The organ systems in your dog's body collaborate to keep the body in a state of homeostasis or balance. Each organ system is made up of a group of organs that work together to carry out specific functions. Organs can be divided into their component tissues, and tissues are made up of cells.

Your dog’s digestive system includes its mouth, pharynx, salivary glands, esophagus, stomach, pancreas, liver, gallbladder, small intestine, and large intestine. 

Once your dog takes a bite of food, it takes 6–8 hours for it to move from its mouth to its rectum. The amount of time it takes to move through its digestive system is influenced by several factors, including its age, size, breed, activity level, type of food, and overall health.

Since the jaw is hinged, it can open widely to capture, hold, or smash its prey. Dogs also have long, pointy canines to grip the prey in their mouth, preventing its escape.

Unlike herbivores and omnivores, dogs aren't able to move their jaws from side to side and they don't have flat molars to grind plant material. Their molars are designed for shearing flesh and tearing it into small, bite-sized pieces. Their incisors are designed to bite chunks of meat off of a carcass and their strong muscles in the esophagus help push their food into their stomach.

The Salivary Glands for Help Swallowing		 	 	 		

Your dog's oral cavity contains four pairs of salivary glands: 

Zygomatic glands: Located in the orbit near the eyeball

Sublingual glands: Located beneath the tongue

Mandibular glands: Located close to the angle of the jaw

Parotid glands: Located between the base of the ear and the mandibular glands

When your dog grips its prey, its salivary glands begin producing the saliva necessary for swallowing. The sight and smell of food also increase saliva production. 

Your dog’s saliva is approximately 99% water and 1% mucus. Its saliva contains bicarbonate and proteins to help it swallow its food and doesn’t contain digestive enzymes. Most mammals have the enzyme amylase in their saliva to begin the digestion of carbohydrates. However, in the dog, amylase is produced in the pancreas.

Saliva also aids in thermoregulation. When a dog is panting, its saliva evaporates on the tongue. This evaporation results in the blood cooling in underlying blood vessels called capillaries. This reduces the body’s core temperature. 

A bolus of food that has been chewed and mixed with saliva, travels from the mouth into the throat. The muscular tube known as the pharynx joins the esophagus, larynx, and trachea to the nasal and oral cavities. The pharynx contracts during swallowing, pushing the food bolus into the esophagus. To stop food from entering the larynx, the epiglottis, a little structure that opens and shuts over the larynx in the pharynx, closes.

When the food reaches the esophagus, a process known as peristalsis causes muscle contractions that force the food toward the stomach's entrance. The epiglottis opens to allow air to enter the lungs through the larynx and trachea once all food has been eliminated from the pharynx.

Although the esophagus plays a minor role in the digestive tract, it is still instrumental in moving the food into the stomach. The muscular lining in the esophagus allows it to expand as necessary to move food through. The structure of the muscles allows food to move in either direction, whether they're eating food or their body is rejecting it through vomiting.

Regurgitation is another process, aside from eating or vomiting, that causes food to move through the esophagus and into the mouth. It's important to note that vomiting and regurgitation are two completely separate ailments. 

Megaoesophagus may lead to regurgitation as can a mass within the esophagus, but that doesn't necessarily mean if your dog is regurgitating it automatically has either of these conditions. However, if your dog is experiencing regurgitation, it should be investigated further.	

The three primary functions of the dog’s stomach are storing food, breaking it up and mixing it with gastric juices, and beginning the process of protein digestion. The stomach has two openings: the cardiac sphincter attached to the esophagus allows food to enter the stomach. The pyloric sphincter attached to the small intestine allows food to exit the stomach.

Like its wild counterpart, the wolf, the domestic dog's thick stomach walls can expand when large meals are consumed. The pH level of the stomach acid ranges from 1 to 8, differing at times of fasting and upon consuming a meal. 

Studies have shown that the pH of a dog’s stomach during fasting can range from 2.7 to 8.3, but within 30 minutes of being fed, the pH changes. Kibble-fed dogs measured The pH levels at 3.9 +/- 0.4. However, raw-fed dogs had stomach acid with a pH of 1 or less, which is equivalent to battery acid. These levels remain until the stomach empties its contents into the small intestines, up to five hours after eating.

Food entering the stomach causes it to distend, activating the secretion of the hormone gastrin.  Gastrin is responsible for the production of gastric juices. The stomach walls are lined with a mucosal membrane, the gastric mucosa, protecting the stomach wall from being damaged by acidic digestive enzymes. Three types of gastric cells secrete gastric juices within the mucosal layer. 

The goblet cells secrete mucus that lubricates the food and aids in the protection of the stomach wall. Chief cells secrete pepsinogen, which converts into pepsin, the primary enzyme to digest protein. The parietal cell secretes the hydrochloric acid that creates the acidic environment within the stomach required for the pepsin to break down protein efficiently.

