Dog Anatomy - Level 2
The Canine Digestive System
Learn the remarkable aspects of how dogs eat and process food, which is vastly different from other mammals, including humans.
By M. Christine Zink, D.V.M., Ph.D.
Dogs are famous for being big eaters. After all they are descended from wolves and everyone knows the expression about “wolfing down your food.’’ They have several unique traits that make them able to handle their love of chow. In fact they have the shortest and fastest-acting digestive tract of all mammals.
Dogs have specialized teeth, like huge canines and the large molars on the sides of the mouth called the carnassial teeth, that are designed to grab and tear food. In contrast, humans use flat-surfaced molars to grind food. We chew our food before we swallow it, which not only prevents us from choking, but also increases the surface area of the food to aid digestion.
Humans secrete an enzyme called amylase into the saliva, which begins to digest food as soon as it’s popped into the mouth. While we “preprocess’’ our food, there would be no point in having that enzyme in dogs, which are known for their ability to gulp down food in an instant. When a dog eats, the food is swallowed in large pieces that go straight to the esophagus, where the dog has a unique protection mechanism.
For at least 15,000 years dogs have evolved as scavengers around human settlements. Early dogs likely scrounged scraps from human trash piles, and until the mid-1800s, when the first dog food was invented, pet dogs often were fed exclusively table scraps. Scavenging dogs needed protection from funky food — human leftovers weren’t always fresh — so the dog’s esophagus is made of muscle that is under voluntary control. If a dog eats something that makes him feel sick, he can decide to regurgitate and rid himself of the offending meal. Very handy!
Most food travels straight to the stomach, where digestion begins. The stomach has a number of special adaptations to help digest those big chunks of food that descend the esophagus. Dogs, like wolves, are facultative carnivores, meaning they are designed to eat primarily animal products like muscle meat, organs, and fat.
Their stomach wall contains special cells that secrete hydrochloric acid, which breaks down all kinds of food, including meat, vegetables and even bones. The cells also secrete pepsinogen, which is converted into an enzyme that specifically breaks down protein. Bicarbonate and mucus protect the stomach from the effects of the acidic gastric contents.
After the food leaves the stomach, it enters the duodenum, the first part of the small intestine. At that point, the gallbladder, a small pouch attached to the intestine, secretes bile acids into the duodenum to break fat down into smaller, more absorbable particles. The pancreas also secretes enzymes such as amylase and lipase, which break down carbohydrates and fats for ready absorption.
Almost all absorption of nutrients and water occurs in the jejunum, which is the second part of the small intestine. In very young puppies, the intestine has a specialized lining that allows the puppy to absorb larger proteins without breaking them down. This is how puppies absorb antibodies from their mother’s milk that can protect them from disease before they are vaccinated. As the gastrointestinal contents move into the third and last part of the small intestine, the ileum, more water and essential vitamins such as B12 are absorbed.
The digestive process then shifts to the large intestine, where bacteria break down anything that has not been fully absorbed.
Finally the intestinal contents enter the colon, where they are formed into feces. The colon secretes water and mucus to make sure the feces are the right consistency so they can be passed easily. The colon can also absorb some water if the feces that are forming are too soft.
The food must move through the gastrointestinal system at the right speed to ensure a balance between nutrient absorption and water secretion and absorption. This is coordinated by the autonomic nervous system, which monitors and optimizes the activities of all canine organs. For example, when your dog is having an all-out game of retrieve with you when you come from work, the autonomic nervous system shuts down digestion to ensure that available energy is being used for contraction of the muscles of the dog’s legs, not of the digestive tract.
The entire digestive process takes about 10 hours — maybe this is why it seems that dogs are always hungry!
So next time you clean up after your canine companion while on a walk, it might make you feel a bit better about the job if you remember how much work went into creating that pile!
M. Christine Zink, D.V.M., Ph.D., presents Coaching the Canine Athlete seminars worldwide, and is a consultant on canine sports medicine, evaluating canine structure and locomotion.
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