The respiration of birds is carried out by the respiratory system of this class of animals; It is responsible for oxygenating the tissues and organs and expelling carbon dioxide from the body of them. The air sacs located around the lungs allow for a one-way flow of air through the lungs, providing more oxygen to the bird's body.
The unidirectional flow of air that moves into the lungs of birds has a high oxygen content, higher than that found in the lungs of any mammal, including humans. One-way flow prevents birds from breathing in “old air,” that is, air that was recently in their lungs (Brown, Brain, & Wang, 1997).
Location of the respiratory system in a bird
Being able to store more oxygen in the lungs allows birds to better oxygenate their body, thus keeping their body temperature regulated while in flight.
In the lungs of birds, oxygen is delivered from the air capillaries to the blood, and carbon dioxide passes from the blood to the capillaries themselves. Gaseous exchange is, in this sense, very efficient.
Morphology of a bird. Example of Vanellus malabaricus. 1-Beak, 2-Head, 3-Iris, 4-Pupil, 5-Mantle, 6-Lesser Coverts, 7-Scapulars, 8-Coverts, 9-Tertiary, 10-Rump, 11-Primary, 12-Vent, 13 -Thigh, 14-Tibia-tarsal joint, 15-Tarsus, 16-Fingers, 17-Tibia, 18-Belly, 19-Flanks, 20-Chest, 21-Throat, 22-Wattle, 23-Eyestripe. Source: Wikimedia Commons
The respiratory system of birds is efficient thanks to the use of a thin surface through which gases and blood flow, which allows greater control of body temperature. Diffusion of air for endothermic purposes is more effective as the surface through which blood and gases flow is thinner (Maina, 2002).
Birds have relatively small lungs and a maximum of nine air sacs that help them with the gas exchange process. This allows their respiratory system to be unique among vertebrate animals.
You may also be interested in the excretory system of birds.
Bird respiration process
The respiration process in birds requires two cycles (inhale, exhale, inhale, exhale) in order to move the air through the entire respiratory system. Mammals, for example, only need one cycle of respiration. (Foster & Smith, 2017).
Birds can breathe through their mouths or nostrils. The air that enters through these openings during the inhalation process passes through the pharynx and then through the trachea or wind tube.
The windpipe is generally the same length as the neck of the bird, however some birds such as cranes have an exceptionally long neck and their windpipe that curls within an extension of the sternum known as the keel. This condition gives birds the ability to produce high resonance sounds.
Inhalation
During the first inhalation, the air passes through the nostrils or nostrils located at the junction between the top of the beak and the head. The fleshy tissue that surrounds the nostrils is known as the wax in some birds.
Air in birds, as in mammals, moves through the nostrils, into the nasal cavity and then into the larynx and trachea.
Once in the trachea, the air passes through the syrinx (organ responsible for the production of sounds in birds) and its current is divided in two, since the trachea of birds has two channels.
The air in the process of respiration of birds, does not go directly to the lungs, first it goes to the caudal air sacs, from where it will pass to the lungs and during the second inhalation it will pass to the cranial air sacs. During this process, all air sacs expand as air enters the bird's body.
Exhalation
During the first exhalation, air moves from the posterior air sacs to the bronchi (ventrobronchi and dorsobronchi) and subsequently to the lungs. The bronchi are divided into small capillary branches through which the blood flows, it is in these air capillaries where the exchange of oxygen for carbon dioxide takes place.
On the second exhalation, air exits the air sacs through the syrinx and then into the trachea, larynx, and finally into the nasal cavity and out of the nostrils. During this process, the volume of the sacks decreases as the air leaves the body of the bird.
Structure
Birds have a larynx, however, and unlike mammals, they do not use it to produce sounds. There is an organ called the syrinx that acts as a “voice box” and allows birds to produce highly resonant sounds.
On the other hand, birds have lungs, but they also have air sacs. Depending on the species, the bird will have seven or nine air sacs.
Birds do not have a diaphragm, so air is displaced into and out of the respiratory system through changes in the pressure of the air sacs. The muscles of the chest cause the sternum to be pressed outwards, creating a negative pressure in the sacs that allows air to enter the respiratory system (Maina JN, 2005).
The exhalation process is not passive, but requires the contraction of certain muscles to increase the pressure in the air sacs and propel the air out. As the sternum must move during the breathing process, it is recommended that, when catching a bird, external forces are not exerted that could block its movement, as it can suffocate the bird.
