(Note: Move your mouse over the above image to highlight the various anatomical parts of the human nose.)
The airway (upper respiratory tract – URT) is part of the respiratory system that forms a pathway between the air we breathe in from the atmosphere, and the trachea and lungs (lower respiratory tract). The upper respiratory tract begins at the nostrils and mouth continuing as the throat and voice box (larynx). The nose is a much larger organ than it first appears and is about 4x the depth of the visible part on the middle of the face. It extends back from the nostrils about 10 cm to the upper throat, where the Eustachian tubes connect (Eustachian tube orifices), coming forwards from both middle ears. In addition to breathing and smelling, the nose warms, humidifies and cleans the inspired air. Two valves just behind the nostrils (alar valves) limit the rate of air flow.
The nose is separated into 2 parts (cavities) by a midline partition (septum), which consists of cartilage and thin bone covered by soft tissue (mucosa), which contains many blood vessels. Three scrolls of bone covered with thick soft tissue rich in blood vessels (inferior, middle and superior turbinates) are attached to the outer wall of the nasal cavity. There are four paired bony air sacs (sinuses) which connect directly to the nasal cavity via small openings (ostia). The maxillary sinus is behind the cheek, below the eye socket. The ethmoid sinuses are a group of air cells behind the nasal bridge between the side wall of the nose and the eye. The frontal sinuses are behind the forehead and the sphenoid sinuses at the very back of the nose in the midline. The frontal, ethmoid and maxillary sinuses drain through the ostiomeatal complex in the side wall of the nasal cavity. This is an important area as any soft tissue thickening can block sinus drainage resulting in recurrent infection (sinusitis). It is believed that the main purpose of the sinuses is to make the skull lighter and improve voice resonance.
Cells under the lining mucosa of the nose and sinuses produce watery (serous) and sticky (mucous) secretions which form a surface liquid blanket. Inhaled particles, bacteria and viruses stick to the mucus, whilst other cells (macrophages) engulf them. The lining cells have small hairs (cilia) which beat backwards and forwards about 10 times per second moving the nasal and sinus secretions towards the back of the nose, like a conveyor belt, and into the throat where they are swallowed. On average humans produce and swallow about 1.5 litres of nasal and sinus secretions daily. Ciliary malfunction or an increase in mucus composition which make the secretions thicker and stickier can interfere with fluid transport and increase susceptibility to sinusitis.
The blood vessels of the nose and sinuses are supplied by the automatic (autonomic) nervous system. Stimulation of sympathetic nerves causes constriction of the blood vessels and tissue shrinkage. Parasympathetic stimulation causes dilation of the blood vessels with tissue swelling (congestion) and watery nasal discharge. Blood from the nose and sinuses drains backwards into the main venous sinus at the base of the brain (cavernous sinus). Infections here can therefore be potentially very serious as untreated they can spread directly via the bloodstream to affect the lower brain structures.
The sense of smell (olfaction) is served by olfactory nerve fibres in the top part of the nasal cavity. Tiny nerve filaments pass through minute holes in the bone at the very top of the nose (cribriform plate) into the front brain cavity. Here they combine to form the olfactory nerves which connect back to the smell centre in the brain.