Author: Dr Peter de Souza
Last modified: 14 December 2021


This is a tutorial to introduce you to the digestive system.   The digestive system is essentially a long tube, which is about six meters long and it runs all the way from the mouth right down to the anus.   You can see how this tube is folded in this abdominal area. This increases the surface area for absorption of nutrients.

As well as this long tube, which is referred to as the gastrointestinal tract or the alimentary tract, you've also got accessory organs of digestion. You can see these other organs coming off the side of the tract. This large one here is the liver. You've got the pancreas sitting behind the stomach. And you've also got the gallbladder sitting behind the liver.


The digestive system consists of this long tube running from the mouth to the anus and that includes the stomach. And then you've got the three accessory organs of digestion – the liver, the gallbladder and the pancreas (which sits behind the stomach).


The purpose of digestion is to absorb nutrients from food into the body. Digestion involves breaking down the food so that it can be easily absorbed.


You've got two types of digestion. You've got mechanical and chemical digestion.   Examples of mechanical digestion are chewing. And then you've got the stomach which can contract and churn. It can churn up the nutrients or the contents of the stomach. And then you've got chemical digestion which involves enzymes produces by various organs and glands. You've also got acid in the stomach and bile in the gallbladder, which help to chemically digest the nutrients from the food.


We’ll start proximally. We'll start at the mouth and work our way down and we'll take a look at some parts of the digestive system.


Digestion begins at the mouth. The mouth is formed by the hard palate and soft palate at the top. You've got the lips at the front, which you can see here. The side walls of the mouth are made up by a muscle called the buccinator muscle. If I just isolate that, you can see this muscle called the buccinator making up the side walls of the mouth. And then you've got the teeth and tongue inside the mouth.


You can see on this model, the upper teeth sit in the maxilla.   This is this bone of the skull. And the lower teeth sit in the mandible.


The function of the mouth is to provide an opening for the food to enter the body. It's used for mechanically digesting the food by chewing.   If you take a look at my tutorial, Muscles of Mastication, you can see how the muscles can bring about chewing movements.


And it also begins chemical digestion.   You’ve got salivary glands which produce saliva obviously. The saliva consists mainly of water, but there's also some enzymes for digestion.   You’ve got things like salivary amylase, which breaks down starches into sugar. You've also got lingual lipase and other antimicrobial enzymes.


You’ve got three major glands, salivary glands. You've got the parotid, submandibular and sublingual glands.   If I just rotate this skull model around, I'll show you the location of these glands.


The parotid gland sits kind of like this over here. And then you've got the submandibular gland, so it lies underneath the mandible in this location. And you've got this sublingual gland, which lies under the tongue in approximately this location.   You’ve got three main pairs of salivary glands to remember.


After food has entered the mouth, it passes into the pharynx. The pharynx is this muscular tube which joins onto the nasal cavity and the mouth. It connects them to the esophagus and the larynx.


The larynx is this structures anteriorly which are commonly referred to as the voice box. Behind, you've got this tube, which runs down at the stomach and this is the esophagus.


I just got rid of the larynx and you can see the trachea anteriorly (which goes into the lungs) and the esophagus, which runs behind it posteriorly.   The pharynx is this bit, which I'm outlining here between the nasal cavities, the back of the oral cavity and just sitting above the trachea and the esophagus.

There are three parts to the pharynx. You've got the nasopharynx, the oropharynx and the laryngopharynx. Food passes out of the oral cavity into the oropharynx and then you've got various muscles, constrictor muscles, which contract and propel the bolus of food from the oral cavity through the oropharynx and into the laryngopharynx. And then it passes down into the esophagus below. During swallowing, the soft palate actually swings up and closes off the nasopharynx.