Physical and geometric constraints explain the labyrinth-like shape of the nasal cavity

TL;DR

This paper presents a theoretical model linking nasal cavity shape to its function, explaining the diversity of nasal geometries across animals through airflow efficiency and exchange trade-offs.

Contribution

It introduces a geometric and physical framework that predicts nasal cavity shapes based on airflow resistance and exchange efficiency, accounting for natural variability.

Findings

Optimal nasal geometries have constant gap width.

Shape is restricted by head geometry.

Humans have smaller nasal cavities than expected, leading to oral breathing.

Abstract

The nasal cavity is a vital component of the respiratory system that heats and humidifies inhaled air in all vertebrates. Despite this common function, the shapes of nasal cavities vary widely across animals. To understand this variability, we here connect nasal geometry to its function by theoretically studying the airflow and the associated scalar exchange that describes heating and humidification. We find that optimal geometries, which have minimal resistance for a given exchange efficiency, have a constant gap width between their side walls, but their overall shape is restricted only by the geometry of the head. Our theory explains the geometric variations of natural nasal cavities quantitatively and we hypothesize that the trade-off between high exchange efficiency and low resistance to airflow is the main driving force shaping the nasal cavity. Our model further explains why humans, whose nasal cavities evolved to be smaller than expected for their size, become obligate oral breathers in aerobically challenging situations.