Respiratory System of Pigeon: Anatomy and Mechanism with Diagram
Respiratory System of Pigeon: Anatomy and Mechanism with Diagram
Due to the aerial mode of life a bird like a pigeon requires more energy. This is done by the breakdown of a large quantity of assimilated food at a faster rate. This is achieved by the extensively modified respiratory system in pigeons.
In pigeons, respiration is pulmonary and simple respiratory organs are present.
They do not show the presence of a true muscular diaphragm.
The respiratory tract and the organs of respiration i.e. lungs and unique system of air sacs, form the Respiratory system in pigeons.
The lungs are proportionately (when compared with other vertebrates) smaller in size but the functional efficiency is increased by the development of air sacs.
Air sacs and respiration in birds are well modified to support the flight offering a good example of flight adaptation in birds
Question: Describe the organs of respiration in Columba.
Answer:
Respiratory tract:
The respiratory tract of the pigeon includes the following organs,
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| Respiratory System of Pigeon (Ventral view): Diagram |
Nares
It starts with the nostrils or external nares which are a pair of slit-like oblique openings at the base of the upper beak, near the cere or operculum, a swollen sensitive pad of skin.
The external nares continue to open as a single opening, directly into the pharynx by internal nares.
Trachea
A slit-like aperture present on the floor of the mouth cavity and near the base of the tongue called glottis leads into windpipe or trachea.
The trachea is an elongated tube that begins from the glottis and runs along the neck region on the ventral side of the oesophagus. But in the middle region, it is slightly displaced to the left by the crop.
The trachea is composed of bony tracheal rings. Near its commencement, the trachea is enlarged into a chamber called the larynx.
This chamber is supported by a cricoid cartilage (composing four pieces) and one pair of arytenoid cartilages.
In birds, the larynx does not function as a voice box ( as in mammals) since the thyroid cartilage and the vocal cords are absent.
The primary function of the larynx is to act as a valve, regulating the flow of air into the trachea.
Near the junction of the neck and the tongue in the thoracic cavity, the trachea is swollen into a chamber called syrinx and then bifurcates into two short bronchi, one for each lung.
The trachea and the bronchial tubes are supported by a series of closely set rings.
All tracheal rings and the first bronchial rings are complete and bony, the rest of the bronchial rings are incomplete, mesially and cartilaginous.
Syrinx
Question: Short note on syrinx in birds.
Answer:
Syrinx or voice box is the characteristic voice organ of pigeons and many other flying birds.
It is formed by the dilation of the last three or four tracheal rings and the first bony ring of each bronchus.
The mucus membrane of the syrinx constitutes a pad-like thickening and is provided with several muscles and membranes.
It has complicated structural construction.
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| L.S. of Syrinx: Diagram |
A bar of cartilage called the pessulus is present at the junction of two bronchi. It extends dorsoventrally inside the tympanum and holds a small fold of mucus membrane called the crescentic or membrane semilunaris.
The inner membraneous lining of the bronchi produces inconspicuous internal tympaniform membranes.
The sound is produced by the vibration of the membrane semilunaris while the pitch of the sound is controlled by the action of the syringeal musculature which includes a pair of intrinsic Syringeal muscles, arising from the lateral sides of the trachea and is attached to the syrinx and a pair of sterno-tracheal muscles which originate from the sternum and are inserted into the trachea.
The position of the syrinx can be changed by the action of these muscles.
(The syrinx is absent in ostriches, storks and some vultures.)
Bronchus
Within the trunk, the trachea bifurcates into right and left bronchi. The first ring of each bronchus is complete and bony while the rest are incomplete mesially.
The left and right bronchi are called primary bronchi or mesobronchi.
In the beginning, each mesobronchus is composed of rings of cartilage that are absent inside the lung.
Each primary bronchus enters the lung through a small space called the vestibulum and extends up to the posterior extremity of the lung.
Within the lung, the mesobronchus sends three short branches into an air sac near its entrance into the lung,
- One enters the anterior thoracic air sac,
- The second is connected with the cervical air sac and,
- the third enters the interclavicular air sac.
The mesobronchus further branches into secondary bronchi. They are further designated as dorsal, ventral and lateral according to their position.
The secondary bronchi i.e. dorsobronchi and ventrobronchi further break up into a network of tertiary or parabronchi and send branches to the air sacs.
