Nonrespiratory functions of the respiratory system
1. What function is served by panting?
2. Is alveolar ventilation increased by panting?
3. Visualize the three patterns of panting in the dog.
4. How do cats purr?
5.
What possible function might be served by purring?6. Does purring signify wellness?
The respiratory system has functions other than providing for alveolar ventilation, and this was noted for respiratory clearance, in which the function was to remove inhaled particles. Also of particular interest among animals are panting and purring. Panting provides for body cooling, but the reason for purring is not known.
Panting
Panting is prevalent among many animal species and has been best described in the dog. It is probably similar for the other animals in which it is observed. The respiratory center of the dog responds not only to the usual stimuli but also to body core temperature. When these inputs are integrated, the dog’s body responds to metabolic needs by regulating alveolar ventilation and to dissipation of heat by regulating dead space ventilation. Dead space ventilation is increased by panting, which provides for body cooling by evaporation of water from the mucous membranes of the tissues involved.
Studies have shown that the three patterns of panting are: (1) inhalation and exhalation through the nose, (2) inhalation through the nose and exhalation through the.nose and mouth, and (3) inhalation through the nose and mouth and exhalation through the nose and mouth. The least amount of cooling is accomplished by inhaling and exhaling through the nose (pattern 1) because the heat and water added to the air during inhalation are partially regained during exhalation. Pattern 2 is more effective because air entering the nose is exposed to a large surface area (nasal conchae) as compared with the mouth, and water is added by the nasal mucosa and nasal glands.
This combination picks up a considerable amount of heat, which is then dissipated mainly by exhalation through the mouth. Pattern 3 is somewhat similar to pattern 2, except that inhalation through the mouth and the nose permits a greater tidal volume, which might be required during times of exertion. The advantage of changing the relative amount of air exhaled through either the nose or the mouth is that the dog can modulate the amount of heat dissipated without changing the frequency or tidal volume associated with panting. Energy is conserved by not changing the frequency (300 pants/min) and hyperventilation (and thus alkalemia) is prevented by keeping the tidal volume constant.Purring
Purring is noted in some members of the feline family and is both audible and palpable in most domestic cats. Studies in the domestic cat have shown that the purr results from a highly regular, alternating activation of the diaphragm and of the intrinsic laryngeal muscles (those within the larynx) at a frequency of 25 times/second during both inspiration and expiration. Contraction of the laryngeal muscles closes the vocal cords. The laryngeal muscles then relax while the diaphragm contracts. Contraction of the diaphragm accomplishes air inflow, which vibrates the vocal cords and results in the purring sound while they are opening (no longer closed by laryngeal contraction), and also contributes to a fraction of the inspiratory phase of the respiratory cycle. The diaphragm then relaxes and the laryngeal muscles contract; this is again followed by laryngeal relaxation and diaphragm contraction. The entire process is repeated 25 times/second until inspiration is completed. The accumulation of small sounds produced with each opening of the vocal cords makes the purring sound. The same sequence occurs during expiration, except that the diaphragm does not contract and air outflow and hence vibration of the vocal cords is accomplished by recoil of the lungs.
The reason for purring in cats is not known. Cats purr when they are contented, sick, and asleep. Purring might provide for more effective ventilation during periods of shallow breathing because of the cyclic intermittent inspiration and expiration, thereby preventing atelectasis.
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