How to Understand the Duct System of the Human Body
Merriam-Webster's Online Dictionary defines a duct as "a pipe, tube, or channel that conveys a substance." Ducts in the human body are usually located inside certain glands, known as exocrine glands. There are several different kinds of exocrine glands throughout the body, but they all share the same main purpose: they contain duct systems to secrete particular substances.
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Instructions
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Get familiar with the different kinds of exocrine glands so you can see what ducts do and why they are important. For example sweat, saliva and mucus are all secreted out of glands through a duct system.
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Know that there are three types of ducts, or "secretory units," and they get their name from their shape. These are acinus cells, which are made up of a ball of cells with a point like a triangle; cords, which are made of sheets of skin cells; and tubules, which are small tubes. Tubules in a gland can come as one straight tube (simple tubule), or they can be branched (compound tubule).
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Learn the two categories of exocrine gland secretions, serous and mucous. Serous secretions are clear and watery, whereas mucous secretions are characteristically thick. Examples of where serous secretions get produced are glands in the pancreas, intestines and the salivary glands; examples of mucous secretions can be found on the tissues of the stomach and the esophagus. Ducts are needed to emit serous and mucous secretions because the glands that don't have ducts, endocrine glands, only secrete substances directly into the blood.
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Imagine a gland is a factory, and ducts are the conveyor belt that moves product through the shop. Most ducts eventually lead out of the gland so the product can be delivered, but there are some ducts that remain tucked inside the gland for the express purpose of moving product around the "factory." Types of ducts inside the gland include the lobar ducts (the main branches), intralobular ducts (found inside the lobules of the gland), and intercalated ducts (which lead to the outside of the gland).
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Understand that blood vessels, lymph vessels and nerves are all present in glands, but these constructs are not ducts. Though blood vessels are a kind of tube, they do not secrete anything, and lymph vessels work in much the same way.
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Tips & Warnings
Viewing pictures of a gland will help to understand a lot. Gray's Anatomy is a world-renowned medical reference, and in most print versions the illustrations are superb.
