Overview of the respiratory system

The respiratory system consists of the paired lungs and a series of air passages that lead to and from the lungs. Within the lung, the air passages branch into increasingly smaller tubes until the very smallest air spaces, called alveoli, are reached (Fig. 18.1).

Three principal functions are performed by this system: air conduction, air filtration, and gas exchange (respiration). The latter occurs in the alveoli. In addition, air passing through the larynx is used to produce speech, and air passing over the olfactory mucosa in the nasal cavities carries stimuli for the sense of smell. The respiratory system also participates to a lesser degree in endocrine functions (hormone production and secretion), as well as regulation of immune responses to inhaled antigens.

The lungs develop in the embryo as a ventral evagination of the foregut; thus, the epithelium of the respiratory system is of endodermal origin. This initial respiratory diverticulum grows into the thoracic mesenchyme. The bronchial cartilages, smooth muscle, and the other connective tissue elements are derived from the thoracic mesenchyme.

The air passages of the respiratory system consist of a conducting portion and a respiratory portion

The conducting portion of the respiratory system consists of those air passages that lead to the sites of respiration within the lung where gas exchange takes place. The conducting passages include those located outside as well as within the lungs.

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