Which structure serves as the primary site of gas exchange in the lower respiratory tract?

A. It stores glucose for later use: Glucose is primarily stored in the liver and muscles as glycogen, not directly in the pancreas. The pancreas regulates blood glucose through hormone secretion rather than storage.

B. It converts glucose into amino acids: The pancreas does not convert glucose into amino acids; its role in glucose regulation is hormonal control through insulin and glucagon.

C. It secretes insulin to lower blood glucose levels: The pancreas contains beta cells in the islets of Langerhans that secrete insulin in response to elevated blood glucose. Insulin facilitates cellular glucose uptake, glycogen synthesis, and helps maintain homeostasis.

D. It secretes glucagon to increase blood glucose levels: Alpha cells in the pancreas secrete glucagon to raise blood glucose when levels are low, but the primary immediate role in lowering high blood glucose is through insulin secretion.

A. Alveoli: Alveoli are tiny sac-like structures in the lungs where oxygen and carbon dioxide are exchanged between the air and blood. Their thin walls and extensive capillary network maximize gas diffusion efficiency.

B. Pharynx: The pharynx serves as a passageway for both air and food. It does not participate in gas exchange but directs air toward the lower respiratory tract.

C. Trachea: The trachea conducts air between the larynx and bronchi. It provides a clear airway but is not involved in the actual exchange of gases.

D. Bronchi: Bronchi branch from the trachea to deliver air to each lung. They act as air passages and do not serve as the main site for gas exchange.