Nasal Anatomy and Physiology
Overview of the External Nose
Regardless of the external appearance of the nose, its internal structure is very similar. The upper part is formed by nasal bones (NB), the middle part by the nasal septum and the upper lateral cartilages (ULC), and the lower part by the nasal septum and the lower lateral cartilages (LLC).
The proportion of the external nasal shape that is due to any of these given elements varies according to gender and genetic factors. Longer nasal bones in one individual may mean shorter upper lateral cartilages, for example. The degree to which the nose projects from the face also varies across the world as do the shape and orientation of the nostrils, the width of the lower nose, as well as many other metrics.
The function of the nose can be split into two:
Sense of smell
The first purpose of the nose is to make the air clean, moist and warm so that it can pass into the lung without causing harm. This is its air-conditioning function and it is very important. Its second function is to allow us to smell our environment, food and to sense danger.
The lungs require air that is at body temperature, is moist and clean and it is the job of the nose to provide this. It has a very rich blood supply, which is close to the surface of the nose, and this helps with raising the air temperature. The cool air is warmed by blood passing through these vessels. It is the rich blood supply that makes the nose bleed so easily when it is hit, picked or operated upon. The nose moistens the air in a couple of ways. Firstly the lining of the nose contains mucous glands that secrete slightly sticky mucus onto its surface. Secondly, the blood vessels in the lining of the nose squeeze out some of their own plasma contents onto the lining. This combination makes the lining of the nose wet so that when the air is drawn into the nose it quickly picks up this moisture and becomes saturated itself. You can see the moisture in your breath if you breathe out onto a cold surface because this moisture condenses as a plume of steam.
The cleaning function of the nose is achieved by the mucociliary clearance system. This system is microscopic in size and works at the surface of the cells that line the nose. Each of these cells has a small tuft of cilia projecting from its surface and these move back and forth and sweep mucus and debris stuck in that mucus backwards towards the back of the nose and then down the throat. The diagram below shows this.
The mucociliary system exists throughout the nose, sinuses and lungs and it keeps these clean by removing debris and pushing it down the throat.
When the system breaks down, the nose and sinuses quickly become infected because trapped viruses and bacteria aren’t swept away. An infective rhinosinusitis may develop. Cigarette smoke is known to paralyse the cilia and it is related to an increased risk of sinonasal infection. Cocaine does the same thing. Some patients are born with cilia that do not work and they commonly have sinus infections.
Sense of smell
The second function of the nose is to allow us to smell. This sense is one of the oldest senses and developed at a very early stage of man’s evolution. It alerts us to danger from bad food, fire, smoke and other environmental hazards. It is absolutely essential to the sense of taste and when it is lost food tastes uninteresting. The organ of smell is called the olfactory organ and it sits high up on the roof of the nose. It sends small nerve bundles called fila olfactoria through the cribriform plate into the olfactory bulb and from here the information passes via the olfactory tracts to the brain (Amygdala, hypocampus and thalamus).
The diagram shows that the olfactory organ is high in the nose and that there are nerve bundles that pass through the anterior skull base into the olfactory bulb through the cribriform plate.
The bulb leads to the olfactory tract that passes backwards into the brain. The olfactory nerve is a special visceral afferent nerve.
Loss of smell commonly happens when the nose itself is blocked when the patient has rhinitis. It also happens after a head injury and, if this is severe, it may never recover. Sinus infections and brain tumours can also cause loss of smell. A complete loss of the sense of smell is called anosmia.
Nasal nerve supply
The diagram below summarises the nerve supply to the lateral nasal wall and to the septum.
Nasal blood supply
The diagram below summarises the blood supply to the nasal septum. It is useful to know this as it helps understand the various methods we use to stop epistaxis.
The blood supply is ultimately derived from either the external carotid artery (superior labial, sphenopalatine, greater palatine) or the internal carotid artery (anterior and posterior ethmoids).
Superficial blood vessels in the mucosa of the nose can be cauterised in the clinic especially if they are in Little's area but vessels further posteriorly are difficult to treat this way.
Should cautery fail then packing the nose with haemostats or balloons will usually stop bleeding but sometimes even this fails and ligation of an artery is necessary. See our page on epistaxis for more details on management.