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David Muir
Edinburgh, UK
The nasal cycle is the alternating congestion and decongestion between nostrils. It usually happens every couple of hours, but can vary quite a bit. The cycle is controlled by the autonomous nervous system, which oversees our unconscious processes like breathing and heartbeat. When a nostril becomes congested, blood is directed to the turbinates – bony projections covered in erectile tissue – and the air flow in this nostril decreases. Simultaneously, the turbinates in the other nostril decongest as blood is directed elsewhere and the air flow in this nostril increases.
High air flow causes nostrils to dry out, potentially exposing them to pathogens and inflammation. When a nostril becomes congested, there is increased blood flow to it, allowing for moistening, rest and repair. Some smell molecules bind easily with olfactory receptors even under high air flow, whereas other molecules are slower to link with receptors, so are more likely to be detected with low air flow. The asymmetric air flow system of the nasal cycle optimises your sense of smell.
Nearly all mammals have two nostrils, so presumably have a nasal cycle. The exceptions are the odontocetes (toothed whales, porpoises and dolphins), which have a single blowhole. Interestingly, the mysticetes (baleen whales) have two nostrils, manifesting as a paired blowhole on top of their head, so there may be a difference in blowhole function between these two groups.
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