Obligate nasal breathing is a term used to describe either a physiological or psychological predisposition or obligation to breathe through the nose 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. In fact, 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.

Humans

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

Choanal atresia

Choanal atresia is a congenital disorder where the back of the nasal passage is blocked, usually by abnormal bony or soft tissue formed during fetal development.-Presentation:It can be unilateral or bilateral....

 in which deaths have resulted from nasal obstruction. In these cases there are cyclical periods of cyanosis

Cyanosis

Cyanosis is the appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface being low on oxygen. The onset of cyanosis is 2.5 g/dL of deoxyhemoglobin. The bluish color is more readily apparent in those with high hemoglobin counts than it is...

. The infant initially attempts to breathe through the nose, and is unable to; hypercapnia

Hypercapnia

Hypercapnia or hypercapnea , also known as hypercarbia, is a condition where there is too much carbon dioxide in the blood...

 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.

Other animals

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

Epiglottis

The epiglottis is a flap that is made of elastic cartilage tissue covered with a mucous membrane, attached to the entrance of the larynx. It projects obliquely upwards behind the tongue and the hyoid bone, pointing dorsally. The term, like tonsils, is often incorrectly used to refer to the uvula...

 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

Vagus nerve

The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...

 results in dorsal displacement of the soft palate

Soft palate

The soft palate is the soft tissue constituting the back of the roof of the mouth. The soft palate is distinguished from the hard palate at the front of the mouth in that it does not contain bone....

 (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

Agonal respiration

Agonal respiration is an abnormal pattern of breathing characterized by gasping, labored breathing, accompanied by strange vocalizations and myoclonus. Possible causes include cerebral ischemia, extreme hypoxia or even anoxia...

.

Many other mammals, such as cats, dogs, and adult humans, have the ability to breathe indefinitely through either the oral or nasal cavity.