Breathing And Exchange Of Gases

Inhaled air travels through the nasal passage, pharynx, larynx, trachea, bronchi and progressively finer bronchioles before reaching the alveoli. NCERT Class 11, Chapter 17 (Breathing and Exchange of Gases) identifies the alveoli as the thin-walled, vascularised air sacs that serve as the primary sites of gaseous exchange between air and blood. Their extremely thin walls and dense capillary network allow oxygen to diffuse into the blood and carbon dioxide to diffuse out efficiently. Bronchioles are conducting passages that carry air toward the alveoli but lack the structure for exchange themselves. The trachea is a rigid, cartilage-supported tube purely for conduction of air and is far too thick-walled for diffusion. Pleural membranes form a double-layered covering that reduces friction during breathing movements and play no direct role in gas transfer. Hence alveoli are the correct answer. A structural plausibility check confirms it: efficient diffusion requires a large, thin, moist, well-perfused surface, and only the alveoli among the listed options combine all these features for oxygen uptake into the blood.