Brief note on Respiratory system of Pigeon with its mechanism of breathing

Avian flight demands more supply of oxygen. As pigeon is a flighting animal the respiratory system of pigeon is more complicated than other groups of vertebrates. Respiration is by means of lungs. Lungs are small in size and supplemented by air sacs which reduce the body weight. Expiration is more active than inspiration.

There is no muscular diaphragm to separate thoracic chamber from abdominal chamber.

The respiratory system of pigeon includes respiratory organs, air sacs and respiratory tract.

(I) RESPIRATORY TRACT:

Respiratory tract includes External nares, Larynx, Trachea, Syrinx and Bronci.

(i) External Nares and Nasal Passages:

There are presences of paired slit like openings; external nares are situated at the base of the beak. The openings are surrounded by soft sensitive membranous cere or operculum. Nostril leads into short nasal sac or nasal passages. Nasal sac opens into the pharynx through internal nares situated dorsal to palatal folds.

(ii) Larynx:

There is a slit like opening situated on the floor of buccopharyngeal cavity called Glottis. It is just behind the root of the tongue. IT communicates Pharynx to the Larynx.

Larynx is the anteriorly expanded chamber at the anterior most margin of trachea. It is greatly reduced in birds. Larynx is supported by a triangular cricoids cartilage. There are no vocal cords in Larynx.

(iii) Trachea:

Trachea is a fairly long, flexible cylindrical tube running backward through the neck; ventral to the gullet pierced into the thoracic cavity beneath the oesophagus and is displaced to the left in the middle region by the crop. On entering into thoracic cavity trachea diatates to form syrynx.

(iv) Syrynx:

Syrynx is the sound producing organ of pigeon called sound box. It is a wide cavity supported by tracheal rings. Wide spacious part of syrynx is called tymopanum mucus membrane of which forms cushion like thickening on either side. At the junction of both bronchi, there is a bar of cartilage called pressulus. It supports a small, vibratory crescentic or semi-lunar membrane which supplements the vibration of tympanic membrane resulting sound production when air passes through it.

(v) Bronchi:

There are two bronchi continued from the syrynx running for a short distance enters into the lungs. Each bronchus made up of cartilaginous rings. The part of bronchus which enters up to the posterior end of lungs is called mesobronchus. These is no brounchioles. They send their branches to the air sacs in form of secondary bronchi and tertiary bronchior para bronci.

(II) RESPIRATORY ORGANS:

Respiratory organs of pigeon include a pair of lungs and air sacs.

(i) Lungs:

Lungs are small, spongy but non-elastic. These are highly vascular reddish coloured, closely attached to thoratic vertebrae and their ribs. Dorsally they posses no peritoneal covering. The ventral surface is free and covered by a thick sheet of fibrous tissue, the pleura or pulmonary aponeurosis (a modified peritoneu). Small fan like costo-pulmonary or inter costal muscles arise from the sternum and vertebral ribs, inseted in the pleural covering. Lungs are solid, respiratory surface is offered by the inner lining of parabronchi. Alveoli are absent.

Parabronchi form loops connecting with each other and also joins recurrent parabronchi. The lung parenchyma has hexagonal areas surrounding parabronchi, they have interlacing blood capillaries associated membrane and they come into contact with blood capillaries for gaseous exchange.

(ii) Air sacs:

There are remarkable, large, thin walled, membranous, non-vascular, non-muscular air sacs present around the lungs of pigeon. Their total volume is several times more than the volume of lungs and they fill up much of the body cavity making the body light. The air sacs arise from the main branches of bronchus which pierce out of the lungs at different places. There are nine air sacs which are of following types.

(a) Inter Clavicular: A single median and triangular air sac connected worth both lungs. It is situated in the angle between two limbs of furcula. It gives off an extra clavicular and clavicular air sacs.

(b) Cervical air sacs:

A pair of small air sacs arise anteriorly one from each lungs lie at the base of neck dorsal to the interclavicular sacs.

(c) Anterior thoracic air sacs:

A pair of air sacs lies ventral to lungs, overlapping the posterior thoracic air sacs.

(d) Posterior thoracic air sacs:

A pair of small posterior thoracic air sacs, lie in front of abdominal sacs.

(e) Abdominal air sacs:

A large posterior air sac arises from outer posterior angle of each lungs lie along the dorsal wall of abdomen.

These air saca acts as the acesseny respiratory organs.

MECHANISM OF BREATHING:

Breathing is the process of in taking of air from outside containing oxygen and expelling of air from body ladened with carbon dioxide. Breathing mechanism in pigeon (Birds) is that of a suction pump. Pigeon performs two different modes of breathing. One type at rest and other type during flight.

(I) Breathing at rest:

When the bird is in rising position, its sternum rises and falls alternately by the activities of abdominal inter costal muscles.

i) During inspiration, the sternum lowered, the air sacs expand and lungs remain compressed.

ii) As a result of which fresh air enters through the nostril, trachea and mesobronchus to the posterior air sacs.

iii) At the same time the air present in lungs moves to the anterior air sacs.

iv) During expiration the sternum is raised air sacs compressed and the lungs expand.

v) Air containing O₂ moves from posterior air sacs to the lungs.

vi) Gaseous exchange takes place, then the air passes to the anterior air sacs and pass out through the mesobronchus and respiratory tract to outside.

(II) Breathing during Flight:

During flight, the skeleton is kept rigid and ribs become immovable trace the wings. The air goes in and out of the lungs by elevation and depression of the back with the synchronized with the strokes of the wings. The faster a bird flies more rapidly it respires. This respiration takes place by –

i) Increasing and decreasing of the thoracic and abdominal cavities by movement of pectrol flight muscles.

ii) Pressure of viscera against the sacs which contract and expand alternately and cause air to circulate back and forth in the lungs.

iii) As the bird flies more rapidly the sternum moving away and towards the vertebral column. Thus air circulation and gaseous exchange become more rapid.

Presence of anastomising and inter-communicating air capillaries in avian lungs accompanied by the presence of ait sacs, increase the efficiency of breathing to a higher degree.