Air Sacks
Birds have a lot of "empty space" inside that allows them to be able to fly. This empty space is occupied by air sacs that inflate and deflate during the bird's respiration process.
When a bird inflates its chest, it is not the lungs that are working but the air sacs. The lungs of birds are static, the air sacs are the ones that move to pump air into a complex bronchial system in the lungs.
Air sacs allow a one-way flow of air through the lungs. This means that the air that reaches the lungs is mostly "fresh air" with a higher oxygen content.
This system is opposite to that of mammals, whose air flow is bidirectional and enters and leaves the lungs in a short period of time, which means that the air is never fresh and is always mixed with the one that has already been breathed (Wilson, 2010).
Birds have at least nine air sacs that allow them to deliver oxygen to body tissues and remove the remaining carbon dioxide. They also play the role of regulating body temperature during the flight phase.
The nine air sacs of birds can be described as follows:
- An interclavicular air sac
- Two cervical air sacs
- Two anterior thoracic air sacs
- Two posterior thoracic air sacs
- Two abdominal air sacs
The function of these nine sacs can be divided into anterior sacs (interclavicular, cervical and anterior thoracic) and posterior sacs (posterior thoracic and abdominal).
All sacs have very thin walls with some capillary vessels, so they do not play an important role in the gas exchange process. However, their duty is to keep the lungs where gas exchange takes place ventilated.
Windpipe
The trachea of birds is 2.7 times longer and 1.29 times wider than that of mammals of similar size. The work of the trachea of birds is the same as that of mammals, it consists in resisting the flow of air. However, in birds the volume of air that the trachea must withstand is 4.5 times greater than the volume of air present in the trachea of mammals.
Birds compensate for the large void space in the trachea with a relatively larger tidal volume and a lower respiratory rate, about one-third that of mammals. These two factors contribute to the lower impact of the air volume on the trachea (Jacob, 2015).
The trachea bifurcates or divides into two primary bronchi in the syrinx. The syrinx is an organ that is only found in birds, since in mammals the sounds are produced in the larynx.
The main entrance to the lungs is through the bronchi and is known as the mesobronchium. The mesobronchus divides into smaller tubes called dorsobronchi which in turn lead to the smaller parabronchi.
The parabronchi contain hundreds of small branches and aerial capillaries surrounded by a profuse network of blood capillaries. The gas exchange between the lungs and the blood takes place within these air capillaries.
Lungs
The structure of the lungs of birds may vary slightly depending on the ramifications of the parabronchi. Most birds have a pair of parabronchi, composed of an "old" lung (paleopulmonic) and a "new" lung (neopulmonic).
However, some birds lack the neopulmonic parabronchus, as is the case with penguins and some breeds of ducks.
Singing birds, such as canaries and gallinaceae, have a developed neopulmonic parabronchus where 15% or 20% of gas exchange occurs. On the other hand, the air flow in this parabronchus is bidirectional, while in the paleopulmonic parabronchus it is unidirectional (Team, 2016).
In the case of birds, the lungs do not expand or contract as they do in mammals, since gas exchange does not take place in the alveoli but in the air capillaries and the air sacs are responsible for the ventilation of the lungs..
References
- Brown, RE, Brain, JD, & Wang, N. (1997). The avian respiratory system: a unique model for studies of respiratory toxicosis and for monitoring air quality. Environ Health Perspect, 188-200.
- Foster, D., & Smith. (2017). Veterinary & Aquatic Services Department. Retrieved from Respiratory System of Birds: Anatomy and Function: peteducation.com.
- Jacob, J. (May 5, 2015). Extension. Retrieved from Avian Respiratory System: articles.extension.org..
- Maina, JN (2002). Evolution Of The Birds And The Highly Efficient Parabronchial Lung. In JN Maina, Functional Morphology of the Vertebrate Respiratory System (p. 113). New Hampshire: Science Publisher Inc.
- Maina, JN (2005). The Lung-Air Sac System of Birds: Development, Structure, and Function. Johannesburg: Springer.
- Team, AN (July 9, 2016). Ask Nature. Obtained from The respiratory system of birds facilitate efficient exchange of carbon dioxide and oxygen via continuous unidirectional airflow and air sacs: asknature.org.
- Wilson, P. (July 2010). Currumbin Valley Vet Services. Retrieved from What Are Air Sacs ?: currumbinvetservices.com.au.