Veterinary Internal Medicine Nursing

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48 | How to understand the gastrointestinal system in 4 easy steps

Laura Jones

We know that gastrointestinal diseases are amongst the most common diseases we see in practice.

Giving great care to these patients starts with understanding how the GI tract works, which is exactly what we’re exploring in the first episode of our brand new series on GI disease.

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So, let’s talk about the GI tract

Together, the gastrointestinal tract and accessory organs are responsible for:

  • Prehending (consuming) food and water

  • Mastication, salivation and swallowing

  • Digesting and absorbing nutrients

  • Maintaining fluid, acid-base and electrolyte balance

  • Evacuating waste products

The gastrointestinal system includes:

  • The oral cavity (and lips/teeth/tongue/salivary glands)

  • The oesophagus

  • The stomach

  • The small intestine (duodenum; jejunum; ileum)

  • The large intestine (descending, transverse and ascending colon)

  • The cecum

  • The rectum and anus

  • The GALT (gut-associated lymphoid tissue)


The exocrine pancreas, liver, and gallbladder are known as accessory gastrointestinal organs since they all play important roles in digestion but are not part of the GI tract itself.

The liver, as we chatted about all the way back in episode 4, is responsible for making bile from bile salts and pigments, and channeling it down to the gallbladder for storage.

After eating, the gallbladder contracts and this stored bile enters the duodenum at the level of the common bile duct, where it helps to break down dietary fats.

The exocrine pancreas is the cells within the pancreas responsible for creating digestive enzymes - amylase, lipase and trypsinogen, which is converted to trypsin in the small intestine. These pancreatic enzymes leave the pancreas and enter the duodenum via the pancreatic ducts, breaking down carbohydrates, fats and proteins respectively.

In this episode, we’ll examine the four main areas of the GI tract and their role in digestion so that we can understand what happens when these areas are impacted by disease.

Starting with the oesophagus

The oesophagus is a muscular tube that transports food from the oropharynx, where it is swallowed, to the stomach, where it is prepared for digestion.

So, your patient eats and forms a bolus of food in their oropharynx, assisted by the muscles in that area. The food then enters the oesophagus and is passed down to the stomach via smooth muscle contractions called peristalsis.

The oesophagus is made up of an upper sphincter, a main body, and a lower sphincter (also called the cardiac sphincter, the joint between the oesophagus and the stomach).

The oesophagus, like all other areas of the GI tract, is made up of several layers, including:

  • Mucosa: The innermost layer which lines the oesophageal lumen

  • The submucosa: A layer containing connective tissue, blood vessels, nerve fibres and small mucous glands

  • The muscularis: The muscular layer containing smooth muscle (dogs) or a mixture of smooth and skeletal muscle (cats)

  • The adventitia: The outermost layer, formed of connective tissue, lymphatic channels, nerve fibres and small vessels.

Common disorders affecting the oesophagus include motility disorders such as megaoesophagus, foreign bodies, and oesophageal strictures. These disorders impact the ability for ingesta to pass through the oesophagus normally, causing regurgitation and inappropriate nutrition, alongside complications such as aspiration.

Then we’ve got the stomach

The stomach is a muscular reservoir where digestion begins. It consists of three major sections: the fundus, or cranial third; the body (aka the middle third); and the pylorus, the distal third.

After being transported to the stomach, ingesta is broken down via a mixture of mechanical and chemical methods.

Ingesta is mixed and broken down through churning movements in the stomach, and gastric acid, released from parietal cells in the gastric wall. Gastric acid is mostly composed of hydrochloric acid, and has a pH of 1-2.5 when a patient eats - so it is very acidic. As it is composed of hydrogen and chloride, excessive losses through vomiting can significantly impact acid-base balance.

After initial digestion in the stomach, the ingesta is turned into chyme - a mixture of liquified food and gastric juices, which can then pass through the pylorus into the small intestine for further digestion and absorption of nutrients.

