tubule

This term is strictly technical and belongs almost exclusively to the realms of histology, anatomy, and cellular biology. It describes a structure that is too small to be seen by the naked eye, creating a mental image of a vast, intricate network of microscopic pipes. While a tube is a general object used for transport, a tubule is a biological component designed for filtration, secretion, or absorption. Using this word in a non-scientific context would be considered an extreme over-formalization or a technical error.