Gas exchange occurs in the pulmonary alveoli. Each
breath renews only a small portion of the total alveolar
air volume. In the resting pig, 10–15% of the alveolar
air is exchanged per inspiration. The normal respiratory
rate (breaths/minute) varies according to age of
the animal (piglets and growing pigs, 25–40; finishing
pigs, 25–35; sows in gestation, 15–20).
The mucosal surface of the respiratory tract provides
a critical interface between the pig and its environment.
The alveolar surface area is very large (in humans
more than 100 m2). This large surface area is continually
exposed to the environment through inspired air.
So, the respiratory tract is equipped with a potent and