Avian Respiratory Physiology
Avian Core
  CVM 6880
Patrick T.  Redig DVM, PhD

            Diseases
            Cannulation
            Endoscopy

1. Functions
   
   1. Gas Exchange
      
   2. Heat Elimination
      
   3. Vocalization
      
   
   
2. Structure
   
   
   1. Upper Respiratory System
      
      1. Nares
         
      2. Operculum
         
      3. Sinuses
         
         1. Supraorbital
            
         2. Infraorbital
            
      4. Choanal Slit (cleft palate)
         
      
      
   2. Respiratory Tree
      
      1. Glottis
         
      2. Trachea
         
         1. Tracheal rings
            
         2. Coiling (swans, cranes)
            
         3. Dual tracheas (penguins)
            
      3. Syrinx
         
         1. Tympanic membranes on medial sides of bronchi
            
         2. Tympanic bulla in male waterfowl
            
      4. Dead space:  tracheal length and dead space volume in bird’s is typically 4.5 times or more greater than that of comparably sized mammals.  They compensate for this by increasing tidal volume and reducing respiratory frequency – see alveolar ventilation equation.
         
      
      
   3. Lungs – non-collapsing parenchyma with distinctive pattern of air passages arranged in an open system – birds lungs do not have blind-sac alveoli as do mammals.  Additionally, birds have no diaphragm.  It is impossible for them to develop a pneumothorax.
      
      1. Bronchi –
      2. Primary intrapulmonary bronchus – empties in caudal group of airsacs – see below.
      3. Secondary bronchi – 4 sets possible – these play in important role in channeling air through the lungs (in conjunction with the airsacs) such that there is very little mixing of incoming air with outgoing air.
         a. Medioventral, b. Mediodorsal, c. Lateroventral, d.  Laterodorsal*
         
         
         
         *Laterodorsal bronchi are not present in older orders of birds (struthioformes – ostriches and sphenisciformes – penguins).  They form the basis for enhanced pulmonary capacity in what is called the “neopulmonic” lung – more about this below.  (Note:  laterodorsal bronchi are not illustrated above).
         
         
      4. Saccobronchi (recurrent bronchi in older literature):  These are parabronchi of similar caliber to the secondary bronchi that are present at the openings in major airsac groups (see below).  These openings are called ostia.  The saccobronchi form the main channels for keeping incoming air separate from outgoing air during the respiratory cycle.
      5. Parabronchi:  These form the tertiary level of branching in the avian respiratory tree.  Parabronchi in the neopulmo are of varying length while those in the paleopulmo are generally similar.
         
         1. Illustration of the paleopulmonic lung (paleopulmo)
            
            
            
            
            
            
         2. Illustration of the neopulmonic lung (neopulmo) (below).  Note:  dotted lines indicate pathways of air through the neopulmo that reduce admixture of incoming and outgoing air.
            
            
            
      6. Further subdivisions:  Each parabronchus has invaginations or cul-de-sacs, referred to as “atria” around the permiter of which are “infundibula”.  Arising from the infundibula are anastamosing networks of air capillaries – tubular channels in which gas exchange occurs. (se below)
      7.  Airsacs:  In general, there are three sets of paired airsacs (anterior thoracic, posterior thoracic, abdominal) and one unpaired (interclavicular).  In some birds (e.g. psittaciformes, some galliformes) there are paired cervical airsacs in the neck region.  Also, diverticula of airsacs “pneumatize” the femur and humerus.  The general arrangement of airsacs is shown in the above illustrations taken from Sturkie.
         
         
         
      8. Arrangement of air capillaries and blood capillaries for gas exchange – the cross-current relationship. Aircapillaries are shown as a circular network arising from the infundibula (I) in the lower side of this illustration.  The blood vessels (a & v) give rise to blood capillaries that are arranged perpendicular to the plan of the air capillary ring.
         
3. Gas flow pattern:  Owing to the anatomic grouping of airsacs and secondary bronchi and their relationship to the neopulmo and paleopulmo, most incoming air transits the lung via the median bronchus and goes into the caudal airsac group without engaging in gas exchange, at least in the neopulmo.  Some of the gas does diffuse into the parenchyma of the paleopulmo.  On expiration, the gas in the caudal airsacs moves caudo-cranially into the parenchyma of the neopulmo and into the paleopulmo.  With the next inspiration, gas in the neopulmo and the paleopulmo passes into the anterior airsacs while new incoming air goes into the caudal airsacs.  With the second expiration, caudal airsac air again goes into the parenchyma; air in the cranial airsacs passes through the paleopulmo and out the trachea.  Thus, two respiratory cycles are required for a bolus of air to pass through the avian respiratory system.  The following illustrations depict this pattern of flow:
   
   
   
   
   Physiological Result
   
   1. More efficient oxygen transfer
      
   2. More efficient carbon dioxide transfer:  End expiratory P_CO2 is between 28 – 30 mm Hg.
      
   
   
4. Clinical Applications
   
   1. Unidirectional air flow either through the trachea or through an opening in the airsac can be used to relief dypsnea due to blockage or to administer gas anesthesia.
      
   2. Susceptibility to and treatment approaches for nebulization therapy for aspergillosis and other avian respiratory pathogens
      
   3. The air sac system provides a convenient access site for endoscopic examination of the coelomic cavity.  The most often used site is the left caudal thoracic airsac which is entered by penetrating the body wall at the last intercostal space.  What feature of avian anatomy that would be of interest endoscopically makes this the primary site of choice?