The nitty gritty details of digestion...

Delilah is not always the most motivating study companion.

      I really have been trying to not get too technical but my brain just doesn’t work that way.  I’ve recently been doing a nice review of the digestive system and thought it might be useful to just write it all down.  So here is the long version of basic canine digestion.  I hope in the future to break this down into pieces and go into more detail about each piece and associated diseases.  It always important to start with how the system does work as opposed to constantly looking at a broken system.  If you are like me, then as you read this light bulbs come on your head and that lead you to further research and wondering about your own personal pet’s health.  You start asking questions like “I wonder if this could be why my dog has this reaction?”.  After a while you just become a paranoid dog mom that spends more time fretting over your dogs (or cats!) than you do any other family member.  Don’t worry, you’re not the only out there.  After all, they do completely rely on us for their health and well-being.  Anyway, here is my take and research on the digestive system of Canis familiaris.  Good luck!

      The mouth is where the whole process of digestion begins and can say much about where an animal should derive its nutrition.  The mouth can tell us quite a bit about the type of food we might expect expect a dog to eat by the overall structure of the mouth, arrangement of teeth and tendons, and type of teeth.  Dogs have a total of 42 teeth in their mouth.  There are six incisors and two canines on both the upper and lower jaws.  In the caudal region (towards the back) of the mouth the dog also has both molars and premolars to increase the area inside the mouth for chewing.  However the upper jaw is slightly longer than the lower jaw to allow a shearing or tearing action when consuming food.  Due to the overlap there is little contact between the upper and lower teeth except towards the rear of the mouth where the molars reside. The lack of contact between the upper and lower teeth means that little to no chewing or grinding or food occurs.  You will often see dogs push larger food items to the back of the mouth and gnaw them with the back teeth to help break down stronger food material.  If you’ve ever fed your dog a marrow bone or bully stick you’ll notice them holding it with their front paws and grinding down with the back jaws.  This is done not only based on skeletal structure but the muscles located within the jaw are much more powerful in the back to aid in crushing harder matter including bone.  ​

---

      Mastication, the process of chewing, is not the only thing that occurs in the mouth to begin the breakdown of food.  Saliva is produced by the parotid and submaxillary glands and released into the mouth to continue the digestive process.  The released saliva is made up of 99% water and 1% mucus, electrolytes and enzymes.   Interestingly, dogs consuming a dry food diet will actually have a more watery saliva while dogs on a meat-based diet have more mucous in the saliva.  This may actually be an indication for the overall effect of a dry food diet on the  mucosa throughout the digestive tract.  The other cool thing about saliva in canines is that they product 10 times the amount of saliva a human does upon parasympathetic (the rest and digest part of the nervous system) stimulation or in other words, when they know dinner is coming.  Thanks to Pavlov we’ve discovered that the thought and anticipation of food is an important first step in healthy digestion.  I think we have all done this experiment unknowingly in our kitchens.  The moment you open the cupboard or the fridge your dog magically appears at your side with that shoestring drool that drops right onto your foot.  I’m not sure Pavlov commented on the perked up ears and big eyes that also accompany the excess saliva but it should definitely be added in my humble opinion.  ​

      Once the food is swallowed it is pushed down the esophagus through a series of muscle contractions passing through the esophagus into the stomach via the esophageal sphincter.  The esophageal sphincter then closes to prevent reflux of stomach acid back into the esophagus.  The stomach is responsible for storing food, mixing or blending the food to create chyme, and begin protein digestion through the release of enzymes from the stomach wall.  Endothelial cells, cells lining the inside of the stomach, are responsible for secretion of mucus, release of enzymes for proteolysis and maintaining electrolyte balance.  One of the most notable secretions from the parietal glands located in the stomach body include hydrochloric acid (HCl) which is stimulated by anticipation and/or presence of food.  The presence of food as well as the the distention of the stomach upon filling with food stimulates the release of gastrin from the stomach wall which then stimulates HCl and mucus secretion as well as peristalsis. ​

The small intestine is really one of the most critical areas where digestion and absorption takes place.  The small intestine is classified into three sections: duodenum, jejunum, and ileum.  The  slurry of partially digested food and enzymes called chyme is released from the stomach into the duodenum through the pyloric sphincter.  The surface of the duodenum consists of circular folds that increase the surface area for better absorption and is lined by small projections called villi.  Mucus is produced by Brunner’s glands located within the walls of the duodenum to protect the lining of the small intestine. It helps propel the food through the intestine and buffer the HCl that enters with the chyme from the stomach.  The chyme is continually mixed throughout its transit by muscular movements that makes sure all food particles are mechanically broken down as much as possible and come into contact with the surface of the intestine to allow for nutrient absorption.  The movement of chyme into the duodenum stimulates the release of pancreatic enzymes such as trypsin, chymotrypsin and other proteolytic enzymes to further break down proteins.  It also releases bicarbonate to help regulate the pH of the mixture by inactivating the HCl.  The pancreas also releases lipase and amylase to break down fats and carbohydrates.  Bile is also released into the duodenum to allow for emulsification of fats so they can be broken down and eventually transported through the intestinal surface.  
​
      The small intestine also releases many enzymes locally from the epithelial cells.  These include disaccharidases for digestion of carbohydrates and aminopeptidases for protein digestion.  The liver also produces bile salts that are stored in the gallbladder for secretion into the duodenum. Presence of chyme in the small intestine leads to release of cholecystokinin (CCK) which causes the gallbladder to release bile which emulsifies fat and activates lipases. This breaks the fat into small water-soluble globules called micelles that can then be absorbed by intestinal cells in the jejunum.  While most of the digestion is completed in the duodenum, absorption mostly occurs in the jejunum where there is a higher concentration of villi.  By the time the the food reaches the ileum most nutrient absorption has taken place.  The ileum is important still since that is generally where Vitamin B12 and remaining biles salts are absorbed.  These two substances require specific transport mechanisms only found in the ileum.    ​

Confused yet? Don't worry. There's more to come...

         The large intestine or colon is aptly named due to its diameter relative to the small intestine.  While most absorption of nutrients occurs in the small intestine, the large intestine is responsible for the absorption of water and electrolytes.  It also houses microbiota that are responsible for the fermentation and further breakdown of fermentable fibers and carbohydrates.  The remaining food materials pass through the ileocecal valve from the small intestine to the large intestine.  Dogs and cats both have a relatively short and simple colon in comparison to both herbivores and omnivores, however the dog cecum is slightly larger than that of its feline companion allowing for a minimal amount of fermentation of fibers.  The bacterial fermentation of fiber in the colon produces short chain fatty acids (SCFAs), gases, and other byproducts based on the materials that pass into the colon.  The SCFAs then serve as an important nutrient source for colonocytes that aid in general intestinal health.  The health of bacteria in the colon is important due to their role in not just maintaining the health of the intestinal wall lining for reabsorption of water and electrolytes but they also play a role in supporting the enteric immune system (now there is a topic for another lifetime).  What remains of indigestible and excess materials is collected in the colon and passed out the anus in defecation.  As a loving dog owner, you always watch the end product which gives you some pretty good insight into how this whole process is functioning in your dog.
​ 
      There are the basics to get us all started.  I will hopefully get back to each system so we can go a little more in detail.  I’m learning so why don’t you too.  I have to finish my dog’s meal for the week so I really must be on way.  Please let me know if you have any questions and I look forward to getting in the habit of sharing more interesting material in the future.

---