Food is made up of three types of 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 in this respect 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 its body weight, so the large dog is 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 intestines, 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 food so that it combines with gastric juices. There is a significant amount of chemical digestion.
- The small intestine. The chyme (partially digested food) then passes through the pylorus 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 pylorus and the first part of the duodenum.
- The digestive glands. The digested food than passes through the small intestine, where chemical digestion continues by means of secretions from the pancreas and liver, which pass into the duodenum through ducts.
- The pancreas is V-shaped in carnivores. This very 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 inactive form (so they do not 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 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 secrete bile into the bile ducts. The ducts carry the bile to the gall bladder, where it is stored between meals. When the chyme 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 hemoglobin) and are in fact eliminated by the digestive tract. Bile salts, on the other hand, play a fundamental role in the digestion of lipids.
- Microorganisms. A dog's intestines, 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.
- Digestion of glucides. Glucides (sugars) are present in foods in many forms of varying complexity, which consist of chains of simpler base molecules named with a form 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 of glucides occurs in the small intestine.
- Digestion of lipids. Lipids (fats) are broken down 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 hydrolyze the lipids, resulting in tiny lipid 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.
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