Avian Respiratory System

• Avian respiratory system


  Differs from that of mammals, has corresponding importance to the veterinarian, indoor flocks of the modern poultry industry are prone to respiratory infections which may be very costly
• Upper respiratory tract


  • Nares
  • Nasal cavity
  • Conchae
  • Paranasal sinus
  • Infraorbital sinuses
  • Larynx
  • Trachea
• Lower respiratory tract


  • Bronchus (primary, secondary, tertiary/parabronchi)
  • Lungs
  • Air sacs
• Nostrils


  Located dorsolaterally at the basis of the upper mandible, circular, oval or slit-like in shape, enclosed by a dorsal and ventral cartilaginous plate, dorsal plate covered by keratinized epithelium (operculum) to keep water/pollen out in diving/flower-feeding birds

  
• Nasal cavity


  Divided by a median septum, in wide communication with the oropharynx through the choana, contains 3 conchae (rostral, middle, caudal) that clean, warm and humidify air
• Paranasal sinus


  Only the infraorbital sinus is present, lies lateral to the nasal cavity, communicates with the nasal cavity through a cleft, lined by squamous epithelium rostrally and ciliated cuboidal epithelium with goblet cells caudally

  
• Larynx


  Supported by cricoid and arytenoid cartilages, glottis located between arytenoids deviates passage of fluids laterally to avoid choking
• Trachea


  Originates at the larynx, connected caudally to the syrinx, lined by pseudostratified ciliary epithelium with goblet cells, composed of cartilage rings connected by elastic membranes, may form a loop in long-necked species
• Syrinx


  Main source of vocalization, formed by terminal trachea and beginning of primary bronchi, located intrathoracically at the level of first and second thoracic vertebrae, laterally compressed, consists of pessulus, syringeal rings, and vibrating tympanic membranes
• Bronchial system


  • Primary bronchus (extra- and intra-pulmonary)
  • Secondary bronchus (mediodorsal, medioventral, lateroventral, laterodorsal groups)
  • Tertiary bronchus (parabronchi)
• Primary bronchus


  Arises from tracheal bifurcation, has extra-pulmonary and intra-pulmonary parts, composed of cartilage rings and smooth muscle to control diameter
• Secondary bronchi


  Arise from primary bronchus, arranged into 4 groups (mediodorsal, medioventral, lateroventral, laterodorsal) with connections to air sacs essential for air flow
• Tertiary bronchi (parabronchi)


  Functional unit of gas exchange, arise from secondary bronchi, paleopulmonic parabronchi form loops that constitute 3/4 of the lung, neopulmonic parabronchi are less regular and more caudal
• Parabronchial structure


  Central lumen surrounded by network of blood and air capillaries, air capillaries converge into infundibula which open into atria, gas flows unidirectionally during both inhalation and exhalation
• Lungs


  • Relatively small, unlobed, bright pink, do not expand, firmer than mammalian lungs, confined to craniodorsal part of body cavity, deeply grooved dorsally by vertebrae and ribs
• The lungs have three surfaces: costal, vertebral, and ventromedial
• Infundibulum


  • Blue arrows
  • Several infundibulum open into atria (*) along the parabronchial lumen
• Gas flow


  • Unidirectional during both inhalation and exhalation (red arrows)
• Lungs


  • Relatively small, unlobed, bright pink and do not expand
  • Firmer than mammalian lungs, soft and velvety to the touch
  • Confined to the craniodorsal part of the body cavity
  • Dorsolateral surface deeply grooved (deep costal sulci) by the thoracic vertebrae and vertebral ribs (4th-7th)
  • Lie immediately above the heart
• Lung surfaces


  • Dorsolateral (costal) surface
  • Dorsomedial (vertebral) surface
  • Ventromedial (septal) surface
• No pleural cavity is necessary, since the capacity for expansion is negligible
• Air sacs


  • Made up of thin walled structure with simple squamous epithelium and a thin layer of connective tissue
  • Walls are poorly vascularized (poor blood supply), do not directly participate in significant gas exchange
  • Extended to the cervical vertebrae, thoracic vertebrae, ribs, sternum, humerus, pelvis, head, and body of femur
  • Responsible for providing the space during the respiratory cycle and thereby function to increase the pulmonary ventilation
• Air sacs in birds

  • Clavicular air sac
  • Cervical air sacs (2)
  • Anterior thoracic air sacs (2)
  • Posterior thoracic air sacs (2)
  • Abdominal air sacs (2)
• Clavicular air sac


  • Single, bilaterally symmetrical
  • Thoracic part fills the space cranial to and around the heart, extends into sternum
  • Extrathoracic part passes between the muscles and bones of the shoulder girdle to pneumatise the humerus
  • Directly connected to the medioventral secondary bronchus, may also have indirect connections via parabronchi to other cranial secondary bronchi
• Anterior thoracic air sacs

  • Paired, lie ventral to the lungs, between the sternal ribs and the heart and liver, and caudal to the clavicular air sac
  • Generally connect to the medioventral secondary bronchi, as well as to parabronchi originating from other cranial secondary bronchi
• Posterior thoracic air sacs


  • Paired, positioned caudal to the lungs and dorsocaudal to the anterior thoracic air sacs, situated lateral to the cranial portions of the abdominal air sacs
  • Directly connected to the lateroventral secondary bronchus and may have indirect connections to other lateroventral, or even cranial medioventral secondary bronchi
• Abdominal air sacs


  • Paired, the largest, extends bilaterally from the caudal side of the lungs to the pelvic cavity
  • Covered by the abdominal wall and intertwined with the abdominal organs
  • Connected to the caudal end of the intrapulmonary primary bronchus and may have more indirect connections to parabronchi from laterodorsal secondary bronchi and the last mediodorsal secondary bronchi
• Air flow through avian lungs and air sacs during respiration

  1. Fresh air travels through trachea -> primary bronchi -> some enters lungs, remaining fills posterior air sacs

  2. Fresh air in posterior sacs enters lungs and undergoes gas exchange, spent air in lungs displaced and flows out through trachea

  3. Fresh air again enters both lungs and posterior air sacs, spent air in lungs displaced but cannot exit trachea as fresh air flowing inward, instead enters anterior air sacs

  4. Spent air in anterior sacs and lungs flows out through trachea, fresh air in posterior sacs enters lungs for gas exchange
• Pressure changes


  • During inhalation: Sternum moves forward & downward, vertebral ribs move cranially to expand sternal ribs & thoracoabdominal cavity, expanding posterior & anterior air sacs & lowering pressure to draw in air
  • During exhalation: Sternum moves backward & upward, vertebral ribs move caudally to reduce thoracoabdominal cavity volume, reducing anterior & posterior air sac volume to expel air