Once all materials have been broken down, the liquified food bolus created by the gastric juices, called chyme, passes through the pyloric valve into the small intestine. When the chyme enters the small intestine, bicarbonate is released to raise the pH levels closer to neutral (7.0) to protect the small intestine from the corrosive properties of the acid.

The Small Intestine for Digestion and Absorption

The longest part of the digestive tract is the small intestine, where most digesting and absorption takes place. Like other carnivores, your dog's small intestine is relatively short at about three times the length of its body. 

The intestinal tract of a cow is almost twenty times longer than its body, while that of a human is roughly four times longer.  Your dog's body can swiftly absorb food due to its short digestive system, which prevents plant material from fermenting.

The three portions of the small intestine are duodenum, jejunum, and ileum. The pancreatic and gallbladder digestive fluids reach the small intestine through openings in the duodenum. 

Trypsin breaks proteins down into individual amino acids

Bile salts activate lipases from the gallbladder to convert fats to fatty acids and glycerol.

It can be hard to tell the difference between the end of the jejunum and the start of the ileum. The duodenum is in a fixed position, but the jejunum and ileum are free-floating within the abdominal cavity. The ileum ends at the cecum, a comma-shaped pouch connecting the ileum and colon. 

The small intestine's lining is covered with microscopic, hair-like projections called villi, where nutrients are absorbed. Villi increase the surface area of the small intestine, further increasing nutrient absorption. 

The building blocks of proteins, amino acids, and simple sugars, pass through the villi and enter the bloodstream from small blood vessels, the capillaries. From this point, they enter the hepatic portal vein and are carried to the liver.

Fatty acids and glycerol are transported to the bloodstream through the thoracic duct's lymphatic system, which then carries them to the heart.  

What is left then moves to the large intestine.

The Large Intestine for SCFA and Absorption

The two main functions of the large intestine include the production of short-chain fatty acids and vitamins as well as the absorption of water, electrolytes, and vitamins.

The large intestine is divided into the cecum, colon, rectum, and anal sphincter. The colon has three sections known as the ascending, transverse, and descending colon. 

The walls of the large intestine are lined with goblet cells which are responsible for secreting mucous to lubricate feces as it moves through the colon and anal sphincter. The gut microbiome can also be found within the large intestine and aids in digestion, absorption, and nutrient development. The microbes found in the gut microbiome can boost the immune system, maintain the balance of good and bad bacteria, and strengthen your dog’s gut.

The cecum is a short pouch that joins the ileum and the ascending colon. It’s a residual organ in carnivores, meaning it doesn’t have a function within their bodies. However, in herbivores, it’s the portion of the intestinal tract where fermentation takes place. 

Vitamins, electrolytes, and water are absorbed within the colon. The primary function of the colon is to prevent water loss. 

The rectum is part of the colon and can be found within the pelvic cavity. The anal sphincter is found at the end of the rectum and is responsible for the passing of feces. 

Waste from the large intestine is stored in the rectum until your dog’s body alerts its need to poop . 

The Pancreas for Hormones and Digestive Enzymes

The pancreas is a small organ with lobes that can be found at the edge of the duodenum. Since it creates hormones and digestive enzymes, it's known as a mixed gland.   

The pancreas secretes several hormones, including insulin, glucagon, and gastrin. Protease, lipase, and amylase are among the digestive enzymes that are secreted into the small intestine through the pancreatic duct.			

The largest gland in your dog's body is the liver. It is located in the abdominal cavity, close to the diaphragm. The liver is deep red because of the amount of blood that it processes and carries out a variety of vital tasks that keep your dog healthy, including:

The formation of proteins necessary for blood clotting, fluid regulation, and immunity

Regulation of amino acids required for the formation of proteins 

Conversion of excess amino acids into urea that will be passed as urine

Conversion of fatty acids and glycerol into phospholipids for the body to use for cell production.

Vitamin storage, especially the fat-soluble vitamins A, D, E, and K

Detoxification of chemicals or medications

Within the lobes of the liver is a little pouch called the gallbladder. The gallbladder's function is to hold the bile produced by the liver. Bile salts included in bile are necessary for the digestion of fat. Chyme, or partially digested food mixed with gastric juices, signals the common bile duct to let bile enter the duodenum as it moves into the stomach.

Your dog’s body is made up of interconnected organ systems that work together to maintain homeostasis, the internal balance necessary for normal health and function. Each system contains organs with specialized roles, and those organs are composed of tissues and cells. Understanding how these systems work, particularly the digestive system, provides an important foundation for understanding what happens to food after it enters your dog's body and how nutrients ultimately support every cell.

Your Dog's Anatomy

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