The correct option is Alveoli.
Explanation
The human respiratory system comprises a conducting zone (trachea, bronchi, bronchioles) that transports air and a respiratory zone where the actual diffusion of gases occurs. The structural design of the respiratory zone maximizes surface area for efficient gas exchange.
Option Analysis
- Alveoli is Correct: Alveoli are the tiny, vascularised, bag-like structures located at the terminal ends of the bronchioles. They are the primary sites of gas exchange, where oxygen diffuses into the blood and carbon dioxide diffuses out. Their thin walls and extensive capillary network facilitate this process.
- Bronchi is Incorrect: Bronchi are the large air passages that branch off from the trachea (windpipe) into the lungs. They serve as conduits to transport air to the bronchioles but are not the site of gas exchange.
- Cilia is Incorrect: Cilia are microscopic, hair-like projections found on the epithelial lining of the respiratory tract. They function to sweep mucus and trapped foreign particles away from the lungs, acting as a defense mechanism rather than a gas exchange structure.
- Cisternae is Incorrect: Cisternae are flattened, membrane-bound sacs found within cellular organelles such as the Endoplasmic Reticulum and the Golgi apparatus. They are involved in cellular metabolism and transport, unrelated to the respiratory system.
Key Takeaway: Alveoli are the functional units of the lungs, specifically adapted as vascularised sacs to facilitate the diffusion of respiratory gases between the air and the bloodstream.