 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-clevicular air sac. Dear students, now we shall discuss the respiratory cycle of a bird. In birds, it takes two ventilation cycles to move air through 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 para-bronchi and air capillaries. Para-bronchi, they are supplied in a large number of blood capillaries. These air capillaries are the sites which are the sites of gas exchange where oxygen is absorbed and carbon dioxide is absorbed from the blood for the outgoing air. Now, the next cycle is the first expirations. The spent air in the lungs flows out of the body through trachea, which is used for the first expirations, and this air from the lungs is replaced by fresh air which is present in the posterior air sacs. The first air entered in the abdominal air sacs enters the lungs during the first expirations. This way, even during the expirations, the lungs get fresh air. Here, students, after the second expirations, the fresh air enters the posterior air sacs. Lungs also receive fresh air from the abdominal sacs, replacing the used air. That which is displaced air from the lungs does not go directly out from the trachea because it cannot exit through the trachea because fresh air is flowing inward. This is why the spent air moves from the lungs and enters the anterior thoracic air sacs. Now, the second expirations, the air moves out of the cranial and anterior thoracic sacs and 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. This way, even during the second expirations, fresh air supplies the lungs to the 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 is the pressure changes in the air sacs. The birds do not have a diaphragm that they keep giving movement in the lungs to create some kind of negative pumping pressure in the lungs. This is not the case. So, due to the pressure changes in the air sacs, the air is in and out of the respiratory system. Cranic 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, expirations are also not passive. It requires certain muscles to contract, to increase the pressure on air sacs and push the air out. Dear students, birds' 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 why the lungs absorb about 90% of the inhaled air in the lungs.