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Digestion is the process by which dogs break down the food they ingest into nutrients which can be used by the cells. This is necessary because whatever type the food is it is made up of substances too complex to be immediately absorbed in the intestine.
The dog’s digestive tract is completely devoted to breaking down the substances in the food (carbohydrates, fats and proteins) into simple molecules which can be absorbed. The digestive process is divided into three sections. The first, concerned with ingestion, comprises tongue, teeth, salivary glands, pharynx and oesophagus. The second, concerned with digestion itself, involves the stomach, small intestine, large intestine and associated glands (liver and pancreas). The third, concerned with evacuation or excretion, is made up of the last part of the large intestine and the anal canal.

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• The dog takes in food through its mouth. Like all carnivores, the teeth of canids have specialised roles in mastication, although today’s dogs scarcely predigest their food before swallowing it. The paired salivary glands secrete saliva into the oral cavity.

The liquid and mucus in the saliva moisten the food and facilitate its movement into the oesophagus. During swallowing, the tongue pushes the food into the oropharynx and then the oesophagus (while the epiglottis closes to prevent the food from entering the trachea).

• Muscle contractions in the oesophagus push the ingested food through the thorax and the diaphragm to the cardia, the upper opening of the stomach.


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Food consists of three categories of nutritional substances: carbohydrates, proteins and lipids. Each one is digested in a different part of the digestive tract by different processes involving different enzymes. There are also differences in digestion depending on the size of the dog. A small dog’s digestive tract accounts for 7% of body weight, whereas a large dog’s accounts for just 3% of its body weight. This means that large dogs are more susceptible to digestive problems.

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• The stomach is located to the left of the abdominal cavity, extending just past the sternum. Because of the dog’s carnivorous diet, the stomach is large in proportion to the gut. The stomach dilates even more just after the dog eats. A completely distended stomach can occupy half of the abdominal cavity. Food undergoes both mechanical and chemical digestion in the stomach. Contractions of the muscles lining the stomach mix the food up with the gastric juices, which work on breaking down the food at the chemical level.


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• The pulpy acidic fluid consisting of gastric juices and partly digested food, known as chyme, is then pushed through the pylorus into the duodenum, the first part of the small intestine. The intestine is delicate, so the pylorus and the first part of the duodenum control the process to ensure the stomach is emptied slowly.

• The chyme then undergoes further chemical digestion in the small intestine by secretions released through ducts by two digestive glands, the pancreas and liver.

• In carnivores, the pancreas is a very long V-shaped organ. It is made up of a cluster of cells known as acini, which produce and secrete digestive enzymes into the pancreatic duct via pancreatic juice after the dog has eaten. The enzymes are inactive when they are secreted (otherwise they would destroy the organs they pass through) and are activated by chemical processes in the intestine. They are therefore precursors of proteases, lipases and amylases. The pancreatic juice also contains bicarbonates to neutralise the chyme, which is acidified in the stomach.

• The liver has various functions, including a digestive one. It is located beneath the diaphragm, on the right of the body. The cells of the liver are arranged in hepatic lobules. They secrete bile continuously, which is conducted through the bile ducts into the gall bladder where it is stored until needed. When the chyme reaches the duodenum, the gall bladder contracts, releasing bile which contains water, mineral salts, bile pigments and bile salts. Bile pigments have no function in digestion (they are by products of the breakdown of haemoglobin) and are actually excreted by the digestive tract. Bile salts, on the other hand, have a fundamental role in lipid digestion.

© Diffomédia/Royal Canin
© Diffomédia/Royal Canin

• As in all animals, the dog’s gut accommodates a large population of essentially bacterial microorganisms which play an active role in digestion. This gut microflora is highly sensitive to variations in food quality (dogs are not strict carnivores), which means that dogs should not have such a varied diet as humans, because this would destroy the flora and could lead to diarrhoea.

This explains why:

- any change in diet must be spread over a period of seven days

- some lactic bacteria (probiotics) mixed in with the chyme have very positive digestive benefits for dogs.