Each of the tertiary bronchus subdivides into a network of fine tubules (air capillaries) very close to the blood capillaries.
Moving ahead posteriorly, the diameter of the mesobronchus gradually decreases and finally opens up as ostium into the abdominal air sac.
Lungs of pigeon
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| Diagram: Lungs,trachea and bronchi of Pigeon |
The pair of lungs in a pigeon is bright red or pink coloured (like the colour of the feet of a pigeon).
They are small (comparatively), compact and non-elastic.
Position:
They lie just below (ventral to) the vertebral column and the ribs.
Dorsally the lungs are held compactly in their place by thoracic vertebrae and the ribs.
The dorsal surface of the lungs has no peritoneal covering and is closely fitted with the interspaces of the ribs.
The ventral surface of the lung is free and has a compact fibrous tissue sheet called the pleura or pulmonary aponeurosis, developed from the peritoneum.
There is a fan-like structure called the Costopulmonary muscles that are inserted on the wall of pleura which originates from the joints of vertebral and sternal ribs.
The alveolar lining is formed by the ramification of tertiary tubules with the distribution of blood vessels.
The bronchus, as the primary bronchus or mesobronchus, gradually decreases in diameter and enters the lung at the antero-ventral end.
Inside, it loses the cartilaginous rings and moves posterior branching as secondary Bronchi. They are designated as dorsal, ventral and lateral according to their position.
Question: write a short note on para bronchi in birds.
Answer:
The secondary bronchi i.e. dorsobronchi and ventrobronchi are further branched as tertiary or parabronchi.
The parabronchi are the major respiratory units of the lung, the sites of gas exchange. They subdivide into numerous finer networks of tubules (air capillaries) very close to the blood capillaries.
Thus, an intricate system of ramifying and anastomosing network of loops forms the main body of the lung.
Lung parenchyma shows the presence of hexagonal units having central parabronchus surrounded by air capillaries and interlacing blood capillaries. The air capillaries open into parabronchus on both ends. There are no blind pouches or alveoli. The para bronchus and the air capillaries join the neighbouring, and none ends blindly.
Thus, no dead spaces in the lungs where there is no airflow. The inner surfaces of air capillaries serve as respiratory membranes through which gaseous exchange takes place (not the alveoli as in mammals).
This makes the avian lung more efficient than any other vertebrate lung.
Compared to the lungs of other vertebrates, pigeon (bird) lungs do not store air.
Air sacs
Question: Write about Air sacs in birds with a neat labelled diagram and explain their function.
Answer:
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| Diagram of Air sacs and Lung (histological scheme) |
They are large, thin-walled, membranous, non-muscular and non-vascular bladder-like structures formed by the dilation of the mucous membrane of the bronchus.
They lie in the viscera and fill up much of the body cavity. Their total volume is several times that of the lungs.
Air sacs arise from the secondary bronchi except the abdominal air sac, which arises at the posterior end of the mesobronchus.
Opening of bronchi into air sacs is called ostia.
All the air sacs remain in communication with the pneumatic cavities of the bone
There are nine major and four accessory air sacs in the pigeon.
Major Air sacs
The nine major air sacs originate directly from the lungs. Amongst them, four are paired and one is unpaired.
The paired air sacs:
1. Abdominal or posterior air sacs:
Position: Among the coils of the intestine. Along the dorsal wall of the abdomen, ventral to the kidney. They are most posterior air sacs.
Size: Largest.
The right air sac is larger comparatively.
The sacs send diverticula to the pelvic girdle, synsacrum, hind limbs and between thigh muscles.
(The pigeon has an abdominal sac that is larger and a posterior thoracic sac that is smaller than most birds.)
2. Posterior thoracic air sacs
Position: Posterior part of the thoracic cavity, just in front of abdominal air sacs. Both the sacs are closely apposed with the lateral wall of the body cavity.
Size: Small. The left sac is slightly larger than the right.
3. Anterior thoracic air sacs:
Position: Ventral to the lungs, one on each side of the thoracic cavity in the anterior part of the chest between lungs and ribs, also close to the pericardium.
They extend back somewhat overlapping the posterior thoracic air sacs.
4. Cervical air sacs:
Position: At the base of the neck and in front of the lungs, dorsal to the interclavicular and alongside the vertebral column.
Each sac sends diverticula into the cervical vertebrae and the skull.
Size: Small.
