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Qouted from Junqueira's Basic Histology textbook :

"Ion and water transport across epithelia can occur in either direc- tion, depending on the organ involved.

(a) Absorption is the pro- cess of transport from an organ or duct’s lumen to capillaries near the epithelial basement membrane and involves movement from the apical to the basolateral cell membrane domains. Absorption occurs for example in the epithelium of the gallbladder and intes- tine where it serves to concentrate bile or obtain water and ions from digested material.

(b) Secretion involves transport in the other direction from the capillaries into a lumen, as in many glands and the choroid plexus. Secretion by epithelial cells removes water from the neighboring interstitial fluid or plasma and releases it as part of the specialized aqueous fluids in such organs. No matter whether an epithelium is involved in absorption or secretion, apical occluding junctions are necessary to maintain tight separation of the apical and basolateral compartments of either side of the epithelium."

I don't understand how these aren't the other way around. If an organ is secreting something shouldn't it be secreting from the lumen, via the epithelial tissue, to the blood vessels ?

I tried researching the topic but didn't really find an explanation, I probably have a misunderstanding in the basic concept.

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  • $\begingroup$ you may be having problems with the word Lumen, Lumen is a term with many uses in biology (basically anything inside a tube is a lumen) in this case the lumen is the fluid and food int the intestines, it can't secrete things, however the body can secrete digestive enzymes into it. A secretion is something a gland or organ produces and releases, oils on your skin, digestive enzymes released into the intestines and urine from the kidneys are also secretions. $\endgroup$ – John Apr 14 at 4:58

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