Respiratory System of Birds: Structure and Elements

He Respiratory system of birds Is in charge of oxygenate the tissues and organs and of expelling the carbon dioxide of the body of the same. The air sacs located around the lungs allow a unidirectional flow of air through the lungs, providing more oxygen to the body of the birds.

The unidirectional flow of air moving into the lungs of birds has a high oxygen content, higher than that found in the lungs of any mammal, including humans. Unidirectional flow prevents birds from breathing"old air,"that is, air that was recently in their lungs (Brown, Brain, & Wang, 1997).

Respiratory system of the birds represented in this drawing

Being able to store more oxygen in the lungs allows birds to better oxygenate their body, thus maintaining regulated body temperature while in flight. In birds' lungs, oxygen is distributed 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.

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. The diffusion of air for endothermic purposes is more effective insofar as the surface through which blood and gases flow is thinner (Maina, 2002).

Birds have relatively small lungs and up to nine air sacs that help them with the gas exchange process. This allows your respiratory system to be unique among vertebrates.

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Breathing process of birds

The process of breathing in birds requires two cycles (inhalation, exhalation, inhalation, exhalation) to move the air through the entire respiratory system. Mammals, for example, only need a breathing cycle. (Foster & Smith, 2017).

Birds can breathe through the mouth or nostrils. The air entering through these openings during the inhalation process passes through the pharynx and then through the windpipe or tube.

The trachea usually has the same length as the bird's neck, however, some birds, such as the cranes, have an exceptionally long neck and their windpipe that coils within an extension of the sternum known as a keel. This condition gives birds the possibility of producing sounds with high resonance.

Respiratory System of Birds: Structure and Elements


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 surrounding the nostrils is known as 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 ​​the birds has two channels.

The air in the process of breathing of the 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 craneal air sacs. During this process, all air sacs expand as the air enters the body of the bird.


During the first exhalation, the air moves from the posterior air sacs to the bronchi (ventrobronchios and dorsobronchios) and then to the lungs. The bronchi are divided into small capillary branches through which the blood flows, it is in these aerial capillaries where the exchange of oxygen by carbon dioxide takes place.

At 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 exits the bird's body.


Respiratory System of the Birds: Structure and Elements 1

Birds have larynx, however and unlike mammals, do not use it to produce sounds. There is an organ called syrinx that is in charge of acting 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 by changes in the pressure of the air sacs. The chest muscles cause the sternum to be pressed outward, creating a negative pressure in the sacs that allows air to enter the respiratory system (Maina J. N., 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 trapping a bird, external forces are not exerted that can block its movement, as it can suffocate the bird.

Air Bags

Respiratory System of Birds: Structure and Elements 2

Birds have a lot of"empty space"inside them, which allows them to be apt to fly. This empty space is occupied by air sacs that inflate and deflate during the breathing process of the bird.

When a bird inflates its chest, it is not the lungs that are working but the air sacs. The lungs of the birds are static, the air sacs are those that move to pump air into a complex bronchial system in the lungs.

The 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 mammals, whose air flow is bidirectional and enters and leaves the lungs in a short period of time, which makes the air never cool and always mixed with the air already breathed (Wilson , 2010).

Birds have at least nine air sacs that allow them to deliver oxygen to body tissues and remove 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 sacks have very thin walls with some capillaries so they do not play an important role in the gaseous exchange process. However, its duty is to keep the lungs ventilated where the gas exchange takes place.


The trachea of ​​birds is 2.7 times longer and 1.29 times wider than mammals of similar size. The work of the trachea of ​​the birds is the same of the one of the mammals, consists in resisting the flow of the 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 vast void space of the trachea with relatively higher tidal volume and lower respiratory rate, about one-third that of mammals. These two factors contribute to the lower impact of air volume on the trachea (Jacob, 2015).

The trachea forks 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 occur in the larynx.

The main entrance to the lungs is given by the bronchi and is known as mesobronchial. The mesobronquio divides in smaller tubes called dorsobronquios that in turn take to the smaller parabronquios.

The parabronchios contain hundreds of small branches and aerial capillaries surrounded by a profuse network of blood capillaries. Gaseous exchange between the lungs and blood takes place within these aerial capillaries.


The structure of the birds' lungs may vary slightly depending on the ramifications of parabronchia. Most birds have a pair of parabronchia, composed of an"old"(paleopulmonic) lung and a"new"(neopulmonic) lung.

However, some birds lack the neopulmonic parabronquio, as it is the case of the penguins and some races of ducks.

Singular birds, such as canaries and gallinaceae, have a neopulmonic parabronchus developed where 15% or 20% of the gas exchange is given. On the other hand, the air flow in this parabronchus is bidirectional, whereas 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, because the gas exchange does not occur in the alveoli but in the aerial capillaries and the air sacs are responsible for ventilation of the lungs .


  1. Brown, R.E., Brain, J.D., & 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.
  2. Foster, D., & Smith. (2017). Veterinary & Aquatic Services Department. Retrieved from the Respiratory System of Birds: Anatomy and Function:
  3. Jacob, J. (May 5, 2015). Extension. Obtained from Avian Respiratory System:
  4. Maina, J. N. (2002). Evolution Of The Birds And The Highly Efficient Parabronchial Lung. In J. N. Maina, Functional Morphology of the Vertebrate Respiratory System (page 113). New Hampshire: Science Publisher Inc.
  5. Maina, I. N. (2005). The Lung-Air Sac System of Birds: Development, Structure, and Function. Johannesburg: Springer.
  6. Team, A. N. (July 9, 2016). Ask Nature. Retrieved from The respiratory system of birds facilitates efficient exchange of carbon dioxide and oxygen via continuous unidirectional airflow and air sacs:
  7. Wilson, P. (July 2010). Currumbin Valley Vet Services. Retrieved from"What Are Air Sacs?:".

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