The canine gastrointestinal tract is composed of organs that are occupied with taking food into the body, digesting food, extracting nutrition, and excreting the non-absorbable parts of the diet from the body. After being swallowed, ingested food (ingesta) reaches the stomach via the esophagus, where it is mixed with gastric acid, and digestion begins. As ingesta passes from the stomach into the intestines, nutrition is absorbed from the food as it moves through intestinal passage through the intestinal wall. The parts of the animal’s diet which cannot be digested are eventually transported into the rectum and expelled through the anus. The liver and pancreas play an important role in this process, by providing enzymes and bile that help to encourage and complete the digestive process. Oral cavity The oral cavity represents the gateway and the beginning of the digestive tract. The meal is separated into portions with the teeth, although dogs tend to swallow their food whole rather than spend time chewing it. After entering the oral cavity every bite is mixed with saliva before being pushed towards the pharynx by the tongue where it can be swallowed. The saliva being mixed with food is excreted by the salivary glands into the mouth, and its production is stimulated by reflex when the dog is anticipates food. As opposed to humans, saliva in dogs merely serves as a lubricant rather than a pre-digestive. Esophagus The esophagus is a muscular tube lined with mucosa that connects the pharynx to the stomach. Linear contractions of the muscle move food towards the stomach. The mucus and saliva reduce friction and help to ease the passage. Stomach The canine stomach is flexible and stretches when food is consumed. Its surface, like all abdominal organs is covered by a serous skin - the peritoneum. The inside of the stomach is lined by gastric mucosa which contains glands that excrete hydrochloric acid and digestive enzymes into the lumen, the space inside the intestines. The stomach serves two primary functions: pre-digestion and killing dangerous microbes. A certain amount of pre-digestion of some food and nutrients takes place here (i.e. proteins), while carbohydrates and fats pass the stomach intact before actually beginning digestion inside the small intestine. But as mentioned before, the stomach also kills dangerous microbes. The acidic environment of the stomach destroys most bacteria, and thereby prevents their spreading into the bowels. The stomach is mostly held in place inside the abdomen by the esophagus and the upper part of the small intestine. There are some further connections, i.e. with spleen and liver through serous skins, which are quite flexible. This flexibility is important as it allows the stomach to change in size depending on how full it is, but it can also lead to dangerous gastric torsion. Small intestine The canine small intestine is composed of the duodenum, the jejunum and the ileum. Ingested food is released in small portions into the small intestine via the pylorus, which is the connection between stomach and small intestine. The major part of digestion - the breakdown of nutrients - takes place inside the duodenum, which contains a basic (by basic we mean the opposite of acidic) environment. Enzymes excreted by the pancreas, and bile excreted by the liver, as well as enzymes excreted by the intestine itself allow for the digestion of proteins, carbohydrates, and fats to take place, thus making them available to the body. The actual absorption of nutrition, which will be transported throughout the body via the blood stream and lymph system respectively, takes place inside the duodenum. The intestinal wall of this section contains a great number of transport mechanisms that allow nutrition to be absorbed into the circulatory system. The internal enfolding of the mucosa and intestinal villi (small finger-like projections) increase the area of contact between intestinal wall and ingesta in order to assure an optimal exploitation. Large intestine The large intestine consists of the cecum, the colon and the rectum. As opposed to the small intestine, the mucosa of this section is not folded. The majority of nutrition is already absorbed before ingesta reaches the large intestine with the actual digestion already complete. The major function of the large intestine is to reabsorb water and electrolytes. The remaining ingesta is further transformed and collected as stool inside the rectum. When reaching a critical amount the stool is expelled through the anus by a passive opening of the inner sphincter muscle and a subsequent active opening of the outer sphincter muscle. This is when the stool passes the anal sacs. The sacs contain glands which excrete a certain substance that helps to lubricate his/her stool and allows the dog to mark his territory. Furthermore, the large intestine is capable of absorbing vitamins (K and E). In dogs, the large intestine is also where flatulence develops, which can result from a low-fiber diet. A dogs diet is a crucial factor for how long ingesta takes to pass the large intestine. If the passage takes a long amount of tim,e the stool becomes hardened as a result of increased water absorption. During short passage insufficient amounts of water are absorbed and the stool becomes loose. Liver The canine liver is separated into four lobes, and is connected to the duodenum through a duct. Bile, which is produced inside the liver and collected in the gallbladder, is excreted to the duodenum where it mixes with the ingesta. Bile is mostly an emulsifier, and helps to dissolve fats contained in the meal. This allows for the formation of many small particles that overall, have a larger surface area, and thus are easier to digest. Bile is also able to neutralize gastric acid, and helps to activate pancreatic enzymes. It is importnat to remember that the liver serves a number of further metabolic processes: Large quantities of body proteins are synthesized here, which are important parts of the immune system or the transport of fat. The liver also stores carbohydrates, and may quickly release them upon demand as a source of energy. It also synthesizes and brakes up fats and fatty acids. Harmful substances such as toxins or metabolites are neutralized here and excreted with the urine or stool. The liver also stores vitamins, iron and other trace elements. Pancreas The pancreas produces digestive enzymes as well as buffer, a substance that neutralizes gastric acid inside the duodenum. Because of the pancreas a basic environment is created which is crucial for the enzymes to become active. The pancreas also produces insulin, a hormone that controls the blood glucose level.
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