Final answer:
The ventilation in birds is accomplished by the expansion and contraction of the thoracic cavity during the process of inspiration and expiration.
Step-by-step explanation:
The ventilation in birds is accomplished by the expansion and contraction of the thoracic cavity during the process of inspiration and expiration.
During inspiration, the diaphragm contracts and moves downward, while the external intercostal muscles contract and move the ribs upward and outward, expanding the thoracic cavity and creating more space for the lungs. This leads to a decrease in intra-alveolar pressure and a pressure gradient that drives air into the lungs. During expiration, the opposite occurs, with the diaphragm relaxing and moving upward, and the external intercostal muscles relaxing, causing the thoracic cavity to decrease in size and push air out of the lungs.
This process of ventilation allows for the exchange of gases, with oxygen diffusing into the bloodstream and carbon dioxide diffusing out of the bloodstream and into the lungs.