There are 3 different regulatory phases of the GIT
Cephalic stage is the stage that is activated when you look at, smell, taste, think about food it causes you to salivate and prepares your body for food incoming, eg HCL, pepsin, mucous blood flow all increase to your stomach in preparation.
Gastric stage is the stage that is activated as food enters the stomach it is greatly regulated massively by a protein known as Gastrin. gastrin stimulates cells to turn over at a faster rate in the stomach which helps combat low pHs caused by HCL in the stomach all of the following causes Gastrin upregulation
Intestinal stage is the stage that is activated as food entering the duodenum
it is good to note that these phases can overlap 3 ways, you can be thinking about food, digesting it in your stomach and having it in our intestines at the SAME TIME although one is nearly always dominant eg only 1/4 food in intestines but 3/4ths of food in stomach = Gastric phase dominant. all timings are noted below in the picture
