Your dog's digestive system starts right off at the mouth and ends at his end — the anus. This system of your dog's body is designed to take in nutrients, utilize those nutrients in the best and most efficient way, and then get rid of waste products. Dogs are extremely good at surviving on even poor-quality food (though you will eventually pay the price with health problems such as skin allergies and bone development abnormalities). Dogs are also very clever at locating food sources, so remember to keep enticing treats off the counter. Close your cupboards, and keep garbage out of reach.
The digestive tract goes through the entire body, starting at the mouth and ending at the rear. Dogs tend to bolt their food and have an expanding esophagus to accommodate the sometimes big chunks of food that come down it.
The normal adult dog has forty-two teeth (twenty-eight for puppies). This holds true for all dogs, from the huge Irish wolfhound to the tiniest Chihuahua. Toy breeds may lose teeth easily due to crowding, and some dogs may be born missing teeth or even with extra teeth, but ideally there should be forty-two. This includes twelve incisors (the small teeth in the very front) that are used for delicately grasping things. The large canine teeth come next — two on the bottom and two on top. The canines are used for biting (both for protection and to catch food), to help keep the tongue in the mouth so it doesn't dry out, and to carry things. Following the canines come the premolars. These teeth get progressively bigger as you go back in your dog's mouth and are used for shearing and chewing. There should be sixteen premolars total, but these are the teeth that seem to be missing most often. The last teeth are the molars — four on the top and six on the bottom.
A dog's teeth are designed to bite, tear, cut, and grind. The incisors (the front teeth) are the biting teeth, while the canines (the fangs) are used to grasp and hold food and tear off muscles and skin. The side and back teeth (premolars and molars) cut and crush food.
Tongue and Salivary Glands
The tongue is one of the strongest muscles in the body. While dogs don't use their tongues to make the wide variety of sounds that people make, their tongues help them to lap up liquids, aid in temperature control when they pant on hot days, and help them express how they feel, such as when they lick you with delight when you walk in the front door.
The salivary glands produce the drool that your dog leaves when trying to convince you to share your hamburger. Saliva also starts the process of digestion and helps food move easily down through the esophagus. It may help to heal wounds in the mouth, and a dog with sores or injuries to the mouth will drool more than usual.
Esophagus and Pharynx
The pharynx is part of the pathway from the mouth to the esophagus. The esophagus is a tube of muscle running from the mouth down to the stomach. It runs through the chest but does not help in breathing at all; it is solely a food tube. Sometimes the muscles or nerves in the esophagus can have problems, and food does not move steadily along but gets trapped. In these situations, the esophagus may stretch into a megaesophagus (think of baggy pantyhose). With a megaesophagus, your dog can't get nutrients into the stomach efficiently and may vomit and aspirate some of the food, leading to pneumonia. This problem can develop as the result of disease later in life or show up as a congenital defect in puppies.
The stomach is a massive muscular food grinder that contains digestive enzymes to help break food down into molecules that your dog's body can use. Food enters the stomach via the esophagus and leaves through the pylorus (a muscular ring) to enter the first part of the small intestine, an area called the duodenum. Just like people, dogs can develop ulcers or get gastritis. Also, due to the way the dog's stomach is attached to the body wall, the stomach may twist. This shuts off the blood flow to the area, causing tissue death and possibly even the death of your dog! This condition, known as bloat or gastrointestinal volvulus, is an emergency situation and usually requires surgery.
If your dog appears bloated, tries unsuccessfully to vomit, and paces uncomfortably, assume she may be bloating. Call your veterinarian immediately! This problem is more common in deep-chested breeds such as Great Danes, Irish setters, and dachshunds.
The small intestines have three parts — the duodenum, the jejunum, and the ileum. These long tubes are the workhorses of digestion. Using their own enzymes, plus enzymes from the pancreas, the small intestines digest food and absorb the nutrients. There is a population of “good bacteria” that work and help in digestion as well. If things are not right, poor digestion or poor absorption (malabsorption) can lead to diarrhea, vomiting, and weight loss.
The small intestines have many projections, or villi, into the open tube that increase the surface area of working cells. The villi secrete enzymes and other substances, absorb nutrients, and help guard against harmful substances and infectious bacteria and viruses.
The pancreas is a small organ that lies next to the stomach and small intestines. It produces the serious enzymes for digesting food, plus some important hormones such as insulin that help the body make use of nutrients. The pancreas is sensitive to stress, such as that caused by feeding a lot of fatty foods, and the resulting inflammation of the pancreas, known as pancreatitis, can be fatal. If the pancreas does not make its usual digestive enzymes, your dog will have diarrhea and have trouble maintaining weight, as he can't utilize the food he is eating very well. Dogs with subnormal amounts of insulin are diabetic and will typically need insulin injections and special feeding regimens.
Liver and Gallbladder
Your dog's liver works to process nutrients and filter out toxins. Together with the gallbladder, which produces bile to help digest fats, the liver is an essential part of your dog's energy production system. Liver problems can show up as malnutrition, life-threatening infections, and even neurologic signs if toxins build up in the blood. Dogs with liver problems may appear jaundiced, with yellow coloring in their skin, eyes, and gums, or they may vomit.
The liver is so efficient that your dog must lose about 75 percent of its working tissues before signs of illness appear. Liver tissue can also heal itself after some damage. Certain drugs can cause liver damage, so if your dog is taking them, your veterinarian will recommend periodic blood tests to make sure the liver is still functioning well.
The large intestines are divided into parts known as the ascending colon, transverse colon, descending colon, and the cecum. The large intestines help to retain water and any nutrients missed by the small intestines. They concentrate waste and produce stool. Problems with the large intestine primarily show up as diarrhea or constipation.