What is the function of the anterior air sacs in the respiratory system of birds?

The respiratory system is one of the major systems of the body. It has a number of very important functions including the provision of oxygen, the removal of carbon dioxide, the removal of excess heat (thermoregulation) and vocal communication. The respiratory system is a complex one and while there are some similarities with that of mammals, there are a number of quite significant differences.

Nasal cavity

The openings to the nasal cavity, the nares, lie at the point of the base of the comb on the top beak or mandible. The nasal cavity occupies a triangular shaped space between the nares and the margin of the eye and within the beak. Between the integument and the nasal cavity lie the lacrimal sinuses that empty into the cavity through the lateral wall.

The lateral wall of the cavity has three conchae, or projections, into the cavity:

  1. Anterior – of squamous epithelium – a single layer of flattened cells
  2. Medial – ciliated columnar epithelium – special cube shaped cells with cilia or hairs that trap foreign material
  3. Posterior – olfactory membrane – that gives the sense of smell

Smell, protection and reduction of electrolyte

The odour or smell of any material or object is a result of minute quantities of special chemicals that are detected by the olfactory membrane that in turn sends a signal via the olfactory nerve to the brain where it will be recognised for what it is. The epithelium of the nasal cavity is well endowed with mucosal glands which produce mucous that helps to keep foreign material from gaining entry to the body through the respiratory system. The nasal glands are small and located on either side. Their action is to supplement the action of the kidneys by reducing the electrolyte content of body fluids, especially when the level is higher than the kidneys can handle (electrolyte – common chemical salts e.g. sodium chloride).

Oropharynx (mouth and pharynx)

The oropharynx consists of the mouth and the pharynx that is located immediately behind it. The palate is part hard and part soft. The choanal opening (from the nasal cavity) is the cleft in the palate. The pharynx begins between the choanal opening and the common opening for the auditory tubes and extends to the rear to include that section of the oral cavity carrying the base of the tongue, the tip of which is located in the mouth.

Behind the base of the tongue is found the rima glottidis or opening into the larynx, which is sometimes called the cranial larynx. This opening is located in a conspicuous mound called the laryngeal prominence. The opening into the larynx is a median slit that is supported on each side by the arytenoid cartilages. These are special cartilages with a shape resembling the mouth of a jug or pitcher. It varies in length from approximately 8.5 mm in the female to 11 mm in the male. During gasping while the bird is in a state of respiratory distress it can open to a width of 7 to 9 mm. There are no vocal cords, epiglottis and thyroid cartilages that are normally found in mammals.


This organ is a long tube with the function of moving the respiratory gases from the upper respiratory system to the organs of respiration – the lungs and air sacs or from the air sacs and lungs to the upper respiratory organs. The trachea in medium sized adults measures between 15 and 18 centimetres. It is held open permanently by 108 to 125 cartilaginous rings each one complete and lapping its neighbour. This arrangement prevents the trachea from collapsing or compressing but allows elongation and flexion. This means that while the trachea is flexible, under normal circumstances it will not collapse and prevent the free passage of air into/out of the lungs.

Most of the muscles in the region are involved in the movement of the head and neck and are not associated with respiration. The trachea is lined with muco-ciliary epithelium which is a special type of epithelium where the hair-like cilia move foreign materials, such as dust, up and out of the trachea. Numerous mucous secreting glands are also found in the tracheal lining.


The syrinx is the vocal organ of the fowl. It is located at the caudal end of the trachea and is suspended within the clavicular air sac. At rest, it is compressed laterally (at the sides). It consists of the pessulus, a wedge shaped cartilage located where the trachea divides into two to form the two bronchi plus the last four specialised cartilaginous rings at the bottom of the trachea. Below this section there are 4-7 thin, flexible syringeal cartilages which are fused at one end to the pessulus but which are free at the other. These are sometimes called the intermediate cartilages or syringeal ears.

At the syrinx the trachea forms two bronchi that then enter the lungs. After the syrinx, the cartilaginous rings of these bronchi are “C” shaped (i.e. an incomplete circle). The walls of the syrinx are two thin, vibrating membranes called the tympanic membranes. The bird makes sound by causing these membranes to vibrate. The tension of these membranes is controlled by muscles and the tension governs the sound produced.


The trachea divides at the syrinx into the left and right bronchi which are called the primary or mesobronchi. It is interesting to note that the combined cross sectional area of the bronchi is more than double that of the caudal end of the trachea from which they arise (30 mm2 v 12.5 mm2). This means that there is a significant reduction of air pressure and resistance to airflow in the bronchi than in the trachea. The cartilaginous rings of the bronchi extend from the syrinx to where the bronchi enter the lungs. Ciliated epithelium with numerous mucous glands lines the primary bronchi.

On entering the lungs, the primary bronchi divide to form four series of secondary bronchi and these, in turn, divide again to form numerous anastomising tertiary bronchi or parabronchi. The secondary and tertiary bronchi are lined with squamous epithelium and not the ciliated epithelium of the primary bronchi. The tertiary bronchi are arranged in layers. They are not blind ending, but join others that in turn lead back to the secondary and primary bronchi. Ultimately, the bronchial system is continuous.

Gas exchange

Leading off from the bronchi in the lungs are a large number of extremely small air capillaries (ducts) that are interlocked with the capillaries of the lung circulatory system. These interlocked capillaries are the lungs’ gas exchange system and are very thin which accommodates gaseous exchange. The blood/gas barrier in this area consists of whatever cell layers separate the two systems – the blood circulatory system and the air supply system of the lungs. These layers are:

What is the function of the air sacs in the bird respiratory system quizlet?

Air sacs in bird function to circulate air through their lungs. They cause unidirectional and continuous flow of air through the lungs of the bird and thus are 30% more efficient than human lungs.

What are the functions of the anterior and posterior air sacs?

During exhalation, fresh air from the posterior air sac moves into the lungs, while stale air from the anterior air sacs is expelled through the bronchus and trachea. This pattern of airflow through the respiratory system creates unidirectional (one-way) flow of fresh air over the gas exchange surfaces in the lungs.

What are air sacs in birds and how does this adaptation help birds in flight?

Air sacs are attached to the hollow areas in a bird's bones. Essentially, their lungs extend throughout their bones. This helps birds take in oxygen while both inhaling and exhaling. This adds more oxygen to the blood, providing a bird with extra energy for flight.

How many air sacs do birds have as part of the respiratory system?

Like mammals, birds have nares, a larynx, trachea and lungs. In addition, they have nine air sacs and a syrinx (vocal center). Unlike mammals, they have no diaphragm and there is a unidirectional air flow that requires two full inspiratory and expiratory cycles to complete.