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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. Some glands are long tubular types(i.e. the sudoriferous or sweat glands). 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. Excretion involves diffusion, osmosis, and active secretion and absorption(correctly, reabsorption here) across tubule cells as urine forms. Depending on the transport process occuring at points along the tubule, different cell types are found. In cross-section most tubule cells appear predominantly as cuboidal epithelia but as you can see here, the tubules are indeed constructed of different epithelia.
As you encounter various tubular structures cut in cross-section, consider them either ducts or tubules as they can look identical. An accurate identification depends on knowing the site.
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