Cilia are cell organelles acquired by eukaryotes more than a billion years ago that induce flow in the surrounding fluid by periodically beating in waves. From a viewpoint of biofluid mechanics, cilia perform four major functions: swimming, feeding, pumping, and sensing. In this paper, we describe the flow produced by cilia and its biological functions. First, the speed and efficiency of ciliary swimming are discussed. It is shown that the friction between cilia and the cell surface is extremely high, with more than 90% of the mechanical power input into the fluid being dissipated by viscous friction within the ciliary layer. Our analysis reveals that swimming efficiency is highest when the number of cilia increases in proportion to the square of the body length, which is consistent with scaling in nature. Next, the swimming and feeding of the colony of choanoflagellates are discussed. The choanoflagellates turn their flagella inwards when exposed to light and flip their flagella outwards when it is dark. Our analysis illustrates that the inward flagellar flow is advantageous for feeding, whereas the outward flagellar flow is advantageous for swimming. Due to their morphological similarities, the pumping function of a choanocyte chamber of a sponge is also addressed. Finally, we discuss nodal flow in mouse embryos, the direction of which determines the left-right axis of the body. The strength of the nodal flow generated by the motile cilia acts equally on the immotile cilia located at the left and right edges of the nodal cavity. Therefore, in order to create a left-right asymmetry, the immotile cilia must sense the direction of the flow, which has been confirmed experimentally. These studies demonstrate that ciliary flow gives rise to a variety of biological functions and that fluid dynamics can provide valuable insights into biology.

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• Biological fluid dynamics
• Emergent biological functions from complex interactions
• Living matter & active matter