Mammalian respiratory system
1. Air enters through the nostrils and is then filtered by hairs, warmed, humidified, and sampled for odors as it flows through a maze of spaces in the nasal cavity
2. The nasal cavity leads to the pharynx, where the paths for air and food cross
3. When food is swallowed, the larynx moves upward and tips the epiglottis over the glottis, allowing food to go down the esophagus to the stomach
4. From the larynx, air passes into the trachea
5. The trachea branches into two bronchi, one leading to each lung
6. Within the lung, the bronchi branch repeatedly into finer and finer tubes called bronchioles
7. The mucus traps dust, pollen, and other particulate contaminants, and the beating cilia move the mucus upward to the pharynx, where it can be swallowed into the esophagus
8. Gas exchange in mammals occurs in alveoli, air sacs clustered at the tips of the tiniest bronchioles
9. Oxygen in the air entering the alveoli dissolves in the moist film lining their inner surfaces and rapidly diffuses across the epithelium into a web of capillaries that surrounds each alveolus