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• Carbohydrate digestion. Carbohydrates are found in food in various forms, some more complex than others. The base substance is designated with the ending –ose, as in glucose or fructose. These form chains to produce much more complex substances. Starch, for instance, is made up of a large number of glucose molecules.

In carbohydrate digestion the larger substances are broken down to facilitate absorption. This is a chemical process involving amylase, an enzyme produced by the salivary glands (in a small quantity) and the pancreas. Most carbohydrates are therefore broken down in the small intestine.

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• Lipid digestion. Lipids (fat) are broken down into triglycerides by lipase (a pancreatic enzyme which only acts on fat) and bile salts from the liver. Bile salts forms an emulsion with the triglycerides, increasing contact with the lipase, which partially hydrolyses the lipids to produce microscopic lipid droplets called micelles.

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• Protein digestion. A protein is a chain of amino acids of varying length. Protein is broken down by enzymes in specific conditions to release the amino acids, which are then absorbed.

Protein digestion begins in the stomach, through the action of acids and protease (an enzyme that only acts on protein) in the gastric juice. Protease secreted by the pancreas continues the process in the small intestine.

Nutrient absorption

• The intestine is where most nutrient digestion and absorption takes place. By necessity it is folded up in loops in the abdominal cavity. If it were to be unfurled it would be six times longer than the dog’s body. All the abdominal viscera (organs) are enveloped by the greater omentum, which helps hold them in position.

The inside wall of the small intestine is folded to increase the absorption surface. The cells that make up the microvilli (microscopic protrusions in the cell membrane) do not all have the same function. The lower cells primarily secrete mucus, whereas the ones higher up towards the tip absorb digested nutrients. Dead cells release other types of enzymes when they break down. The absorption method differs depending on the type of chyme.

• Carbohydrate absorption. The -ose form of carbohydrates are found in the small intestine. They are absorbed by the intestinal cells and enter the blood vessels, which are numerous in the small intestine.

• Lipid absorption. The micelles are absorbed by the intestinal cells, which reconstitute them into triglycerides. These are then bound to proteins and other molecules, entering the lymphatic vessels of the small intestine.

• Protein absorption. Peptides present in the intestinal lumen are composed of amino acid chains of varying lengths. These are hydrolysed by protease enzymes to break them down to just two or three amino acids which can then be absorbed by the intestinal microvilli and enter the bloodstream.

Absorption of other nutrients

Water and mineral salts are also absorbed in the gut. Water is only partially absorbed in the small intestine in a process involving sodium ions and glucose molecules or amino acids. Different parts of the gut absorb mineral salts by different processes. Calcium, for instance, is absorbed in the duodenum through the action of a protein responsible for transporting nutrients.

The blood vessels in the small intestine come together to form the portal vein, which goes to the storage depot, the liver.

Evacuation of faeces

The following sections of the large intestine are responsible for evacuation: the caecum, colon, rectum and anal canal. The total length of a dog’s large intestine is about 70 cm, although it varies depending on the breed.

• The caecum is a very short section of the large intestine that has the same function as the colon, which is located at the back, under the loins. These two sections absorb any nutrients that have not already been absorbed in the small intestine, especially water. Any remaining digesta is partially digested by the gut microflora, but this is of secondary importance in dogs. The resulting nutrients are absorbed as in the small intestine. The caecum and colon also play a role in the formation, storage and evacuation of faecal matter.

• The rectum and anal canal are located in the pelvic cavity. As in all carnivores, their function is to store and evacuate faecal matter.

• The evacuation of faecal matter occurs in three phases. The first is of an essentially behavioural nature: the dog looks for a place to defecate. It will tend to want to do so well away from its living space. The second phase is mechanical preparation characterised by muscle contraction, which causes the animal to adopt a typical position. The third phase, actual evacuation, occurs with the strong contraction of the large intestine.

Learn more

Assessment of the quality of faeces

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