The

“The digestive system is made up of the digestive tract”
This sentence is about a general idea. Why ‘the’ is used twice here?
“Organs that make up the digestive tract are the mouth, esophagus, stomach, small intestine, large intestine—also called the colon—rectum, and anus.”
Why ‘the’ is used here only before mouth?
“In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digestion”
Here again ‘the used only before mouth?
Sometimes I hear a fact that in some sentence you can or cannot use ‘the’. This makes the case more confusing. Please help!