Where glandular or secretory structures are found, ducts and tubules are also. Consider these the miniature pipes or plumbing that carry fluids from a site of production to a surface.
Ducts are typically smaller tubes associated with exocrine structures.
They can be constructed of a variety of epithelia depending on location. In cross section, they all appear similar with a lumen or space enclosed by a circle of cells. Tubules are typically larger or longer structures but this distinction is not all that clear. As such, their ducts are often described as tubules. In the kidneys, a complex filtering apparatus is constructed in part by a highly coiled tubule that varies in structure along its length.