Lumen: A luminous term referring to the channel within a tube such as a blood vessel or to the cavity within a hollow organ such as the intestine. Lumen is a luminous term because it is Latin for light, including the light that comes through a window. When a hollow organ is cut across, you can see light through the space that has been opened. So the word "lumen" came to mean this space.
For example, in arteriosclerosis, fat deposits narrow the lumen of the artery. For another example, cells called enetrocytes within the intestinal wall help transport water from the lumen of the intestine into the bloodstream.
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