COMPARATIVE ANATOMY: RESPIRATION IN BIRDS, REPTILES AND MAMMALS
Comparative anatomy of respiratory systems in calotes (lizard), columba (pigeon) and oryctolagus (rabbit).
Calotes is a poikilothermic terrestrial lizard. It is commonly known as garden lizard. Columba and oryctolagus are homoiothermic animals. Columba is commonly known as pigeon adapted for aerial mode of life. Oryctolagus is herbivorous mammal. It is commonly called rabbit. All these three animals are grouped under Amniota.
Living cells require energy for performing metabolic activities. This energy is provided by the oxidation of food materials present within the cells. During oxidation the oxygen is consumed and releases carbon dioxide and water. Carbon dioxide is harmful to the protoplasm. Hence to keep the oxidation continuously, there is a constant need for the supply of oxygen to the cells and the removal of carbon dioxide from them. Oxygen is obtained from the environment (water or air) and in return carbon dioxide is added to the environment. The exchange oxygen of the environment with the carbon dioxide of the body is known as 'respiration'. In all vertebrates, some organs of the body are specialised for the exchange of gases. These are called 'respiratory organs or breathing organs'. The surface of these organs at which the exchange takes place is called respiratory surface.
Vertebrate respiratory organs include the gills and the lungs. Both of these organs develop from the pharynx. Lungs arise in the embryo as an endodermal diverticulum from the ventral wall of the pharynx. The diverticulum soon divides into two parts, which form right and left lungs. A windpipe or trachea connects the lungs with the pharynx. Anterior part of the trachea is modified into the larynx. The larynx communicates with the pharynx by a slit like opening the glottis. The laryrnx functions as sound producing organ in tetrapods except in birds. The birds have their sound-producing organ known as 'syrinx'. The trachea bifurcates into two branchi. Each primary branchus further divided inside the lungs as secondary bronchi, tertiary bronchi and bronchides. The bronchides are connected to the alveoli.
In vertebrates the blood plays a vital role in respiration. It carries oxygen from the lungs to the cells for oxidation and collect carbon dioxide for removal from the body. There are two phases of respiration. The exchange of gases takes in the lungs is called 'external respiration'. In the tissues the gaseous exchange takes place and it is termed internal respiration. The respiration performed by the lungs is called 'pulmonary respiration'.
|Calotes (Garden lizard)||Columba (Pigeon)||Oryctolagus (Rabbit)|
|1. Paired external nostrils are small and oval apertures situated dorsally at the tip of the snout.||1. Paired external nostrils are oblique slits present far above the mouth and lie at the base of the upperbeak.||1. Paired external nostrils are oblique slits lying a little above the mouth.|
|2. External nostrils are uncovered and are not connected to the mouth.||2. Same as in calotes.||2. External nostrils are partly covered by fleshy tip of the snout and are connected with a cleft in the upper lip.|
|3. External nostrils possess valves.||3. Valves are absent.||3. Valves are absent.|
|4. Nasal chambers are small and do not have the conchae.||4. Nasal chambers are small and conchae are present.||4. Nasal chambers are long and have conchae.|
|5. Nasal chambers remain separate throughout as there are two internal nostrils.||5. Nasal chambers remain separate throughout as these are two internal nostrils.||5. Nasal chambers join posterior as there is a single internal nostril.|
|6. Paired internal nostrils open into the anterior part of the buccal cavity.||6. Paired internal nostrils open into the posterior part of the buccal cavity.||6. Internal nostril opens into the pharynx.|
|7. Glottis is without epiglottis.||7. Same as in calotes.||7. Glottis is covered by a cartilagenous plate-epiglottis.|
|8. Larynx is supported by a pair of arytenoids and a median single cricoids cartilaginous plates.||8. Larynx is supported by the same cartilagenous plates as in calotes. But the cricoid is sub-divided into four pieces procricoids.||8. Larynx is supported by four cartilagenous plates. Median ventro-lateral Thyroid, ring-like lower cricoid and paired arytenoid cartilages on the dorsal side. It is also lined by ciliated epithelium.|
|9. Laryrnx is a voice-box by possessing paired vocal cards which are functional.||9. Larynx is poorly developed and nonfunctional, ie. The sound is not produced.||9. Larynx possesses a pair of vocal cards and their vibrations produce sound.|
|10. Larynx has two paired muscles - innermusculus compressor laryngis and outer musculus dilater laryngis. It opens into the trachea.||10. Same as in calotes. Larynx opens into the trachea.||10. Same as in Columba and the larynx opens into the trachea.|
|11. Trachea is supported by complete cartilaginous rings.||11. Trachea is supported by incomplete bony rings present dorsally.||11. Trachea is long, thin walled tube, supported by incomplete cartilaginous rings present dorsally.|
|12. Bronchi have complete cartilagenous rings.||12. Median incomplete cartilagenous rings are present in the bronchi.||12. Bronchi have incomplete median and dorsal cartilaginous rings.|
|13. Bronchides are not formed.||13. Bronchides are formed in a complex manner.||13. Bronchides are formed by repeated branching of bronchi.|
|14. Syrinx is absent.||14. The sound producing organ Syrinx is present at the bifurcation of the trachea.||14. Syrinx is absent.|
|15. Lungs have orange colour. These are elongated bodies of fairly large size.||15. Lungs have pinkish colour. These are ovoid organs relatively small in size.||15. Lungs have pinkish colour. These are irregular organs of large size.|
|16. Lungs are hollow sacs with thin elastic wall. These are having low septa and shallow alveoli on the inner surface.||16. Lungs are compact spongy organs. They are slightly distensible.||16. Lungs are soft, spongy and elastic organs.|
|17. Lungs are not differentiated into lobes.||17. Same as in calotes.||17. Lungs are divisible into lobes. The right lung has four lobes Anterior azygos, anterior, posterior and posterior azygos. The left is divisible into two lobes-anterior & posterior.|
|18. Lungs are enclosed by peritoneum.||18. Lungs don't have peritoneum on the dorsal side.||18. The lungs are completely covered by peritoneum. The space between the lungs is called mediastinum where heart is present.|
|19. Air sacs are absent.||19. Lungs are associated with thin, transparent air sacs. There are nine air sacs. These are, median interclavicle, paired cervical, paired anterior thorasic, paired posterior thorasic and paired abdominal air sacs.||19. Air sacs are absent.|
|20. The inter costal muscles help during the processes of inspiration and expiration.||20. The inter costal and abdominal muscles help during the processes of inspiration and expiration.||20. The inter costal muscles and peripheral muscles of diaphragm help during the processes of inspiration and expiration.|
|21. Gaseous exchange occurs through the entire inner surface of the lungs.||21. Gaseous exchange occurs in the blood capillaries present in the lungs. Double respiration takes place since the air is preserved in the air sacs.||21. Gaseous exchange takes place in the alveoli present within the lungs.|
|22. Some deoxygenated residual air is left over in the lungs. Hence aeration of blood is not very efficient.||22. Residual air is not left over in the lungs.Hence aeration of blood is very efficient.||22. Some deoxygenated residual air is always left over in the lungs. Hence aeration of blood is not so efficient.|