Birds require large amount of oxygen due to their flight activity in accordance to which their respiratory system is comparatively more complex and developed.

1. Vaishnavi Solanki
IEHE , Bhopal

2.  Birds require large amount of oxygen due to their flight
activity in accordance to which their respiratory system is
comparatively more complex and developed .
 The following description is mainly based on the
respiratory system of pigeon –
Respiratory tract - it includes nares , nasal sacs ,
glottis , larynx , trachea and syrinx .
1) Nares & Nasal sacs –
 A pair of external nares are situated at the base of upper
beak which are overlapped by a soft, swollen sensitive pad of
skin called cere.
 The nostrils open into nasal sacs which open into pharynx
through internal nares.

3. 2) Glottis & Larynx --
 A median slit – like glottis lies just behind the root of tongue
which leads into larynx.
 Larynx is reduced and is not capable of sound production.
It is supported by a pair of arytenoid cartilage and one
cricoid cartilage .
 The cricoid cartilage is often made of pieces called
procricoids. In Pigeon , there are 4 – procricoid cartilages.
 Larynx opens into trachea.
3) Trachea -
 It is long, cylindrical tube which is as long as the neck and
is supported by complete, bony rings.
 Upon entering into the thoracic cavity, it expands to form
syrinx and then divides into two small bronchi.
 The bronchi are also supported by rings out of which the
first ring is complete & bony while the rest of the rings are
incomplete on inner side & cartilaginous.

4. Pigeon : Respiratory system in ventral view .

5.  Syrinx is the sound producing organ of birds and is their
characteristic feature.
 It is absent in ostrich, stork and some vultures. It is situated
at the end of the trachea, where it divides into bronchi.
 The syrinx is formed of an enlarged chamber called tympanum
whose walls are supported by last three or four rings of trachea
and the first ring of each bronchus collectively constituting
syringeal arches.
 The tympanum is lined by soft cushion- like external tympanic
membrane.
 A pair of membranous folds called internal tympanic
membrane project into the cavity of syrinx from median and
lateral walls of each bronchus.
 At the junction of two bronchi, a bony ridge called pessulus
extends dorso-ventrally.

6. The bronchial rings opposite the syrinx project into the
tympanum as external labia.
The internal tympanic membrane and the external labium on
each side functionally form a valve between the lung and the
syrinx.
 Syrinx is operated by syringeal muscles arising from the
sides of trachea and inserted into the syrinx.
The voice is produced by the vibrations of the tympanic
membranes as the air expelled from the lungs passes between
them during expiration.
The pitch of the voice is altered by changing the tension of
these membranes controlled by the syringeal and sterno-
tracheal muscles.

7. Pigeon: Syrinx A- External view ; B – In longitudinal section

8. Lungs :
 In pigeons and other birds, the lungs are small, more or
less solid, indistensible and less elastic.
 They are pressed dorsally against the thoracic vertebrae
and ribs and there is no peritoneal covering on this side.
 The ventral surface is free and is covered
by a thick sheet of fibrous sheet called pulmonary
aponeurosis or pleura.
 It is formed by the modification of peritoneum.
 Small fan-like costo-pulmonary muscles arise from the
junction of the sternal and vertebral ribs and are inserted
into the pleura.

9.  A bronchus enters into each lung from its antero-ventral side
and is called primary bronchus.
 Inside the lung, the primary bronchus repeatedly divides to
form a system of tubes called bronchial intercom.
 Upon entering into the lung the cartilaginous rings of primary
bronchus are lost. Now it is called mesobronchus.
 In pigeon, the mesobronchus enlarges into the vestibule and
extends for back, where it divides into two main branches out
of which one branch goes to abdominal air sac and the other
branch goes to posterior thoracic air
sac.
 From the mesobronchus throughout its way in the lung, arise
a number of thinner tubes
called secondary bronchi.
There are two sets of secondary bronchi 4-6 ventral
endobronchi and 7-10 dorsal endobronchi. Both of these
bronchi rebranch into still thinner tubes called tertiary bronchi
or parabronchi.

10.  These parabronchi meet with the parabronchi of other
secondary bronchi and form interconnecting loops.
 The parabronchi further divide into still thinner tubes called
air capillaries or bronchioles. Each parabronchus is
surrounded by a zone of capillary loops.
There are no alveoli. As the parabronchi and air capillaries
join with their neighbours, none of them ends blindly. Thus,
there is no dead space in the lungs.
The inner vascular surfaces of the air capillaries serve as
respiratory membranes through which
gaseous exchange with blood occurs.

11. Associated with lungs are air sacs which are actually
dilated extensions of the bronchial tubes.
Air sacs are extremely thin-walled having very few nerves
and blood vessels.
Internally these are lined with squamous epithelium and
connective tissue and externally these are covered with
peritoneum. Air sacs open into lungs through the ostia.
Air Sacs :

12. -- Histological structure of lung & origin of air sacs :

13. There are in all four paired and one unpaired air sacs in
birds which are named according to their location in the
body ---
(1) Interclavicular—
 It is a single, median somewhat triangular air sac
connected with both the lungs. Two branches arise from
each lateral side of interclavicular air sac.
 First branch is called clavicular sac. The second branch
is called axillary or extra-clavicular sac which goes in
the arm pit and then opens in the air cavity of humerus
bone.
(2) Cervical — These are a pair of small sized air sacs
arising from the anterior part of each lung.
 These are situated at the base of the neck dorsal to the
interclavicular air sac and along side the vertebral column.

14. (3) Anterior thoracic -
 An anterior thoracic air sac arises from the antero-lateral
side of each lung which lies in the anterior part of thoracic
cavity ventrally.
 Anterior thoracic air sacs of both the sides extend back to
overlap the posterior thoracic air sacs.
(4) Posterior thoracic -
 A posterior thoracic air sac arises from the postero-lateral
side of each lung which lies in the posterior thoracic cavity
close to the side walls of the body.
(5) Abdominal -
 A large abdominal air sac arises from the outer posterior
angle of each lung.
 Both the abdominal air sacs lie along the dorsal wall of the
abdomen in the coils of the intestine.

15. Among the air sacs described above, in posterior thoracic
and abdom inal air sacs, the air is filled during inspiration
hence these are called inspiratory air sacs
 while in remaining air sacs, the air is filled during expiration
hence called expiratory air sacs.
 A tough but elastic oblique septum covers the ventral walls
of the thoracic air sacs. Along the mid- dorsal line, the
oblique septa of the two sides unite.
 The heart, liver, stomach, intestine and abdominal air sacs
thus lie posterior to the septum, while lungs and other air
sacs infront of the septum.
 Air sacs serve the function of accessory respiratory organs.
They do not take part in gaseous exchange instead act as a
reservoir.

16.  At each expiration they forcibly expel the air
through lungs thus leaving no residual air in the lungs.
 Thus, by removing the residual air they ensure complete
renewal of air in the lung at each breath.
 In addition to this, air sacs are connected with the pneumatic
cavities of the bones and provide lightness and buoyancy during
flight.
 Air sacs also serve the function of regulation of temperature by
removing water vapours.