Mouth

The process of digestion begins in the mouth. Saliva is secreted by four pair of saliva glands. In dogs, the saliva flow increases with sight and smell of the food. The type of food and its moisture content affect saliva amount and composition. The pH of dog saliva varies between 7.32 and 7.80

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Like many other animals, dogs lack the alpha-amylase enzyme that would initiate the process of starch digestion. One of the reasons your dog is not able to get maximum nutrition from starch rich foods.

Another important function of saliva is evaporative cooling that help dogs to cool down.

Esophagus

It is a short, muscular tube leading from the mouth to the stomach. It contains only strained muscle that lends itself to more rapid passage of peristaltic waves. It transports food from the mouth to the stomach.

Stomach

The stomach of a dog has two major functions:

1. It acts as a temporary storage for the food, allowing consumption of discrete meals rather than many small meals. Also, it controls the rate of entry of ingesta into small intestine.

2. It participates in the initial stages of digestion by secreting hydrochloric acid and pepsinogen.

Small Intestine

Main digestion occurs in the first segment of the small intestine, known as duodenum. Most of the enzymatic digestion occurs in the small intestine. The resulting monomeric units then are absorbed as they are released from the food along with water, vitamins, and minerals. Ingestion of large volumes of water by the dog induce acid secretion in the stomach and lower the pH to values as low as 1.5 in the start of the small intestine.

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Pancreas

It secrete enzymes. The two major functions of pancreas: (1) exocrine— secreting enzymes into the gut in addition to bicarbonate salts that optimize luminal pH for pancreatic and intestinal enzyme function, and (2) endocrine— secreting hormones into the blood. Pancreatic enzymes include inactive proteases, lipases, and amylases.

The pancreatic juice of dogs possesses antibacterial properties.

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Liver

It produces bile which is stored and concentrated in the gallbladder, then released into the intestine in response to the presence of lipids or their digestive end products in the duodenum. Emptying peaks are found 30 minutes after a meal, and the emptying decreases 2 hours after food ingestion.

Large Intestine

The primary role of the large intestine of dogs is to absorb electrolytes and water and to serve as an environment for microbial fermentation of nutrients that escape digestion and absorption by the small intestine.

The large intestine consists of the cecum, colon, and rectum. The colon makes up the majority of the large intestine and consists of three sections— the ascending, transverse, and descending colon.

The large intestine has no villi, and its surface is flat. Straight, tubular glands, contain mucus cells in their deepest portions and both mucus and epithelial cells near the surface.

The mucus is alkaline (from bicarbonate) and its function is to protect the large intestinal mucosa from mechanical and chemical injury. The mucus provides lubrication to facilitate passage of feces, and the bicarbonate ions neutralize acids produced by bacterial fermentation.

• The large intestine is relatively small in dogs - around 0.6 m

• The residence time of undigested food in the large intestine of the dog and cat is approximately 12 hours

• Only about 8 percent of the total digestion of food occurs in the large intestine of the dog

Factors affecting digestibility in dogs

1. Ingredient sources and absolute nutrient concentrations influence digestibility measurements.

2. Ingredient particle size reduction generally improve digestibility, and therefore food utilization, but will results in lower throughput of food in the manufacturing process, higher production costs, and reduced flowability.

3. Processing conditions in the preconditioning chamber, the pellet mill-extruder-retort process, or the drying oven could impact diet nutritive value.

4. Animal factors affect digestibility. These include breed, age, gender, activity level, and physiological state.

5. Housing and environmental factors could impact nutrient digestibility, but research with dogs housed in metabolism cages or covered kennel runs indicated that digestibility values were similar, regardless of housing system used.