Food molecules ingested by the dog are generally too complex to allow them to be absorbed in the intestine or used by the cells. Digestion provides the dog with nutrients, simpler molecules that can be used by the cells.
To this end, the canine digestive tract is devoted entirely to breaking down food molecules (glucides, lipids and proteins) and absorbing nutrients. Anatomically speaking, it can be divided into three sections. The first section, where ingestion takes place, includes the tongue, teeth, salivary glands, pharynx and oesophagus. The second, responsible for digestion, comprises the stomach, small and large intestines and their associated glands (the liver and pancreas). The third, dedicated to elimination, is made up of the lower end of the large intestine and the anal canal.
Ingestion of food.
§ The mouth. A dog ingests its food through its mouth. As in all carnivores, the teeth of canines are specialized for their different roles in chewing. These days, however, whether the dogs eat household food or commercial food , they usually just gobble it down with hardly any chewing, which means there is little or no mechanical pre-digestion.
The paired salivary glands secrete saliva into the mouth cavity. The liquid and mucosal components of saliva moisten the food and help it to pass through the oesophagus. When the food is swallowed, the tongue pushes the food into the oropharynx, the epiglottis closes (preventing the food from entering the trachea) and the food is directed into the oesophagus.
§ The oesophagus. The arrival of the rest of the meal, along with muscular contractions of the oesophagus, pushes the food through the thoracic cavity and the diagram to the entrance of the stomach, known as the cardia.
Digestion of Nutrients
Food is made up of three molecules: glucides, proteins, and lipids. Each is digested in a different part of the digestive tract, through different processes involving different enzymes. It is interesting to note the differences between dogs of different sizes: While a small dog’s digestive tract represents 7% of its body weight, a large dog’s digestive tract represents only 3% of it’s body weight, so the large dog, such as a German shepherd Dog, s more susceptible overall to digestive problems.
§ The stomach is on the dog’s left side behind the chest wall, extending slightly beyond the sternum. It has a large volume compared to the instestines, due to the dog’s meat- based diet. When a dog eats the volume increases even more.- The totally distended stomach can occupy half of the abdominal cavity. In the stomach, food is subjected to both mechanical and chemical digestion. Contraction of the tunica muscularis (muscular tissues surrounding the stomach) mix the foods that it combines with gastric juices. There is a significant amount chemical digestion
§ The small intestine. The chime (partially digested food) then passes through the pyrolus into the duodenum, the first portion of the small intestine. Because the intestine is delicate, the stomach empties into it slowly, a process controlled by both the pyrolus and the first part of the duodenum.
§ The digestive glands. The digested food then passes through the small intestine, where chemical digestion continues by means of secretions from the pancreas and the liver, which pass into the duodenum through ducts.
§ The pancreas is V-shaped in carnivores. This elongated gland is made up of groups of cells called acini that produce digestive enzymes and secrete them into the pancreatic duct as pancreatic juice. Secretion occurs only after the dog has eaten.
The enzymes are secreted in an active form (so that they destroy the organs through which they must pass) and are activated by chemical processes in the intestine. They are thus precursors of proteases, lipases and amylases. Pancreatic juice also contains bicarbonates that neutralize the intestine’s contents, which were acidified in their passage through the stomach.
The German Shepherd is particularly vulnerable at the level of the pancreas. Consequently, to prevent the occurrence of the chalky, putty-coloured diarrhea from which it frequently suffers, it should be fed on products whose protein, lip and carbohydrate components are particularly easy to digest.
§ The liver has many functions, including digestion. It is found behind the diaphragm, on the dog’s right side. Liver cells are organized into hepatic lobules that continuously secret bile into the bile ducts. The ducts carry the bile to the gall bladder, where it is stored between meals. When the chime arrives in the duodenum, the gall bladder contracts and releases the bile. The bile then comes in contact with the partially-digested food in the duodenum
Bile contains water, mineral salts, bile pigments and bile salts. Bile pigments have no digestive function (they are products of the breakdown of haemoglobin) and are in fact eliminated by the digestive tract. Bile salts, on the other hand, play a fundamental role in the digestive of lipids.
§ The digestive micro-flora. A dog’s intestine, like those of other animal species, contain a large population of microorganisms (mainly bacteria) that actively participate in digestion. The intestinal flora is highly sensitive to changes in the type of food. This means that unlike humans (omnivores), dogs (carnivores) cannot eat different foods at every meal. This would destroy the intestinal flora and cause diarrhea.
