Final answer:
Air molecules cross a series of respiratory structures including the external nares, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and finally alveoli where they engage in gas exchange with the pulmonary capillaries.
Step-by-step explanation:
Pathway of Air to the Pulmonary Capillaries
When an air molecule travels to a pulmonary capillary, it crosses several structures beginning at the external nares (nostrils). It then moves through the nasal cavity and passes into the pharynx. From the pharynx, it enters the larynx, followed by the trachea.
The trachea divides into two bronchi, one for each lung, which further branch into smaller bronchioles. Ultimately, these bronchioles lead into terminal bronchioles. The terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolar sacs. Each alveolar sac contains 20 to 30 spherical alveoli, resembling a bunch of grapes. Air continues into the atrium of the alveolar sac and then into the alveoli, where gas exchange occurs with the pulmonary capillaries.
This exchange of gases is facilitated by the thin-walled nature of the alveoli which are lined with capillaries. The mucous glands in the airways secrete mucus to keep the passages moist and flexible, ensuring efficient gas exchange. The vast number of alveoli (approximately 300 million per lung) markedly increases the surface area for gas exchange to about 75 m² in adults, comparable to half the size of a tennis court. This large surface area and the thin alveolar walls enable rapid diffusion of oxygen and carbon dioxide between the alveoli and the circulating blood.