The unpaired air sac
1. Inter-clavicular or median air sac:
Position: It is situated at the angle between the two limbs of the furcula.
Size: large, triangular.
It is unpaired and connected to both the lungs via ducts. Although the sac is unpaired in adults, it is formed by the fusion of two sacs which is evident by the presence of two ducts.
Each side of the air sac gives off two extensions,
i. Axillary or extra-clavicular air sac (in the armpit),
ii. Humeral air sac.
Accessory air sacs
These air sacs originate as paired diverticula from the interclavicular air sac. They are,
i. Clavicular air sacs
ii. Humeral air sacs.
These sacs are communicated with the air cavities of the bones.
A layer of fibroid tissue called oblique septum encloses the ventral wall of both the thoracic air sacs. This septum extends up to the pericardium and joins the similar septum of the other side along the middle line to divide the body cavity into two chambers
In one chamber there are present, lungs, thoracic and interclavicular sacs and in the other chamber, the heart, liver, stomach, intestine and abdominal air sacs.
Function of air sacs
The air sacs are essential components of aerial life.
The air sacs are not directly respiratory in function as they do not have a capillary network. On one hand, they communicate with bronchi and also with pneumatic cavities of bones thus providing the following functions.
Lightness
They probably act as balloons providing lightness and buoyancy by reducing the specific gravity of the body. This view is not much supported now.
Flight
Their function in flight is supported by the fact that the best fliers amongst birds possess the most highly developed air sacs.
Accessory respiratory organs ( bellows)
The air sacs do not have capillaries so they do not take part in gaseous exchange but serve as reservoirs and help in air flow during respiration. They act as bellows forcing complete renewal of air in the lungs leaving no dead space or uninspired air.
Temperature regulation
Pigeons' skin does not show the presence of sweat glands. Internal perspiration occurs i.e. water vapour diffuses from blood into the cavities of air sacs and passes out through the lungs, thus decreasing the body temperature.
Minimize mechanical friction
The positioning of air sacs between the muscles (especially the flight muscles) acts as pads minimising the friction and increasing the flexibility of wings during flight.
Balancing
The arrangement of air sacs on both the sides of body maintains the proper centre of gravity during flight. If in any case the equilibrium is altered, it is restored by shifting the contained air from one side of the body to the other.
Regulate moisture content of the body
Water is evaporated from the walls of air sacs in birds, thus maintaining the water content of the body.
Act as resonator
The pitch of the sound is controlled to some extent by the forceful expulsion of the air from the air sacs, acting as a resonator.
Mechanism of respiration in birds (pigeon)
Question: Describe the mechanism of respiration in birds (Columba).
Answer:
Pigeons ( birds) show a double respiration process where there is a double supply of oxygenated air to the surface of the lungs for improved aeration.
There occur two inspiratory and two expiratory cycles for a given volume of air to move through its complete path.
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| Avian Breathing Cycle: Tracing the passage of one bolus of air (shaded) |
(Explanation of the above diagram: One bolus of air-shaded blue, which is inhaled at Inhalation 1 and exhaled at Exhalation 2.
The preceding bolus, one step ahead of the shaded bolus, would pass through the lungs into the anterior air sacs at Inhalation 1. and then be exhaled at Exhalation 1. While the shaded bolus passes from the lung into the anterior air sacs in Inhalation 2, a succeeding bolus is inhaled into the posterior air sacs and when the shaded bolus is exhaled in Exhalation 2, this succeeding bolus is forced forward into the lungs. Thus air moves forward through the lungs during Inhalation 1 and 2 and Exhalation 1 and 2 alike.)
The breathing mechanism is different during rest and flight.
Breathing at rest
During inhalation,
- 1. the sternum is lowered,
- 2. Air sacs expand
- 3. Lungs are compressed.
At the same time the previously entered bolus of air moves from the lungs to the anterior air sacs.
- 1. Sternum is raised,
- 2. The air sacs are compressed.
- 3. Lungs expand.
Air high in O2 content moves from posterior air sacs into the lungs, and the stale air of anterior sacs moves into the mesobronchus, and trachea, then out through the nostrils.
Breathing during flight
During flight, the sternum remains immovable. With the elevation and depression of the back, synchronized with the stroke of the wings, the air goes in (inhalation) and out (exhalation) of the lungs.
The faster the bird flies, the more rapid the air circulation and gaseous exchange in the lungs.
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