Like the oesophagus, the stomach and small intestine are divided into four layers: the mucosa, submucosa, muscular, and serosa. Diseases affecting the stomach include acute or chronic inflammation, ulceration, neoplasia, infectious disease, and foreign bodies, which we’ll discuss more in future episodes of this series.

What about the small intestine?

The small intestine runs from the pylorus to the ileocaecal colic junction (ICCJ). It’s divided into three sections: the duodenum (or proximal third), jejunum (middle third), and ileum (distal third).

The small intestine is the major site of chemical degradation or absorption of nutrients from chyme. Bile and pancreatic enzymes from the accessory GI organs enter the GI tract and help break down fats, carbohydrates, and proteins into fatty acids, sugars, and amino acids for absorption.

The small intestine is lined with finger-like projections or villi, which increase the surface area for absorbing nutrients. Each villus is lined with a thin layer of epithelial cells called enterocytes. Beneath this is a lymphatic vessel called a lacteal, which absorbs fatty acids, and a capillary, which absorbs glucose and amino acids.

Many diseases affect the small intestine, from simple and self-limiting enteritis to chronic inflammation, infectious disease, and neoplasia. We’ll discuss all of these later in the series.

And then there’s the large intestine

The large intestine extends from the end of the ileum to the anus. It contains the colon - divided into the descending, transverse and ascending portions - and the caecum.

The caecum is of varied importance and function depending on the species—in cats and dogs, for example, it plays a much less significant role than in rabbits.

The colon primarily absorbs water, along with some nutrients and electrolytes, concentrating ingesta into solid waste for excretion. It does this by the high sodium content of its cells, which then pull water into them via osmosis, concentrating the faeces.

At the end of this process, the patient is left with solid faecal waste (in normal cases) to be excreted, and digestion is complete.

What about the microbiota?

As we know, the GI tract is not a sterile cavity. It’s home to a complex population of microorganisms called the microbiota. We’re only just beginning to understand the GI microbiota and its role in health and disease, and over the last few years, procedures like faecal microbiota transplantation (FMT) have been performed increasingly commonly.

So, what IS the microbiota?

When a mammal is born, its GI tract is sterile, but within 24 hours, microorganisms begin to populate it. 

The microbiota is formed of hundreds of different species of bacteria, fungi, protozoa, parasites, and viruses, and it varies depending on the individual host, species, environment, diet, and health.

These microorganisms have - in normal circumstances - a mutually beneficial relationship with us as hosts; we supply them with nutrients from the food they help us break down, and in turn, they do a whole host of things to help us, including:

  • Acting as a defensive barrier against pathogens

  • Facilitating nutrient breakdown and energy release

  • Providing metabolites of nutrients for our enterocytes

  • Regulating the immune system

  • Metabolising other substances that the host cannot, such as certain drugs

Why do we need to know about the microbiota?

We manipulate the microbiota all the time - any time a patient is prescribed a prebiotic or given a diet containing prebiotics, we’re positively impacting the microbiota by feeding those organisms fermentable fibre.

We also regularly see problems with the microbiota—these problems are collectively referred to as dysbiosis. They are usually seen when an opportunistic bacteria disrupts the balance of microbes within the GI tract, impacting intestinal homeostasis and all of those helpful functions we’ve just mentioned.

Over the next few years, we’ll see more evidence about the microbiota emerge, and I’m sure we’ll be performing more faecal transplants and learning more about the impact of diet selection on gastrointestinal health.

So that’s an overview of the four main organs impacted by GI disease—the oesophagus, stomach, small and large intestines! Now that you’ve refreshed yourself on how these organs work, we’ll spend the next few episodes looking at what happens when they don’t—the common diseases we see and how we can best care for these patients.

Did you enjoy this episode? If so, I’d love to hear what you think. Take a screenshot and tag me on Instagram (@vetinternalmedicinenursing) so I can give you a shout-out and share it with a colleague who’d find it helpful!

Thanks for learning with me this week, and I’ll see you next time!

References and Further Reading