air sac
noun [ C ] biology, anatomy specialized uk/ˈeə ˌsæk/ us/ˈer ˌsæk/
气囊（鸟类体内从肺部延伸至骨骼以及其他身体部分的气囊，用于呼吸）
a space filled with air that continues from a bird's lungs into one of its bones or another body area, used in breathing
（昆虫的）气囊
a wide part like a bag in the tubes an insect uses for breathing
肺泡
an alveolus (= a very small air bag in the lungs)