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Introduction to The Dog's Digestive System

Viral papilloma in a dog, by Joelmills, wikipedia, CC BY-SA 3.0

Understanding your dog's digestive system is important so you can better understand problems and recognize them at an early stage. When your dog eats, his digestive system works in such a way that food is broken down in easy-to-digest pieces so your dog can absorb life-sustaining nutrients and discard waste.

In this article, we will look at the whole digestive process from the point of entry to the exit route.

The Mouth

In humans, the digestive process starts in the mouth. When we chew food in smaller pieces, saliva flows and, salivary amylase, a special enzyme meant to help us with the digestion of starch (carbohydrates), is produced. Digestion in dogs, on the other hand, doesn't start in the mouth, meaning they don't secrete enzymes like we do when chewing.

Dogs gulp down food in big chucks and often swallow it whole. They also have fewer taste buds than us. Their teeth are designed to grab, hold, tear and crush meat and bones, and their jaws, while muscular, are like a stiff hinge, preventing the side-to-side movement meant to grind food as we do. Their stomach can digest food without the need of salivary enzymes. Their saliva, released at the sight and smell of food (that's why dogs drool and smacks their lips) is used only to lubricate the food for easy swallowing and that's about it.


[adinserter block="4"]Once the dog swallows the food, it enters the esophagus, a tubular passageway that connects the mouth to the dog's stomach. Food is moved down this tube by the esophagus muscle's peristaltic (wave-like) contractions. Sometimes, food from the esophagus is brought back up (regurgitated). When this happens, the material brought up appears as if shaped like a tube. This shape is due to the shape of the esophagus, which is a tube-like structure. What you'll likely see is a slimy mixture of food and saliva.

Because of the lack of bile (that yellow, stomach acid you see when your dog vomits on an empty stomach) the regurgitated food may appear appealing to a dog, so they might eat it back up. I know, yuck!

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Once the food arrives from the esophagus to the stomach through the cardiac sphincter, it's exposed to hydrochloric acid, a seriously corrosive fluid. This is where the food starts breaking down. The stomach grinds the food through folds called gastric folds, while the acid and enzymes secreted by the stomach's lining help break down food into partially digested parts. Protein is digested in the stomach courtesy of protein-digesting enzymes (proteases). Most food will exit the stomach within 12 hours and head towards the small intestine.

Small Intestine

After being exposed to the stomach, the food, now in liquid form, passes through the pyloric sphincter and goes into the small intestine, a tube-like structure connecting the stomach and large intestine. This is the longest part of the digestive tract. The first portion of the small intestine is the duodenum which measures about ten inches long in a 40 pound dog.

The liver and pancreas will release additional digestive enzymes into the duodenum to further aid digestion. This production of enzymes meant to break food down into more easily assimilated particles is known as the process of hydrolysis. The enzymes involved include proteases (for furter protein digestion), amylase (for digestion of carbohydrates), and lipase (for digestion of fat).

Now, after being broken down to its simplest form, the food passes into the middle section of the small intestine, known as the jejunum, where finger-like projections called villi help assimilate nutrients for the body. It's estimated that about 95% of nutrients are absorbed in the small intestine. Lastly, the remaining content is emptied into the ileum, the last and shortest section of the small intestine.

Large Intestine

[adinserter block="5"]After the dog's body has assimilated the nutrients, the remnants enter the large intestine through the last section of the small intestine, the ileum. The large intestine, also known as the colon, measures about sixteen inches in a forty-pound dog and its diameter is larger compared to the small intestine. It basically connects the dog's small intestine to the rear end. Here, any remaining water and nutrients are extracted and absorbed and the waste is formed into stools, ready to be soon passed.

Stools are made of 60 to 70 percent water, the rest consists of undigested food, dead bacteria, and inorganic material. Dogs who can't digest food properly are often (but not always) attracted to eating their own feces (coprophagia) because they contain food, and therefore, they smell and taste like food.

Rear End

At this point, the stools are ready to exit the dog's rear. As the stools exit, the dog's anal glands secrete a special watery/pasty fluid that gives the stool a particular scent that is used to mark territory and provides identifying information to other dogs. When these glands become inflamed, they cause discomfort which often manifests with itchiness, biting and scooting behaviors. Our trip into the dog's digestive system ends here.

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