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Minggu, 12 Mei 2013

Respiratory System of Aves


Respiratory System of Birds

Differences between avian and mammalian respiration. Respiration in birds is much different than in mammals.
Birds have a larynx, but it is not used to make sounds. Instead, an organ termed the "syrinx" serves as the "voice box." Birds have lungs, but they also have air sacs. Depending upon the species, the bird has seven or nine air sacs. The air sacs include:
·                     Two posterior thoracic
·                     Two abdominal
·                     Two anterior thoracic
·                     Two cervical (these are not present in some species)
·                     One interclavicular
The air sacs of birdsextend into the humerus (the bone between the shoulder and elbow), the femur (the thigh bone), the vertebrae and even the skull. Birds do not have a diaphragm; instead, air is moved in and out of the respiratory system through pressure changes in the air sacs. Muscles in the chest cause the sternum to be pushed outward. This createsa negative pressure in the air sacs, causing air to enter the respiratory system. Expiration is not passive, but requires certain muscles to contract to increase the pressure on the air sacs and push the air out. Because the sternum must move during respiration, it is essential that it is allowed to move freely when a bird is being restrained. Holding a bird "too tight" can easily cause the bird to suffocate. Because birds have air sacs that reach into the bones, and have no diaphragm, respiratory infections can spread to the abdominal cavity and bones. Bird lungs do not expand or contract like the lungs of mammals. In mammalian lungs, the exchange of oxygen andcarbon dioxide occurs in microscopic sacs in the lungs, called 'alveoli.' In the avian lung, the gas exchange occurs in the walls of microscopic tubules, called 'air capillaries.' The respiratory system of birds is more efficient than that of mammals, transferring more oxygen with each breath. This also means that toxins in the air are also transferred more efficiently. This is one of the reasons why fumes from teflon are toxic to birds, but not to mammals at the same concentration.When comparing birds and mammals of similar weight, birds have a slower respiratory rate. Respiration in birds requires two respiratory cycles (inspiration, expiration, inspiration, expiration) to move the air through the entire respiratory system. In mammals, only one respiratory cycle is necessary.


These air sacs fill a large proportion of the chest and abdominal cavity, and also connect to the air spaces in the bones. Two primary bronchi, leading from the trachea, and a number of secondary bronchi, leading from the primary bronchi, feed air into the abdominal air sacs . Some of the secondary bronchi, which spread over the lower surface of each lung, channel air to theanterior air sacs. Also leading from the primary bronchi are between 7 and 10 dorsal bronchi, which spread over the back and sides of each lung, and ventral bronchi, which spread over the front of each lung. Finally there is a network of tertiary bronchi, which connects the ventral bronchi with the dorsal bronchi. Arising directly from the tertiary bronchi are air capillaries, which are responsible for the gas exchange between the ventilation system and the blood that transports oxygen to the body cells where it is needed for respiration.
The passage of air through a bird's lungs is similar to water flowing through a sponge: Air flows directly through the lungs and into the adjacent air sacs. The theory is that the air sacs might very well function like bellows to drive air through the lungs. Unlike mammalian lungs, in which the volume of air changes with each inhalation and exhalation, avian lungs maintain a constant volume of air. Because of the air sacs, the lungs maintain a constant volume of air. Because of the air sacs, the lungs inflate but do not deflate to take in more oxygen: they hold air.
If its lungs inflated and deflated with every breath, a bird in flight would be constantly gaining and losing altitude. Besides the advantage of keeping them on course, the avian respiratory system enables birds to maintain a higher energy level than mammals, because mammals lose energy just by breathing. Although the rate of respiration varies somewhat with the age of the bird, it is, in general, inversely proportional to body size -- the smaller the bird, the faster its breath rate.






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