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The major function of the respiratory system is to exchange oxygen and carbon dioxide between the body and the environment. The gas exchange process itself takes place in the respiratory division within the lungs. The rest of the respiratory tract – the nose, pharynx, larynx, trachea, bronchi and bronchioles – essentially serve as passageways for air to flow in and out of the lungs, and constitute the conducting division.
The nasal cavity is lined with a ciliated mucus membrane. The sticky mucus traps inhaled particles, while the beating of cilia drives debris-laden mucus toward the throat to be swallowed. Inhaled bacteria are destroyed by lysozyme in the mucus. Additional protection against potential pathogens is provided by lymphocytes and antibodies.
There are three folds of tissue arising from the wall of the nasal cavity, called nasal conchae, or turbinates. These structures serve to increase the contact surface with inhaled air, enabling the nose to RAPIDLY warm, moisten and cleanse it. The roof of the nasal cavity has olfactory nerve cells in its lining and is responsible for the sense of smell.
From the nose, inhaled air turns 90 degrees downward as it reaches the pharynx. This turn is another trap for large dust particles, which, because of their inertia, crash into the posterior wall of the throat, and stick to the mucosa.
The pharynx houses several tonsils. These immunocompetent tissues of the immune system are well positioned to respond to inhaled pathogens.
In addition to inhaled air, which is on its way to the lungs, the pharynx also passes food and drink from the mouth to the esophagus. Because aspiration of food or drink into the lungs may potentially be life threatening, there are mechanisms in place to prevent this from happening. The larynx is most critical in this regard. The opening of the larynx is guarded by a tissue flap called the epiglottis. During swallowing, the larynx is pulled up and the epiglottis flips over, directing food and drink to the esophagus. More importantly, the vocal folds also close to protect the airway.
From the larynx, air passes to the trachea, the windpipe, which then splits into two primary bronchi, supplying the two lungs. In the lungs, primary bronchi branch into smaller and smaller bronchi and bronchioles, forming the bronchial tree with millions of air tubes, or airways. The airways have a layer of smooth muscle in their wall which enables them to constrict or dilate. In response to the body’s higher demand for air, such as during exercise, the airways dilate to increase air flow. On the other hand, in the presence of pollutants in the air, the airways constrict to minimize their entry to the lungs.
The larynx, trachea and bronchial tree are lined with ciliated columnar epithelium, which produces mucus and functions as a mucociliary escalator: the mucus traps inhaled particles, while the cilia beating moves the mucus up toward the throat, where it is swallowed.
The last component of the conducting division, the terminal bronchioles, branch into several respiratory bronchioles which mark the beginning of the respiratory division. The respiratory bronchioles end with microscopic air sacs called the alveoli, each of which is surrounded by blood capillaries. This is where the gas exchange process takes place. The alveolar wall is composed mainly of type I – thin squamous cells which allow rapid gas diffusion. Inhaled oxygen moves from the alveoli into the blood in the capillaries, while carbon dioxide relocates from the blood to the alveoli to be exhaled out of the body. There is also a small number of type II cuboidal cells secreting a surfactant, whose function is to lower the surface tension at the air-liquid interface and prevent the alveolus from collapsing at the end of each exhalation. The alveoli also house a large number of macrophages, ready to engulf any inhaled particles that managed to get past previous barriers to the lungs. The debris-laden macrophages then ride the mucociliary escalator up to the throat to be swallowed and digested.