Unidirectional pulmonary airflow in vertebrates: a review of structure, function, and evolution

Area of Science:

• Comparative physiology
• Evolutionary biology
• Respiratory system anatomy

Background:

• Unidirectional pulmonary airflow allows consistent lung gas flow during breathing.
• This trait was historically observed in birds and recently identified in crocodilians and squamates.

Purpose of the Study:

• To review current knowledge on the functional anatomy, fluid dynamics, and significance of unidirectional pulmonary airflow.
• To explore the evolutionary implications of this respiratory mechanism in various vertebrate groups.

Main Methods:

• Review of existing literature on avian and reptilian respiratory systems.
• Analysis of fluid dynamics principles, particularly convective inertia and aerodynamic valving.
• Discussion of blood-gas exchange efficiency and evolutionary hypotheses.

Main Results:

• Convective inertia in aerodynamic valves is the primary mechanism for unidirectional airflow in birds.
• Inspiratory aerodynamic valving is likely crucial for flow control in non-avian reptiles.
• The presence of this trait in non-volant ectotherms refutes the flight-adaptation hypothesis.

Conclusions:

• Unidirectional airflow in reptiles necessitates a re-evaluation of its evolutionary drivers.
• This respiratory pattern may offer advantages in gas mixing, washout, and hypoxic conditions.
• Further research is needed on blood-gas exchange mechanisms in reptile lungs with unidirectional airflow.