|The Respiratory System|
HOW VERTEBRATES VENTILATE THEIR LUNGS
The fish species which ventilate their air bladder with surface air are air gulpers. They take a volume of air into the mouth and pharynx and then use the pharyngeal muscles to push the air back into the air bladder or lung.
Some salamanders are lungless and use only their skin and oropharyngeal space space for respiration. These salamanders ventilate their oropharyngeal cavity with rapid movements of the throat muscles.
Lunged amphibians also use the throat musculature to ventilate their lungs. A combination of rapid throat movements and alternating periods of nasal valve closure ensure adequate ventilation of the lungs. With the nasal valve opened the movements of the throat muscles refreshes the air volume of the oropharyngeal cavity. When the valve is closed oropharyngeal air is ventilated into the lungs.
Although some reptiles can employ the throat musculature to aid in lung ventilation, the primary means of ventilation for most species is the use of intercostal muscles for ribcage expansion and contraction.
Turtles are unique because the ribcage cannot be moved. Turtles employ muscle groups of the shoulder region, just anterior to the lungs. Breathing movements of the turtle can be observed lateral to the base of the neck.
Some lizards, crocodilians, and birds have a membranous septum which serves to incompletely separate the lung area from the remainder of the organs of the body cavity. It is called the oblique septum because it runs obliquely from the posterior lateral walls of the coelomic cavity to the medial side of the lungs in the anterior coelomic cavity. In crocodilians a diaphragmaticus muscle of the body wall can pull on on the liver and oblique septum to work somewhat like the diaphragm of mammals. The pubic bones of the pelvic girdle are also moved ventrally to allow the abdomen to expand when the crocodilian inhales.
Birds use muscles of the chest wall to expand and contract the ribe cage and produce movements of the sternum. It has also been discovered that in some species of birds the pelvic bones can be moved downward to facilitate the movement of the abdominal organs posteriorly in the coelom during inhalation. These movements are adequate for ventilation, but when in flight the movements coinciding with the wingbeats may be the primary ventilation means.
Mammals employ the diaphragm and rib cage expansion for inhalation, and rib cage contraction and abdominal wall muscle contaction for expiration.
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