Q.Comment on the role of air sacs in increasing the respiratory efficiency in birds.

Step 1. A bird's lungs are solid, spongy and fixed rigidly to the ribs, so unlike mammalian lungs they cannot themselves expand and contract to draw air in and push it out.

Step 2. Because the lungs cannot change volume, the actual movement of air is instead driven by nine interconnected air sacs (paired cervical, a single interclavicular, paired extraclavicular, paired clavicular, paired thoracic and paired abdominal sacs), which act like a set of bellows as the sternum and ribs move.

Step 3. This air-sac arrangement allows air to keep flowing through the lung's air capillaries during both inspiration and expiration, rather than the in-and-out, partly "stale" airflow of a simple in-out lung, so gas exchange with the blood is close to continuous and unusually efficient for the bird's high metabolic rate during flight. …

Explain why rigid, non-expandable bird lungs need the air sacs to drive continuous airflow, and note the a …

• Assuming bird lungs expand and contract on their own like mammalian lungs, without needing the air sacs to move air through them. …