Airflow through the nasal passages is normally asymmetrical because of alternating changes in nasal resistance in each nostril. The mechanism involves changes in sympathetic tone to the venous erectile tissue of the nasal mucosa; increased sympathetic vasoconstriction causing resistance to fall.
The total nasal resistance to airflow remains fairly constant as changes between the nasal passages tend to be reciprocal so that the patient is usually unaware of the phenomenon.
This nasal cycle has a periodicity of approximately 2 hours and is found in 80% of the population. The reason for its existence is uncertain. A simple explanation is that it permits one side of the nose to go through a rest period and recover from the minor trauma of conditioning the inspired air.
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