Respiratory System

    

The avian respiratory system delivers oxygen from the air to the tissues. It also removes carbon dioxide. The respiratory system plays an important role in thermoregulation. The system is different from that of other vertebrates in that the chicken has relatively small lungs and includes nine air sacs that play an important role in respiration. It is also different because there is no diaphragm and there are no sweat glands. Thus, the respiratory system is the cooling system and is highly efficient.

Image of the respiratory system of the chicken

Source: http://www2.ca.uky.edu/smallflocks/_images/Avian_respiratory_system.gif

Air sacs

The air sacs permit a unidirectional flow of air through the lungs. This permits a “fresher” flow of air so that more oxygen is available to diffuse into the blood. The air sacs are lined with squamous epithelium and are not vascularized very well. They do not take part in oxygen exchange, but rather enhance its efficiency and allow for high metabolic rates. The system of nine air sacs helps to keep the volume of air in the lungs consistent. These include cervical, clavicular, cranial and caudal thoracic air sacs. Some of these are paired. They have very thin walls with few blood vessels.

Image of the air sacs in a chicken

Source: http://www.poultryhub.org/wp-content/uploads/2012/04/Air-sacs-Copy.jpg

Lungs

            The parabronchial lungs of the chicken are subdivided into large number of extremely small alveoli or air capillaries. Air capillaries are the sites of the exchange of gases between the lungs and the blood. The lungs are relatively rigid compared to other species. The lungs are unlobed and lie near the thoracic vertebrae. They are continuous with the abdominal air sac. They fill the dorsal body cavity and are tightly attached to the dorsal and lateral body wall.

                                                           Image featuring the lungs 

Source: http://partnersah.vet.cornell.edu/avian-atlas/sites/agilestaging.library.cornell.edu.avian-atlas/files/avian_atlas_assets/39_lungs%20x750.jpg
Image of the lungs (taken in the chicken dissection) 

Trachea

            The trachea is generally as long as the neck. It is also called the windpipe and comes after the pharynx. In the chicken, the trachea is made up of tracheal rings that are cartilaginous circles joined by annular ligaments. At the syrinx, the trachea bifurcates (or splits) into two primary bronchi. The syrinx is a unique avian organ of phonation.

Image featuring the trachea (taken at the chicken dissection)

Ventilation - Inspiration

            Air flow is drivien by change in pressure within the respiratory system. Air mostly enters the caudal air sacs during inspiration, whereas a lesser part flows through the parabronchi/air capillaries into cranial air sacs. During inspiration, the sternum moves forward and downward while the vertebral ribs move cranially to expand the sternal ribs and the thorocoabdominal cavity. This expands the posterior and anterior air sacs and lowers the pressure, causing air to move into those air sacs. Air, from the trachea and bronchi, moves into the posterior air sacs. Simultaneously, air from the lungs moves into the anterior air sacs.

Ventilation - Expiration

            During expiration, the major part of inspired air streams from the reservoirs through the parabronchi/air capillaries into major distal airway. There, it mixes with the deoxygenated respiratory gas stored in cranial air sacs during the inspiratory phase. During expiration, the sternum moves backward and upward and the vertebral ribs move caudally to retract the sternal ribs and reduce the volume of the thorocoabdominal cavity. This reduces the volume of the anterior and posterior air sacs, causing air to move out of those sacs. Air from the posterior sacs moves into the lungs. Simultaneously, air from the anterior sacs moves into the trachea and out of the body. Thus, air moves unidirectionally through the lungs and, as a result, is higher in oxygen content than air in the alveoli of humans and other mammals.

Image of ventilation and direction of air flow (including inspiration and expiration)

                                                                           

Source: http://people.eku.edu/ritchisong/birdrespiration.html