Olfaction and the fluid mechanics of the nose

Smell is a weird sense. In everyday life, we tend to underestimate its importance, because the post-industrial-revolution world is focused around sight. Smell is the only sense not connected to the rest of the brain through the thalamus; instead it is connected to the limbic system, where most of our emotions rest. Smell is critical for taste, and also for finding a partner. However, it is not very well understood.

The olfactory tract in humans has a shape that privileges retro-nasal smell (smelling what’s inside your mouth) in contrast to ortho-nasal smell (smelling things outside). This is especially noticeable if you compare the shape of a dog’s head, which looks like a long rectangle and privileges ortho-nasal smell, to the shape of a human head, which looks more like a tall rectangle. Even accounting for the unnatural size of our brain does not explain these differences. The nose has other functions, like conditioning the air before it enters the body as well as cleaning it.

In this project we investigated what fluid dynamics could tell us about the shape of noses in different animals, in particular of carnivorous mammals. This led to an article where we showed that fluid mechanics imposes two opposing geometrical requirements: narrow nasal geometries humidify and heat air best, but have high resistance and require more powerful lungs. These two constraints are best satisfied in carnivores by having labyrinth-like cavities, and humans fall slightly off the curves, but are still easily rationalized. My old colleague David Zwicker is still working on understanding smell.