Parabronchi: Structure and Function in Avian Respiration

Parabronchi are the gas-exchange tubes in the avian lung, analogous to mammalian alveoli but structurally distinct. They form a continuous, unidirectional airflow system connecting the primary bronchi to the air sacs.

Key Features

• Anatomy: Narrow, parallel tubes with highly vascularized walls.
• Gas exchange: Occurs across the blood-gas barrier between parabronchial lumen and surrounding capillaries.
• Cross-current flow: Air flows perpendicular to blood flow, enabling efficient oxygen extraction—even at high altitudes.
• No dead-end sacs: Unlike mammalian alveoli, parabronchi allow continuous airflow during both inspiration and expiration.

Physiological Advantage

This design supports the high metabolic demands of flight and explains birds' superior hypoxia tolerance compared to mammals.