What is it all about?

The gastrointestinal (GI) system encompasses anatomy, embryology, physiology, pharmacology, biochemistry, immunology and pathology. We learn about what goes into the gut, how it is moved along the gut, how it is broken down, what it gets turned into, where the products go and how we get rid of the unwanted stuff. Understanding the basic science of normal gut behavior makes learning about the diseases a lot easier.

Where does it happen?

We all know that the gut begins at the mouth and ends at the anus. But the GI system isn't just the esophagus, stomach and intestines; it also includes the liver, biliary tract, pancreas and gallbladder.

At the cellular level, a vast array of different cell types and functions interrelate to produce the puzzle that is the working GI system. From hydrochloric acid (HCl) secretion by the oxyntic cells in the stomach to phagocytic Kupffer cells in the liver, the GI system does a lot more than just break down food for us.

It is important to learn some specifics about motility, and also about the mucosal surfaces, if we are going to understand disorders such as achalasia or ulcerative colitis. Things going wrong in the gut can have a knock-on effect throughout the body. For example, liver damage leading to reduced drug metabolism can result in toxic metabolites entering the systemic circulation.

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General principles



* The gut is composed of a basic structure throughout its length: mucosa, submucosa, muscular layer and adventitia (or serosa).


* It is innervated through intrinsic submucosal and mucosal nerve plexuses (Meissner's and Auerbach's plexuses). This activity is moderated by extrinsic innervation.


* Food is moved through the tract by gravity and peristalsis, which is a wavelike movement involving coordinated contraction of muscle in one area and relaxation of muscle in the next.


* The major functions of the gut are: ingestion, breakdown and digestion of food; storage of waste; secretion of exocrine, endocrine and paracrine substances; and excretion of waste products.


* The gut defense mechanisms include: sight, smell and taste; the vomit reflex; stomach acid; a natural flora; IgA secretions and Peyer's patches in the gut wall.


* Dietary fat is mostly composed of triglycerides (esters of free fatty acids and glycerols). The essential fatty acids are linoleic acid and α-linoleic acid. Dietary fat provides 37 kJ (9 kcal) of energy per gram.


* Vitamins are either fat soluble or water soluble. The fat-soluble vitamins are A, D, E and K. These are stored in fatty tissue and are more abundant than stores of water-soluble vitamins.


* The GI tract is derived from the endodermal germ layer. It forms passively and depends on the cephalocaudal and lateral folding of the embryo.


* Between weeks 7 and 11 of gestation, the gut herniates and continues developing outside the abdominal cavity.


* The gut is divided into foregut, midgut and hindgut, which all have separate blood supplies: the celiac artery, superior mesenteric artery and inferior mesenteric artery, respectively.



What are the hard bits?

The GI system is fairly straightforward, although learning about the biochemistry related to liver metabolism can be tricky. Also, remembering principles about the breakdown of bile or the innervation behind peristalsis can be confusing.

The actions of the drugs can be difficult to recall because many drugs give the same desired effect but act on different receptors! Don't panic. Just draw out simple diagrams to show you the different sites of actions (e.g. of H2-antagonists and proton pump inhibitors). Flow diagrams and spider diagrams can also make learning easier.

What are the good bits?

Everyone has a gut, so it is easy to get a mental picture of what is going on. The pathology is pretty logical and diseases of this system tend to be more straightforward than diseases of other systems (e.g. the kidney or endocrine organs).

What relevance does this have when I'm on the wards seeing patients?

GI stuff is general medicine and is quite common, so it is very useful to know. Many common presentations in the emergency room will be gut symptoms. For example, vomiting, diarrhea, abdominal pain or jaundice. In fact, you might well be personally familiar with many of the GI signs and symptoms! A sound knowledge of the basic science behind the GI system will stand you in good stead, even if only to allow you to put together a differential diagnosis. Learning the drugs used to treat the common GI diseases is also very valuable.