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13. Gaseous Exchange eLearn.Punjab

membrane. The nasal cavity leads into the throat or pharynx by two internal openings. The pharynx
is a muscular passage lined with mucous membrane. The air is channelized from the pharynx into
the larynx.

The larynx or voice box is a complex cartilaginous structure surrounding the upper end of the
trachea. One of the cartilages, the epiglottis has a muscularly controlled, hinge-like action and
serves as a lid which automatically covers the opening of the larynx during the act of swallowing so
as to prevent the entry of food or liquids into the larynx. The opening of larynx is called glottis and
is also lined with mucous membrane. In the glottis the mucous membrane is stretched across into
two thin edged fibrous bands called vocal cords, which help in voice production, when vibrated by
air.

Fig. 13.8 Events in the throat associated with breathing (a) and swallowing (b). The commonly held belief that the epiglottis closes
downward upon the larynx when food is swallowed is not quite true. The closure is probably never complete; the degree of closure
is determined partly by the backward movement of the tongue during swallowing (which forces the epiglottis into a more or less
horizontal position) and partly by the upward movement of the larynx (which brings it up under the epiglottis). Food does not enter
the partly open larynx and obstruct breathing primarily because the epiglottis diverts the food mass to one side of the opening and
safely down the esophagus.

The trachea or wind Pipe is a tubular structure lying ventral to the oesophagus and extends to the
chest cavity or thorax where it is divided into right and left bronchi. In the wall of trachea there
are a series of C shaped cartilage rings which prevent the trachea from collapsing and keep the
passage of air open. Each bronchus on entering the lung divides and subdivides progressively into

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