The Anatomy and Physiology of the Chicken

In all animals, the respiratory system is responsible for the absorption of oxygen, the release of carbon dioxide, the release of heat, detoxification of certain chemicals, the rapid adjustment of the acid-base balance and vocalization.
Although avian and mammal respiratory systems are very similar in their functions, their anatomy is quite drastically different. Just as with mammals, birds also have two symmetrical lungs that are connected to a trachea. Mammalian lungs contain bronchi, which lead to aveoli. The aveoli in mammalian lungs have only one opening -meaning they don't go all the way through the lung. The avian lung has parabronchi which are continuous tubes that allow for air to pass through the lung in one direction. These parabronchi are laced with capillaries. These capillaries are where the gas exchange occurs.

Diagram 5 shows the illustrated respiratory system of the chicken


The avian respiratory tract begins with the Glottis, which closes when feed is passed down the throat to prevent the inhalation of feed. The glottis connects to the trachea, which is made up of cartilaginous rings to prevents its collapse from the negative pressure caused by inhalation.
The syrinx is the vocal organ of birds. Vocalizations are made by air pressure on a sound valve and modified by muscle tension. The avian lungs are relatively small and they do not expand. The lungs are firmly attached to the rib cage. Birds have an incomplete diaphragm. Instead of being able to expand and deflate lungs, birds have air sacs, which inflate and deflate accordingly. In birds, there are nine such air sacs.
On first inhalation, air flows in from the trachea and bronchi, primarily into the caudal thoracic air sacs.
On exhalation, air moves from the caudal thoracic air sacs into the lungs.
With the second inhalation, air moves from the lungs into the cranial thoracic air sacs.
With the second exhalations, air moves from the cranial thoracic air sacs back into the trachea and out.

The next page will discuss the skeletal system.​