Chapter XVII

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“Respiration” refers to the exchange of and oxygen between the cells of the body and the atmosphere. Air taken in through the nasal cavity passes over the turbinates or nasal conchae, thin bony plates that help filter incoming air and regulate its temperature and humidity. Inhaled debris is trapped by mucus, which is then moved toward the stomach by microscopic hair-like cellular projections called “,” where the unwelcome particles are disposed of. The sinuses also play a role in respiration by moisturizing the air and producing (the four sinuses in the human head are the maxillary, ethmoid, frontal, and sphenoid).

Air then passes through the (Grk. “throat”) past the epiglottis, which prevents food material from entering the lungs (named because of its placement “over” or “on top of” the glottis, the vocal apparatus of the larynx). Air continues into the larynx (Grk. “upper ”), which guards the entrance to the trachea and also functions as the voice organ, and then through the trachea itself. This structure was known in antiquity by the Greek term arteria (“artery”), to which the adjective tracheia (“”) was added in order to distinguish it from other, lesser arteries (recall that doctors in antiquity thought that arteries carried , not blood). The noun arteria was dropped, leaving only “trachea” (cf. the loss of the noun in the phrase toxikon pharmakon).

The trachea branches into the two (Grk. “air tubes”), which are separated by the trachea’s (Lat. “keel, ridge”) and which divert air into the right and left lungs. The lungs are divided into lobes and are coated in a serous double membrane known as the (Grk. “side, rib”). The visceral pleura encloses the lobes of the lungs themselves, while the parietal pleura lines the interior of the chest cavity (the small space between these membranes is known as the pleural and contains fluid that provides lubrication for the movement of the lungs in the thoracic cavity).

The bronchi split into smaller tubes called bronchioles (or bronchioli, dim. of Grk. bronchus). The bronchioles terminate in clusters of sac-like structures called “” (dim. of Lat. “belly, hollow space”). It is from these that oxygen passes into the blood by transmission into capillaries, where it is transferred to the pulmonary veins and then sent to the heart to be pumped throughout the body. Also of importance in respiration is the (Grk. “barrier”), a sheet-like muscular structure that contracts downward to form a vacuum in the lungs, drawing in air; when it relaxes, it ascends and pushes the air outward in the process of exhalation.