Phases of Digestion – Gastrointestinal System

00:00 Okay.

00:00 Now, that we have the overall functions of the GI system down, let’s talk about the various phases of digestion.

00:09 And the phases are going to be very important for controlling things like secretions.

00:12 So let’s go through these kind of in order.

00:16 The first is the cephalic phase.

00:17 The cephalic phase involves chemoreceptors and mechanoreceptors that are located in both the mouth and the nasal cavity.

00:27 And these are enacted by things like tasting, chewing, swallowing, even smelling of food.

00:32 But you also start to enact the cephalic phase even thinking about food.

00:39 So if you’re hungry and you’re starting to think about wanting to eat, the cephalic phase is already engaged and you’re starting a digestion process.

00:48 The gastric phase is the one that you think about most when you eat because that’s when the food stuff actually enters the stomach.

00:55 And this is linked to how the stomach distends when food is added to it so it gets larger as well as various constituents of food stuff such as peptides and amino acids also stimulate this particular phase.

01:11 Finally, in the intestinal phase, this is when the food stuff has left the stomach and entered into the small intestine.

01:16 And here, it’s normally linked to a bunch of digestion constituents such as large proteins, fats and sometimes even carbohydrates as they’re broken down, it starts to stimulate hormonal responses as well as some neuroresponses involved in digestion.

01:36 So let’s take a visual look at these phases.

01:40 So the very first thing you want to think about is as you look at a sandwich to eat.

01:46 Here, there’s going to a neuroresponse that happens first.

01:51 There are going to be some sensory afferents as you start to eat that particular sandwich.

01:57 This is primarily done by the vagus nerve or cranial nerve number X.

02:01 And this is one of the classic cephalic phase responses.

02:07 So that’s parasympathetic excitation and it not only increases salivary secretions, but also engages the stomach to start secreting both acid and pepsinogen, which will be converted to pepsin.

02:23 The gastric phase involves a lot of reflex responses.

02:28 Both of these are local reflexes in nature.

02:31 And these are called vago-vagal responses.

02:34 And these local reflexes enact or cause a lot of the responses in the stomach whether it be from distension or acid or some other amino acid.

02:46 But in addition to that, there is some gastrin and histamine that is stimulated at this time as well.

02:54 And both gastrin and histamine will increase acid production.

03:00 Finally, in the intestinal phase, this is mostly a nervous system response as well as various hormones that are released that will travel to the pancreas and other places in the small intestine to start this digestion process.

03:13 So again, we have our three different phases, our cephalic phase, our gastric phase, and our intestinal phase, and they all involve various aspects of either the neural system, hormonal system or local reflexes.