Airsacs and Mechanism of respiration in birds
The lungs are small and attached to ribs and are rigid; they do not expand or contract during the respiratory cycle. Large discrete air sacs are present. Both inspiration and expiration are active.
Respiratory system beginning from external nares leads to nasal cavities, which open into pharynx. The air also passes through the mouth in to the respiratory system.
Trachea begins with glottis and divides into two bronchi. At the division is the syrinx, which is the vocal organ in birds. The length and volume of the trachea is greater in birds than in mammals.
- Each lung contains one main intrapulmonary primary bronchus, which divides into these sets of secondary bronchi.
- The Secondary bronchi divide into tertiary bronchi or parabronchi.
- The three sets of secondary bronchi are
- Mediodorsal
- Medioventral
- Lateroventral
The parabronchi connect the mediodorsal and medioventral secondary bronchi and also connect some secondary bronchi to the airsacs.
Those parabronchi (structures that contain gas exchange tissues in their walls) that passes between mediodorsal and medioventral secondary bronchi are nearly parallel and are called as paleopulmonic parabronchi.
Those that pass from mediodorsal and lateroventral secondary bronchi and intrapulmonic primary bronchus to the caudal airsacs are called neopulmonic parabronchi. Exchange of gases between lungs and blood occur in parabronchi.
Airsacs
Airsacs are large thin-walled airsacs arise from some secondary bronchi. They lie outside the lungs in the body cavity. They function as airways. Since they are avascular, no gasesous exchange occurs.
There are 9 airsacs ; (5 anterior and 4 posterior ) unpaired inter clavicular sac, paired cervical and anterior thoracic airsacs) are anterior air sacs. Paired posterior thoracic and large abdominal sacsare the posterior air sacs.
Diverticula from some of the airsacs are connected to many of the bones. Hence, many of the bones in birds are pneumatic and the humerus is an important pneumatic bone.
The humerus a bone is connected to interclavicular air sac. Lungs can be ventilated via humerus and birds can suck air through broken and opened humerus.
Gas volume of the airsacs is approximately 10 times more than lungs.
Mechanics of respiration
There is no diaphragm in birds. So, there is no division of abdominal and thoracic cavities.
Pressure occurring during respiratory cycle is referred to as thoraco-abdominal pressure. Respiration is caused by changes in body volume.
During inspiration, the transverse and dorso-ventral diameter of thorax increase because of the ventral cranial movement of sternum and lateral movement of ribs.
During inspiration, some gas moves into the medio dorsal secondary bronchi, pass through the paleopulmonic parabronchi and enter into the cranial airsacs.
The pressure increase in expiration in all air sacs and air is forced out of the air sacs, which moves through lungs to outside. Thus, gas flows through the avian lungs during both inspiration and expiration.
During expiration, gas from the caudal airsacs passes again through the neopulmonic parabronchi and lateroventral secondary bronchi in the opposite direction and then moves into the mediodorsal secondary bronchi and passes through the paleopulmonic parabronchi in the same direction as during inspiration.
Gases from the cranial airsacs move into the medioventral secondary bronchi and out of the lungs without passing through the gas exchange area.
The unidirectional movement of gas through paleopulmonic parabronchi, which is the gas exchange portion of avian lungs, increases efficiency of ventilation.
Gas exchange can occur in the paleopulmonic parabronchi during both inspiration and expiration.
The remainder the gas moves through the neopulmonic parabronchi, lateroventeral secondary bronchi into the caudal airsacs.
During expiration these diameters decrease. In air sacs pressure drops and volume increase during inspiration and air move through the lungs into the airsacs.
Respiratory rates in birds
Respiratory rates of resting birds vary from 5/min in ostrich to 100 in small birds. Respiratory rates during flight are 3 – 18 times greater than at rest. Chicken, male 17 and female 27, Pigeon 28, House sparrow 59, and Goose 14/min.
Transport of blood gases in birds
The difference in gas transport and diffusion between mammals and birds are not great. O2 dissociation curve is same as in mammals except that it is to the right of mammalian curve. i.e. avian blood is less saturated with O2 than mammalian blood at same temperature and partial pressure and release of O2 to tissues is greater than mammalian blood.
PO2 for the arterial blood ranges 90 – 96 mm Hg. O2 saturation is 88 – 90% in arterial blood and 40% in venous blood which are lower than mammalian blood. PCO2 fowl blood is 28 – 34 mmHg. Arterial pH is generally higher in birds (7.44 to 7.58).