Obligate nasal breathing
Obligate nasal breathing describes a physiological necessity to breathe through the nose (or other forms of external nares, depending on the species) as opposed to the mouth. The term may be misleading, as it implies that the animal has no choice but to breathe through its nose; however, it is commonly used to describe cases where effective breathing through the mouth is possible but not preferred. Alternately, the term has been defined by some as the ability to breathe through the nose while swallowing. While this ability is a common trait of obligate nasal breathers, clearly this definition does not require that nasal breathing in any way be obligatory to the animal. Even in obligate nasal breathers such as horses, rabbits, and rodents, there is a potentially patent path for air to travel from the mouth to the lungs which can be used for endotracheal intubation. It has been suggested that obligate nasal breathing is an adaptation especially useful in prey species, as it allows an animal to feed while preserving their ability to detect predators by scent.
Human infants are commonly described as obligate nasal breathers as they prefer breathing through their nose rather than mouth. Most infants, however, are able to breathe through their mouth if their nose is blocked. There are however certain infants with conditions such as choanal atresia in which deaths have resulted from nasal obstruction. In these cases there are cyclical periods of cyanosis. The infant initially attempts to breathe through the nose, and is unable to; hypercapnia occurs, and many babies instinctively begin to cry. While crying, oral ventilation occurs and cyanosis subsides. There is variation in the length of time until a baby begins oral breathing, and some will never cease attempts at nasal breathing. It has also been suggested that infants may not be able to sustain oral breathing for significant lengths of time, because of the weakness of the muscles required to seal the nasal airway and open the oral airway.
Horses are also considered obligate nasal breathers. The term is more accurate in horses, because their normal anatomy prevents them from breathing orally. The epiglottis rests above the soft palate while the animal is not swallowing, forming an airtight seal. Oral breathing can only occur with significant anatomical abnormalities or pathological conditions. For example, denervation of the pharyngeal branch of the vagus nerve results in dorsal displacement of the soft palate (DDSP), and it has been suggested that this leads to a clinical syndrome which may include oral breathing. However, significant respiratory dysfunction including airway obstruction is observed with DDSP, and the animal cannot function normally in this state.
Rabbits and rodents are also obligate nasal breathers. Like horses, the normal anatomical position of the epiglottis causes it to be engaged over the caudal rim of the soft palate, sealing the oral pharynx from the lower airways. Rabbits with advanced upper airway disease will attempt to breathe through their mouths; this is agonal respiration.
Many other mammals, such as cats, dogs, and adult humans, have the ability to breathe indefinitely through either the oral or nasal cavity.
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