Flow physics guides morphology of ciliated organs

Organs that pump luminal fluids by the coordinated beat of motile cilia are integral to animal physiology. Such organs include the human airways, brain ventricles and reproductive tracts. Although cilia organization and duct morphology vary drastically in the animal kingdom, ducts are typically classified as carpet or flame designs. The reason behind the appearance of these two different designs and how they relate to fluid pumping remain unclear. Here, we demonstrate that two structural parameters—lumen diameter and cilia-to-lumen ratio—organize the observed duct diversity into a continuous spectrum that connects carpets to flames across all animal phyla. Using a unified fluid model, we show that carpets and flames represent trade-offs between flow rate and pressure generation. We propose that the convergence of ciliated organ designs follows functional constraints rather than phylogenetic distance and offer guiding design principles for synthetic ciliary pumps. The ducts of many fluid-pumping organs feature cilia. Two structural parameters organize the different types of ducts into a continuous spectrum between ciliary carpet and flame designs depending on the fluid-pumping requirements.