Flow Physics Explains Morphological Diversity of Ciliated Organs

Ciliated organs that pump luminal fluids are integral to animal physiology. For example, millions of short epithelial cilia direct mucus flow to continuously clear pathogens out of the human airways. However, many other ciliated organs in the animal kingdom admit drastically different morphology and cilia organization to this familiar ciliary carpet archetype, and it is unclear how this structural diversity relates to the fluid pumping abilities of ciliated organs in general. Here, we apply morphometric and mechanistic analyses to ciliated ducts across all animal phyla. We find that two structural parameters, lumen diameter and cilia-to-lumen ratio, organize the observed duct diversity into a continuous spectrum that connects ciliary carpet designs to ciliary flame designs where long beating cilia can fill most of the luminal space. Using our unified fluid model, we further establish that carpets and flames, respectively, maximize flow rate and pressure generation, which is consistent with physiological requirements for bulk transport and filtration, whereas intermediate designs along the morphological spectrum constitute optimally efficient hybrids. We propose that convergence of ciliated organ designs follows functional constraints rather than phylogenetic distance, and we present universal design rules that can also guide the design of synthetic ciliary pumps.