This phenomenon explains why:
-it is imperative to have an eight-day transition period when changing a dog’s food;
-certain lactic acid bacteria found have highly beneficial (“probiotic”) effects on canine digestion when mixed with the food.
The German Shepherd is particularly sensitive to the intestinal spread of harmful bacteria, which are a major cause of chronic diarrhea, which should be prevented by means of specific diet.
§ Digestion of glucides. Glucides are present in foods in many forms of varying complexity; which consist of chains of simpler base molecules named with a from ending in “-ose” e.g., glucose and fructose. Starch for example, is a huge molecule made up of many glucose molecules.
Digestion breaks glucides down into smaller molecules so they can be absorbed. A number of enzymes are involved in this chemical reaction. These enzymes, known as amylases, are produced by the salivary glands (in small quantities) and the pancreas. Most breakdown off glucides occurs in the small intestine.
§ Digestion of lipids. Lipids (fats) are broken on into triglycerides through the action of pancreatic lipase (an enzyme specific to lipids) and biliary salts from the liver. The biliary salts form an emulsion with the triglycerides, thus increasing contact with the lipases. The lipases partially hydrolyse the lipids, resulting in tiny droplets known as micelles.
§ Digestion of proteins. Proteins are made up of large or small chains of amino acids. Only amino acids can be absorbed, so proteins are broken down by enzymes under specific conditions.
Digestion is begun in the stomach by the acids and proteases (enzymes specific to proteins) of the gastric juice. It is continued in the small intestine by pancreatic proteases.
Absorption of Nutrients
§ The intestine is responsible for most digestion and absorption of nutrients. Six times longer than the dog’s body, it forms loops that are folded up in he abdomen. All of the abdominal viscera are enclosed by the greater omentum, or epiploon, which holds the organs in position.
The inside service of the small intestine is also folded, which increases the surface are available for absorption. The cells that make up the villi (the smallest folds of the intestinal wall, at the cellular level) have different functions. The lower cells primarily secrete mucus, while the upper cels absorb digested nutrients. Dead cells also release other types of enzymes as they break down. The absorption process differs depending on the type of digested matter present.
§ Absorption of glucides. It is mainly the basic forms of glucides, the “-oses”, that are present in the small intestine and are absorbed by the intestinal cells, whereupon they enter the small intestine’s numerous blood vessels.
§ Absorption of lipids. The micelles are absorbed by the intestinal cells, which alter their various components to reconstitute triglycerides. The triglycerides are attached to proteins and other molecules and are taken up by the small intestine’s lymphatic vessels.
§ Absorption of proteins. Amino acids are absorbed by the intestinal cell in a complex process. Other peptides consisting of amino-acid chains of various lengths are also present in the intestinal lumen. The shortest, chains of two or three amino acids, can be absorbed by an active transport system. They are then hydrolyzed by enzymes within the cells to form the amino acids to pass into the bloodstream.
Absorption of Other Nutrients
Water an mineral salts are also absorbed in the intestine. Water is only partially absorbed in the small intestine by means of a mechanism involving sodium ions an glucose molecules or amino acids.. Mineral salts are absorbed in various parts of the intestine, through differing mechanisms. For example, calcium is absorbed in the duodenum by means of a transport protein.
The intestine’s blood vessels join to form the portal vein, which leads to the liver, where nutrients are stored.
Elimination of Faeces.
Digested matter then passes through the various portions of the large intestine; the caecum, the colon, the rectum and the anal canal. The total length of the large intestine is about 70cm in German shepherds.
§ The caecum and colon. The caecum which is very short, has the same function as the colon, which is dorsally located in the loins. The caecum and the colon absorb any nutrients that were not absorbed by the small intestine (particularly water). The remaining material is partially digested by the intestinal microbial flora, but this is a secondary function in dogs. The resulting nutrients are absorbed as in the small intestine.
The large intestine also forms, stores and evacuates faecal material: the stools.
§ The rectum and the anal canal are located in the pelvic cavity. As in all canivores, they store and evacuate faecal matter.
§ Defaecation. Faecal matter is eliminated in three steps. First, in an essentially behavioural step, the dog “looks for a place”. Dogs tend to leave their living area to defaecate. Next there is a stag of mechanical preparation: the dog assumes a particular attitude through contraction of various muscles. Finally, evacuation occurs when the large intestine contracts forcefully.
( with permission copied from the Royal Canin Encyclopaedia/Aniwa publishing)