Anatomy & Function of the Nose/Sinus

Nasal physiology is greatly dependent on the physical structure of the nose. Seemingly individual aspects of the nasal cavity collectively affect nasal function.

Nasal function is primarily 3 fold:

• filtration of particles,
• warming if the air we breathe and
• humidification of the air inhaled.

The nasal physiologic functions, such as warming and humidification, are vital for upper airway function. It has been estimated that an adult inspires up to 10,000 liters of air daily. Filtration of environmental particles occurs first in the nasal cavity. The largest particles are filtered by vibrissae (nose hairs).

Nasal resistance is an important factor in considering airway resistance. In adults, nasal resistance can contribute up to half of total airway resistance. Through heat exchange, the nasal mucosa maintains the nasal cavity at a range of 31–37° Celsius. The nasal cavity maintains the inhaled air moisture and warmth by the 3 turbinates (upper, middle and lower turbinate) on either side of the nose. These turbinates may get swollen or engorged due to allergies or infections leading to nasal obstruction and/or runny nose.

Between these 3 turbinates are small ostia (windows) where the nasal cavity drain (open) into the main nasal passage. Below the middle turbinate is where most of the sinuses drain (open) into. The nasal septum divides the inner nose into two cavities. The septum, with the turbinates, regulates the flow of air and creates resistance.

Anatomy of the sinuses (also called paranasal sinuses): The human skull contains four major pairs of hollow air–filled–cavities called sinuses. These are connected to the space between the nostrils and the nasal passage. It helps insulate the skull (cushion/shock absorber during trauma), reduce its weight, and allow the voice to resonate within it (resonance for the voice).

The four major pairs are:

1. Frontal (in the forehead),
2. Maxillary (behind the cheek bones),
3. Ethmoid (between the eyes) and
4. Sphenoid (behind the eyes).

The sinuses contain defenses against foreign bacteria (germs). If a disruption occurs it may affects the normal host defenses, those defenses may allow bacteria, which are normally present in the nasal passages, to enter any of the sinuses. Once there, the bacteria may stick to the lining cells and cause a infection. When the microscopic hairs (cilia) of the mucosa fail to propel the germs or infection out of the sinuses, sinusitis will result.

The nasopharynx is the posterior portion of the nose where the right and left nasal cavities rejoin. The sinuses, eustachian tubes and nasolacrimal (tear) ducts connect with the nasal cavity. The nasopharynx is the posterior most structure of the nasal cavity and has the eustachian tube draining into it. Any blockage of this eustachian tube opening in the nasopharynx (e.g. nasopharyngeal cancer) would lead to fluid accumulation in the middle ear cavity, as the ear cannot drain fluid through the eustachian tube, resulting in middle ear effusion and hearing loss.

The sense of smell is another important role of the nose. The nose aids in the smell of the food intake, flavor perception, and detecting and warning about toxic or irritating substances. Odor particulates contact with the mucous lining of the nose and travel through the mucous lining to nerve receptor cells that communicate the odor to the brain. The brain processes the information as noxious stimuli or pleasant sense based on learnt response.

The blood supply of the nose is like the rest of the face, with an abundant rich multiple supply from the internal and external carotid vessels. The arterial supply to the nose may be principally divided into:

1. Branches from the internal carotid artery, namely the branches of the anterior and posterior ethmoid arteries from the ophthalmic artery, and
2. Branches from the external carotid artery, namely the sphenopalatine, greater palatine, superior labial, and angular arteries.

Internally, the lateral nasal wall is supplied by the sphenopalatine artery postero-inferiorly and by the anterior and posterior ethmoid arteries superiorly. The nasal septum also derives its blood supply from the sphenopalatine and the anterior and posterior ethmoid arteries with the added contribution of the superior labial artery (anteriorly) and the greater palatine artery (posteriorly). The Kiesselbach plexus, or the Little’s area, represents a region in the anteroinferior third of the nasal septum, where all 3 of the chief blood supplies to the internal nose converge. The Little’s area is worth mentioning as this is the area where children frequently have nose bleeds, and treatment is easy with manual nasal compression (press one’s nose and cold compress).