Describe the physiology of the nose.

The Functions of the Nose

• The nose is far more than an airway.
• Its functions are respiration and air-conditioning, protection and defence, olfaction, vocal resonance, and reflex regulation — and every one of them is lost or impaired by the mouth-breathing that follows nasal obstruction.
• 1. Respiration and air-conditioning
• The nose accounts for about half the total resistance of the entire respiratory tract, most of it at the nasal valve. This resistance is not a fault but a function: it slows and directs the airflow, creating the back-pressure needed for normal alveolar ventilation and for the elastic recoil of the lung

The Nasal Cycle and the Mucosa

• The nasal cycle is a normal physiological phenomenon present in a majority of people: alternating congestion and decongestion of the two sides, with a period of 2 to 7 hours, under hypothalamic autonomic control
• The total airway resistance stays constant.
• Its clinical importance is threefold: a patient with a mildly deviated septum notices obstruction only when the cycle congests the narrow side, giving alternating blockage; it confounds rhinomanometry and acoustic rhinometry unless the nose is decongested first; and it explains why a patient may report the obstruction moving from side to side.
• The turbinates contain venous sinusoids (cavernous tissue) whose filling and emptying, under autonomic control, produce these changes rapidly
• Posture affects them — the dependent side congests on lying on one side, and both sides congest on lying supine, which is why nasal obstruction is characteristically worse at night
• Other influences — exercise decongests (through sympathetic activity); alcohol, hot drinks and emotion congest; and pregnancy and hormonal changes cause congestion — rhinitis of pregnancy
• Mucociliary clearance is considered separately, but the essential physiology is that cilia beat 10 to 20 times per second in a double layer of mucus, moving secretion toward the nasopharynx at 1 cm per minute, clearing the nose in about 20 to 30 minutes
• Ciliary function is impaired by — drying, which is the commonest and most correctable; extremes of temperature and of pH; smoking; infection, particularly viral; topical decongestants and preservatives (benzalkonium); and anaesthetic agents. Optimal function requires a pH near 7 and adequate hydration
• The practical consequence for treatment is that saline irrigation, humidification and stopping smoking support the nose’s own defences.
• Nasal resistance is a function, not a fault, providing the back-pressure needed for normal alveolar ventilation
• Half the total airway resistance is nasal, and most of that is at the nasal valve
• Mouth breathing delivers cold dry unfiltered air to the lower airway.
• The nose uses about a litre of water daily to humidify inspired air.
• Particles above ten micrometres are filtered out almost entirely, while smaller ones reach the lower airway
• Secretory IgA is the principal immunoglobulin of nasal secretion, alongside lysozyme, lactoferrin and interferon
• The sinuses produce nitric oxide, which is bacteriostatic and improves ventilation-perfusion matching when inhaled
• Only a small fraction of air reaches the olfactory cleft in quiet breathing, which is why sniffing is needed to smell
• Rhinolalia clausa is a blocked voice and rhinolalia aperta an excessively nasal one, and the two indicate opposite problems
• The nasal cycle keeps total resistance constant, which is why a healthy person is unaware of it
• Alternating blockage is reassuring, indicating the cycle acting on a mild deviation rather than a progressive lesion
• Decongest before rhinomanometry, or the nasal cycle makes the measurement uninterpretable
• The dependent side congests on lying, which is why obstruction is characteristically worse at night and on one side
• Exercise decongests the nose through sympathetic activity, and alcohol and hot drinks congest it
• Rhinitis of pregnancy is hormonal, resolves after delivery, and should not be treated with prolonged decongestants
• Cilia clear the nose in twenty to thirty minutes, moving mucus at about a centimetre a minute toward the nasopharynx
• Drying is the commonest impairment of cilia and the most easily corrected.