Answer:
In birds, air flows through the lungs in a unidirectional manner through a bronchial system that is made up of primary bronchi, secondary bronchi, and tertiary bronchi called parabronchi, which are the functional units of gas exchange.
Step-by-step explanation:
The respiratory system of birds has peculiar characteristics both in its structure and in the way its breathing is carried out. The respiratory system of birds is made up of the lungs, air sacs, trachea, bronchi, blood vessels and nerves. Compared to mammals, birds lack structures such as the ethmoid labyrinth and the thoracic cavity is not limited by the diaphragm. When the bird breathes in, the air is drawn directly from the bronchi to the posterior air sacs, where the air is stored. As the posterior air sacs expire they deflate, this causes the air to pass into the lungs, where oxygen is absorbed. The consumed air (which contains carbon dioxide harmful to the organism) is then lodged in the anterior air sacs, and the animal inhales fresh air again to send to the posterior air sacs. After exhaling the consumed air expelled from the anterior sacs (to the outside) and bringing fresh air from the posterior air sacs to the lungs. This dynamic in the respiratory system of birds, allows air movements unidirectionally through the lungs generating a higher oxygen content.