The respiratory system of birds is highly developed as they require sufficient oxygen for their flight activities. The respiratory system comprises the respiratory tract, the lungs and the air sacs.
They are being described below:
Respiratory tract: The respiratory tract includes the nostrils, nasal sacs, glottis, rudimentary larynx, trachea and the Syrinx.
There is one pair of slit like openings at the base of the upper beak which are known as the nostrils. The nostrils are overlapped by cere. The external nostrils open into the nasal sacs which communicate to the pharynx. The glottis is present near the base of the tongue and it opens into a rudimentary larynx situated at the anterior most part of the trachea.
The larynx does not function as a sound producing organ in birds. Its place as a vocal organ is taken by the Syrinx described later. The larynx is however supported by triangular cricoid cartilage.
A pair of procricoids is present in some birds. The arytenoids are frequently ossified in birds. The larynx opens into the trachea. The trachea is a long, cylindrical and flexible tube. The tracheal rings are usually complete, and they are frequently ossified. Now, the trachea divides into two bronchi, each bronchus enters into the lung of its corresponding side. At the division of the trachea into bronchi the Syrnix is present which is sound producing organ in birds. The actual sound-producing elements are membranes which vibrate by the passage of air.
The broncho-tracheal syrinx is very common. Here, the last rings of the trachea are united to form a resonating chamber known as tympanum. The folds of membrane, internal and external tympanic membranes extend into the cavity from the median and lateral wall of the bronchus. In certain cases, there is an internal skeletal element, the Pessulus. It bears a semilunar membrane on its lower surface. In the tracheal type of Syrinx the lateral portions of the last tracheal rings disappear and the associated membrane forms the vibratile part. In the bronchial syrinx the membranes are present between two successive rings of each bronchus. When the bronchial wall is shortened the membranes are forced as folds into the tube.
There are some muscles between the trachea and the bronchi. Associated with the inner and outer walls of the bronchus there are present the internal tympaniform membrane and the external tympaniform membrane respectively. These membranes are the mucous membranes. An interclavicular air space is enclosed beneath the semilunar membrane, by the tympaniform membranes. The tympaniform membrane is controlled and regulated by a pair of intrinsic syringeal muscles present between the trachea and the syrinx. These are inserted into the syrinx.
There is one pair of sternotracheal muscles also arising from the sternum and inserting into the trachea. The sound is produced by the vibrations of the above membranes. The pitch of the voice is altered by the associated muscles i.e. intrinsic syrengial and sternotracheal muscles.
There is one pair of bright red and spongy lungs in birds. The bronchus enters into the lungs and is known as pulmonary bronchus. The lungs lie in the pleural cavities. The lungs in the case of birds are closely connected with the ribs and vertebral column and as such, they can undergo very little change in shape. The ventral surface of lung is covered by a peritoneal membrane known as pleura. The costopulmonary muscles are attached to the pleura. These muscles arise from the junction of vertebral and sternal ribs.
The bronchus enters the medioventral side of the corresponding lungs and loses its cartilaginous rings. The bronchus continues as a main trunk to the distal end of the lung and it known as mesobronchus. From the mesobronchus arise the secondary bronchi which are called entobronchi, ectobronchi, laterobronchi and dorsibronchi according to their position. These secondary bronchi give off small tubes of uniform diameter known as parabronchi. Thus a series of parabronchial tubes with looped air capillaries carry on respiration instead of alveoli. There are 8 entobronchi, 6-10 ectobronchi, nearly 25 dorsibronchi (in chick) and a very few laterobronchi.
Presence of air sacs is an important feature of the respiratory system of birds. Four secondary bronchi do no stop at the lung wall of each side, rather, they pass through the walls of the lungs and are continued as thin-walled sacs. The air sacs are thin walled, membranous, non-muscular and nonvascular structures present in all the living species of birds. There are recurrent bronchi leading from the air sacs to the parabronchi and the air capillaries of the lungs. In Pigeon the following air sacs are present —
a) Interclavicular. There is a single interclavicular air sac lying anterior to the furcula. From each side of this air sac arise two tubular air sacs known as axillary air sacs. One of these tubular sacs, the posterior one enters the humerus bone through a pneumatic foramen.
b) Cervical: There is one pair of cervical air sacs. The cervical sac of either side lies at the base of the neck. They are dorsal to the interclavicular sac. They have some small branches leading to the neck region.
c) Anterior thoracic. There is one pair of anterior thoracic air sacs lying at the ventral side of the lung. They are in close contact with the ribs. An oblique septum covers its ventral wall.
d) Posterior thoracic: There is one pair of posterior thoracic sacs also which overlap the posterior end of the corresponding lung.
e) Abdominal: There is one pair of abdominal air sacs each one arising from the distal end of its lung. They lie along the dorsal wall of the abdomen. It is ventral to the kidneys and is in close contact of the small intestine.
Functions of Air Sacs
The following are the important functions of the air sacs —
1. The air sacs act as balloons and reduce the specific gravity of the bird in air. They help in maintaining equilibrium during flight.
2. A complete change of air in the lung is made possible with every breath with the help of air sacs. Thus, they increase the availability of oxygen. They force air into the lungs at each expiration to renew the air.
3. As they extend between the muscles, they naturally reduce the friction during flight.
4. They aid in the inspiration and expiration processes, and, permits the air to pass twice over the respiratory surfaces of the lungs. It has been suggested that interclavicular, cervical and anterior thoracic air sacs are expiratory and they are supposed to be more active during flight. The posterior thoracic and abdominal air sacs are inspiratory and they are supposed to be more active when the bird is not flying.
5. They are helpful in reducing the bodily temperature in correlation with absence of sweat glands. The water vapour diffuses from the blood into the cavities of the air sacs finally passing out through the lungs.
6. The air sacs probably allow movement to the heart, which is rigidly set in the thoracic region.
Mechanism of Respiration
When the bird is at rest, the respiration is brought about by Costopulmonary or intercostal muscles. This muscle is inspiratory.
By the contraction of muscles the ribs are raised and the sternum area is lowered. Naturally, the area of thoracic and abdominal cavities is increased. As a result, pressure on the lungs is reduced and air is drawn in into the lungs. The air passes into the air sacs also.
During inspiration the path of entering air is, nostrils—? glottis —? trachea —? bronchi —?mesobronchi —? secondary bronchi —?parabronchi —? air capillaries.
Exchange of gases takes place in the air capillaries.
But, the process of expiration is brought about by the movements of thoracic and abdominal muscles, and as such, these muscles are expiratory. The areas of thoracic and abdominal cavities decrease and as a result pressure on lungs increases. The air goes out of the lungs and finally through the external nostrils. By the compression of air sacs, the air goes into the recurrent
bronchi and from there, it goes into air capillaries. This is actually a secondary supply of fresh air to the air capillaries.
During flight, the mechanism is quite different. The area of the abdominal and thoracic cavities is increased or decreased by the movement of the pectoral muscles of flight. The pressure of viscera against the air sacs causes air circulate. The respiration takes place also by the movement of sternum towards the vertebral column and away from it. When the speed of the bird is faster, rate of respiration is rapid.