A role for the nasal cycle in respiratory defence.

This review describes the phenomenon of the nasal cycle, which consists of periodic congestion and decongestion of the nasal venous sinusoids. The hypothesis is put forward that the nasal venous sinusoids participate in respiratory defence by generation of plasma exudate. This hypothesis is based on recent studies, which have shown that the nasal venous sinusoids have a fenestrated endothelium and that the nasal cycle is increased during periods of nasal infection; and also on a series of older observations in the literature, which link the generation of nasal fluid to the decongestion of nasal venous sinusoids. It is proposed that the periodic congestion and decongestion of nasal venous sinusoids may provide a pump mechanism for the generation of plasma exudate, and that this mechanism is an important component of respiratory defence.