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, while their overall shape can adhere to the geometric constraints imposed by 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.

鼻腔是呼吸系统的重要组成部分，可加热和加湿所有脊椎动物中的吸入空气。尽管具有这种共同的功能，但鼻腔的形状在不同动物之间差异很大。为了理解这种可变性，我们在这里通过理论上研究气流和描述加热和加湿的相关标量交换来将鼻部几何形状与其功能联系起来。我们发现，对于给定的交换效率，具有最小阻力的最佳几何形状在其侧壁之间具有恒定的间隙宽度，而它们的整体形状可以遵守头部施加的几何约束。我们的理论定量地解释了天然鼻腔的几何变化，我们假设高交换效率和低气流阻力之间的权衡是塑造鼻腔的主要驱动力。我们的模型进一步解释了为什么在鼻腔内有氧运动变得比预期的要小的人，在有氧挑战性的情况下会成为强制性的口腔呼吸器。