Translations:Benutzer:Arian/Klett-Mini-Test/490/en

Birds extract oxygen from the air both in inhaling and in exhaling. On inhalation, the air enters partly into the lungs and, unused, partly into the air sacs distributed throughout the body and into the partly hollow bones. On exhalation, this stored air passes through the lungs — an open tube system, not the blind-alley-like pulmonary alveoli — from inside outward. Heaviness is transformed in the bird's body into lightness: the skull, streamed through for the most part directly by the outer air; the body ventilated by the air sacs; the air-filled cavities that penetrate large portions of the skeleton, especially in the great bird species; and finally the mobile, ventilated plumage. The lightness is further underscored by the fact that the highly concentrated food — whether of plant or animal origin — undergoes rapid digestion and is excreted again after only a brief sojourn.