Avian Respiratory System
The respiratory system of a bird is much more complicated than a mammal's respiratory system. The demands on the muscles for flying, requires a much more efficient system than the mammal's system. A simplified avian respiratory animation is shown in the Flash animation below. You can view the inhalation phase of respiration by clicking on the [INHALATION] button, the exhalation phase of respiration by clicking on the [EXHALATION] button or the entire respiration sequence by clicking on the [LOOP RESPIRATION] button. A description of the avian respiratory system is also provided below the animation.
Unidirectional Air Flow
Before we look at a bird's respiratory system, let's review a typical mammal's respiratory system. During the exhalation phase of respiration, oxygen depleted air is expelled out of the body. If we took a snapshot at the very moment that exhalation ends and inhalation begins we would see oxygen depleted air in the bronchial tubes and windpipe. When the inhalation phase begins, the system is already partially full (windpipe and bronchial tubes) of oxygen depleted air. This oxygen depleted air is mixed with fresh oxygen rich air drawn from outside the body. This results in a mixture of air that is somewhere between the oxygen rich air from outside and the oxygen depleted air from the lungs. This mixture results in a very inefficient (though sufficient) system because the lungs have to use a mixture of "good" and "bad" air. This is a direct result of the bidirectional air flow in the human's respiratory system. Air flows into and out of the lungs using the same pathway. The bird's system is very different and much more efficient because it uses a unidirectional air flow. The air flow into the lungs takes one path and the air flow out of the lung takes another path. This results in a system that provides "good" oxygen rich air to the lungs every time instead of the mixture of "good" and "bad" air in the mammal's system.
Air Sacs
This unidirectional flow of air in a bird is possible due to in part to air sacs which are not found in the mammal's respiratory system. Air sacs provide temporary storage locations for air and do not perform any role in gas exchange (Oxygen, Carbon Dioxide etc). Most birds have nine (9) air sacs.
- One (1) Interclavicular Air Sac
- Two (2) Cervical Air Sacs
- Two (2) Anterior Thoracic Air Sacs
- Two (2) Posterior Thoracic Air Sacs
- Two (2) Abdominal Air Sacs
These air sacs are grouped into two functional groups.
- Anterior Air Sacs -(1) Interclavicular, (2) Cervicals, and (2) Anterior Thoracics
- Posterior Air Sacs - (2) Posterior Thoracics and (2) Abdominals
Let's look at the bird's respiration starting with inhalation.
Inhalation
The inhalation part of the respiration is shown in the animation by clicking the [INHALATION] button.
During inhalation, air moves through the trachea, through the bronchia and mostly into the Posterior Air Sacs (Posterior Thoracics and Abdominals). This is shown by the red arrows during the [INHALATION] part of the animation.
At the same time, air flows from the lungs into the Anterior Air Sacs (Interclavicular, Cervicals, and Anterior Thoracics). This is shown by the blue arrows during the [INHALATION] part of the animation.
Exhalation
The Exhalation part of the respiration is shown in the animation by clicking the [EXHALATION] button.
During exhalation, air moves from the Posterior Air Sacs into the lungs. This is shown by the red arrows during the [EXHALATION] part of the animation.
At the same time, air moves from the Anterior Air Sacs out of the trachea and out of the body. This is shown by the blue arrows during the [EXHALATION] part of the animation.
Respiration
The combined inhalation and exhalation can be viewed in the animation by clicking the [LOOP RESPIRATION] button. As shown in the animation, it actually takes two complete cycles to move air completely through the respiratory system.
- Cycle One
- Inhalation - Air from outside the body into the Posterior Air Sacs
- Exhalation - Air from Posterior Air Sacs to Lungs
- Cycle Two
- Inhalation - Air from the lungs to the Anterior Air Sacs
- Exhalation - Air from the Anterior Air Sacs out of the body
Summary
The system appears to be inefficient because it takes two cycles. However, it is actually more efficient because air flowing into the lungs (from the Posterior Air Sac) is always oxygen rich blood due to the unidirectional flow of air through the system.