Definition

1. Noun:
   • A membranous, air-filled extension of the lungs in birds: A structure in birds that stores air and aids in respiration.
   • A thin-walled extension of the tracheae in insects: A part of the respiratory system in insects that holds air.
   • A tiny, thin-walled sac in the lungs of mammals: The terminal dilation of an air passageway (alveolus) where gas exchange occurs.

Examples of Usage

• Noun:
  • The bird's air sacs allow for a continuous, unidirectional flow of air through its lungs.
  • An insect's air sacs can expand and contract, helping to move air through its body.
  • Oxygen and carbon dioxide are exchanged in the microscopic air sacs of the human lung.

Advanced Usage

• "Pulmonary air sac": Specifically refers to the air sacs within the respiratory system of birds.
  • The fossil showed evidence of a complex system of pulmonary air sacs, similar to modern birds.

Variants and Related Words

• Alveolus (pl. alveoli): The precise anatomical term for the tiny air sac in mammalian lungs where gas exchange takes place.
  • Smoking damages the alveoli, the delicate air sacs in the lungs.
• Saccule: A general term for a small sac, which can be related in structure.

Synonyms

• Alveolus (specifically for the mammalian lung structure).
• Respiratory sac.
• Gas-exchange sac.

Related Phrases

• Air sac system: Refers to the entire network of air sacs, especially in birds.
  • The avian air sac system is highly efficient for flight at high altitudes.

Noun

1. any of the membranous air-filled extensions of the lungs of birds
2. any of the thin-walled extensions of the tracheae of insects
3. a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways