Which statement correctly describes particles that reach the alveoli for absorption and their possible absorption pathways?

Which statement correctly describes particles that reach the alveoli for absorption and their possible absorption pathways?

• A. Large particles reach alveoli and are absorbed directly into blood
• B. Particles (≤5 μm) cannot enter alveoli
• C. Particles of any size are equally absorbed
• D. Small particles (≤1 μm) reach alveoli; can diffuse into blood or be phagocytosed and absorbed via lymph

Answer: (D)

The key idea is that alveolar absorption depends on particle size. Only very small particles can reach the alveolar region and cross the thin air-blood barrier. When particles are small enough (on the order of or smaller than about 1 μm), they can diffuse across the alveolar epithelium into the pulmonary capillary blood, leading to systemic absorption. Alternatively, these tiny particles can be taken up by alveolar macrophages (phagocytosed) and then absorbed via the lymphatic system. Thus, small particles reaching the alveoli can either diffuse into blood or be cleared via macrophages into the lymphatics. The other statements fail because large particles don’t reach the alveoli, particles of intermediate size aren’t deposited in the alveolar region with the same absorption pathways, and absorption is not uniform across all particle sizes.

The key idea is that alveolar absorption depends on particle size. Only very small particles can reach the alveolar region and cross the thin air-blood barrier. When particles are small enough (on the order of or smaller than about 1 μm), they can diffuse across the alveolar epithelium into the pulmonary capillary blood, leading to systemic absorption. Alternatively, these tiny particles can be taken up by alveolar macrophages (phagocytosed) and then absorbed via the lymphatic system.

Thus, small particles reaching the alveoli can either diffuse into blood or be cleared via macrophages into the lymphatics. The other statements fail because large particles don’t reach the alveoli, particles of intermediate size aren’t deposited in the alveolar region with the same absorption pathways, and absorption is not uniform across all particle sizes.