In other words, how do the stomach and intestines detect what substances are within themselves?

Article Index. The digestive system is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. Inside this tube is a lining called the mucosa. ... Two solid organs, the liver, and the pancreas produce digestive juices that reach the intestine through small tubes.

That is the beauty of the body. Amazing thing isn't it. Receptors in the stomach and intestines use a variety of receptors to determine what is available and what is needed. Search "Physiology of the body".
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