Reason for Superior Gas Exchange Efficiency in Birds
Gas exchange is more efficient in birds than mammals due to:
- Larger surface area of avian lung
- Thinner mean harmonic thickness of avian lung
- Large number of air sacs
- Both larger surface area and thinner harmonic thickness — Correct Answer
Correct Answer: Both larger surface area and thinner harmonic thickness
Gas exchange is more efficient in birds than mammals due to two anatomical factors related to the parabronchial lung:
Factor 1: Larger Surface Area of Avian Lung
Avian parabronchial lungs have a larger total diffusion surface area relative to body mass compared to mammalian alveolar lungs. More surface area = more area for O₂ and CO₂ exchange per breath.
Factor 2: Thinner Mean Harmonic Thickness
The air-blood barrier (the tissue layer across which gases must diffuse) is thinner in birds than in mammals. A thinner barrier means gases can diffuse more rapidly (Fick's Law: rate of diffusion ∝ 1/thickness). This is called thinner mean harmonic thickness of the blood-gas barrier.
Why Air Sacs Are NOT the Reason
Air sacs are avascular (no blood supply) — no gas exchange occurs there. They serve only as reservoirs/bellows to push air through the parabronchi. Their large number does NOT improve gas exchange efficiency.
Overall Result
Larger surface area + thinner diffusion barrier → Fick's Law of Diffusion is maximally satisfied → Birds can sustain high metabolic demands of flight and can survive at high altitudes (e.g., migrating geese over the Himalayas).
