches. The trachea separates at the bottom into two branches (mainstem bronchi), leading to the left and right lungs.
As the mainstem bronchi enter the lungs, they divide into smaller and smaller tubes. The entire network of these bronchi is called the bronchial tree. The smallest tubes are called bronchioles, which end in tiny air sacs known as alveoli. The alveoli are lined with small blood vessels (capillaries). This allows oxygen to pass from the inhaled air into the bloodstream, while allowing carbon dioxide in the blood to be eliminated into the airway and exhaled.