 Dear students, in this topic we shall discuss the mechanism of lung ventilation in birds. The birds respiratory system consists of lungs and air sacs. The number of air sacs in birds may be 7 or 9. These air sacs include two posterior or abdominal air sacs, two anterior, thoracic or cranial air sacs, two posterior, thoracic or cordal air sacs, and two cervical air sacs, which may not be present in some birds. And they include one intra-cleavicular air sac. Dear students, now we shall discuss the respiratory cycle of a bird. In birds, it takes two ventilation cycles to move air to the entire respiratory system. In this way, the inhaled air remains in the respiratory system for two complete inspiratory and expiratory cycles before it is fully used and then exhaled. During the first inspiration, the inhaled air enters through the nostrils and passing through the larynx, it enters the trachea. From the trachea, air does not directly enter to the lungs. Air rather travels to the posterior abdominal air sacs. Simultaneously, a small amount of air passes through the posterior air sac to the lungs through the ventro bronchi and dorsal bronchi. So, this is a difference in the bird's inspiration, that the first air that is inspired goes into the abdominal air sacs and during that inspiration, through the abdominal air sacs, through the ventro and dorsal bronchi, a portion of the air is sent to the lungs. In the lungs, the ventro and dorsal bronchi divide to form small diameter parabronchi and air capillaries. These parabronchi are supplied in large quantities by blood capillaries. These air capillaries are the sites which are the sites of gas exchange where oxygen is absorbed and carbon dioxide is added to the blood for the outgoing air. Now, the next cycle is the first expirience. The spent air in the lungs flows out of the body through trachea, which is used by the lungs to go out directly through the first expirience. And this air from the lungs which is out is replaced by fresh air which is present in the posterior air sacs. The first air entered in the first expirience in the abdominal air sacs enters the lungs through the first expirience. In this way, even during the expirience, the lungs get fresh air. Here, students, after this, when there is a second expirience, the fresh air again enters the posterior air sacs. Lungs also receive fresh air from the abdominal sacs replacing the used air. That which displaced air air lungs say that does not go directly out from the trachea because it cannot exit through the trachea because fresh air is flowing inward. This means the spent air moves from the lungs and enters the anterior thoracic air sacs. Now, second expirience is that the air moves out of the cranial or anterior thoracic sacs and it moves out and goes out and the air in the lungs also goes out. This flows out through the trachea. Fresh air which is present in the posterior abdominal sacs again enters the lungs for gas exchange. If there is a second expirience, the fresh air also supplies the lungs. Dear students, now we shall see how the air flow is maintained during these inspiratory and expiratory cycles. Air is moved in and out of the respiratory system through pressure changes in the air sacs. The mechanism of movement of air sacs is the pressure changes in the air sacs. The air sacs do not have any diaphragm to create any negative pumping pressure in the lungs. So, the pressure changes in and out of the air sacs are caused by air in and out movement. The thoracic muscles which cause the sternum to be pushed outward, this creates a negative pressure in the air sacs causing the air to enter the respiratory system. Similarly, expirience is also not passive. It requires certain muscles to contract to increase the pressure on air sacs and push the air out. Dear students, bird's car, your respiratory system head that is more efficient than mammals. It transfers more oxygen with each breath. Second directional air flow of fresh air ensures that our fresh air passes over the respiratory epithelium during both inhalation as well as exhalation. It enables near continuous state of gas exchange within the lungs and this is the reason that the lungs absorb about 90% of the inhaled air from this fresh air flow.