Structure and Function of the Gastrointestinal Tract in Dogs

Below is information about the structure and function of the canine gastrointestinal tract. We will tell you about the general structure of the stomach and bowels, how it works in dogs, common diseases that affect the gastrointestinal tract and common diagnostic tests performed in dogs to evaluate the gastrointestinal tract.

What Is the Gastrointestinal Tract?

The gastrointestinal (GI) tract or system is responsible for processing and extracting nutrients from food and collecting and passing waste material from the dog’s body. It is a very long and winding tube, beginning in the mouth and ending at the anus, through which food is swallowed and collected, then broken apart and digested. It is also where the nutrients from food are absorbed into the body. The GI tract includes the mouth, teeth, tongue, pharynx, esophagus, stomach, small intestine and large intestine.

Where Is the Gastrointestinal Tract Located in Dogs?

The GI tract is a large system that travels the length of the body. It starts at the mouth, extends into the throat, through the chest and abdominal cavities, and ends at the anus.

• Mouth. Food is initially grasped by the teeth and tongue and enters the mouth. As the food is swallowed, it passes into the back of the mouth, which is known as the pharynx. Both food and air pass through the pharynx on their way into the body.
• The esophagus is the connecting tube between the pharynx and the stomach. As food leaves the pharynx it enters the esophagus and travels down the neck and through the chest. The esophagus passes through the diaphragm (the muscle that separates the chest from abdominal cavity) and ends at the stomach.
• The stomach lies in the front of the abdominal cavity, just behind the liver. It is situated between the esophagus and the small intestine, lying predominantly on the left side of the body.
• The small intestine is located within the abdominal cavity and extends from the stomach to the junction of the small and large intestine.
• The cecum is a small dead-end pouch that lies near the junction of the small and large intestines. The colon begins in the lower portion of the right side of the abdomen and travels forward along the right side, then crosses the midline, and proceeds back down the left side. This last portion of the colon (descending colon) leads into the rectum and then empties through the anus. The rectum is the terminal portion of the large intestine that passes through the pelvis and leads to anus.

What Is the General Structure of the Gastrointestinal Tract?

For most of its length, the GI tract is a long hollow tube lined by different types of cells. The walls of the tube are composed of glands, nerves and muscles. Structurally, the cell type, muscle thickness, glandular elements, and nervous supply differ in the various functional regions, as does the diameter and shape of the tube.

• The esophagus is a rather straight tube that is lined with muscles that force food down the neck and through the chest towards the stomach. In a medium-sized dog it is about 15 to 18 inches long and an inch in diameter when collapsed. It is divided into cervical (neck), thoracic (chest), and abdominal portions.
• The stomach is a large sac-like dilatation of the GI track and is made up of several distinct areas. The inlet or opening from the esophagus into the stomach is called the cardia. The exit or outlet of the stomach that leads to the small intestine is the pylorus. The stomach is shaped somewhat like a large lopsided kidney bean that lies across the front of the abdomen. The left side of the stomach, closest to the cardia is larger than the right side and is called the fundus. The smaller right-sided portion of the stomach that ends at the pylorus is called the body of the stomach. The lining of the stomach contains glands that produce acids and enzymes that digest food and the walls of the stomach contain muscles that mix and move the food. The glands of the stomach also produce mucous, which protects the stomach from being digested by its own acid and enzymes.
• The small intestine is the longest portion of the GI tract. It is a circular hollow tube that is approximately three to four times the length of the animal’s body. The interior lining of the small intestines has numerous microscopic, finger-like projections called villi. These villi stick out towards the center of the intestine and greatly increase the surface area available for digestion and absorption.

The small intestine consists of the duodenum, the jejunum and the ileum. The duodenum is the first and most stationary part of the small intestine. Within the duodenum, openings are present that allow digestive juices to enter the intestines from the pancreas and gallbladder. The jejunum is the longest part of the small intestine and is free to move into whatever unoccupied space is available within the abdomen. The ileum is the short, terminal portion of the small intestine.

• The large intestine is wider and shorter than the small intestine. It includes the cecum, colon, rectum, and anal canal. The cecum is a comma-shaped pouch that lies at the junction of the ileum and colon. The colon is shaped like a question mark. It is thin-walled and baggier than the small intestines. The rectum is the last few inches of the colon and leads directly to the anal canal. The anal canal is the short, terminal part of the GI tract that lies just inside the anus. It is only about one-half inch in length. The anus has two muscular sphincters that act as a door, holding the stool (fecal material) inside the body until it is appropriate to defecate.

What Are the Functions of the Canine Gastrointestinal Tract?

• The esophagus acts like a conduit, a tube that moves ingested material from the mouth to the stomach. Wavelike contractions (called peristalsis) move food from the mouth down the neck, through the chest and into the stomach. The esophagus has a tight sphincter muscle where it meets the stomach. This lower esophageal sphincter is a ring of thick muscle that acts as a door, and it prevents acid reflux or movement of acid from the stomach back up into the esophagus.
• The stomach has three basic functions that assist in the early stages of digestion and prepare the food for further processing in the small intestine. First, it serves as a short-term storage area, allowing the animal to consume a large meal quickly and processing it over a longer period of time. Second, substantial chemical and enzymatic digestion begins in the stomach, particularly of proteins. Third, the stomach’s contractions mix and grind food with secretions, liquefying or blending the food, a necessary step before the food is delivered to the small intestine.
• The small intestine is where the absorption of almost all nutrients into the blood occurs. When in the small intestine, food particles are exposed to enzymes and bile, which convert the food to even smaller particles capable of being absorbed into the blood. In addition to absorbing food particles, the small intestines also the absorb other materials such as water, electrolytes and other molecules. The small intestines provide nutrients to the body and play an important role in water and acid-base balance.
• The large intestine participates in the last phase of digestion. It has three very important functions. It recovers the last available water and electrolytes from the food,; forms and stores feces, and works with bacteria to produce enzymes capable of breaking down difficult-to-digest material.
• The rectum and anal canal are basically collecting spaces, where the feces are stored until it is appropriate for them to be defecated.

What Are Common Diseases of the Gastrointestinal Tract in Dogs?

There are many primary disorders that affect the GI tract. Vomiting and/or diarrhea are commonly seen with gastrointestinal disease. Regurgitation (the effortless evacuation of fluid, mucus and undigested food from the esophagus) is commonly seen with esophageal disease. Some common diseases of the GI tract include:

• Congenital defects. They may lead to defects in swallowing or movement of food through the esophagus, inability to properly digest food, or inability to defecate.
• Infectious agents including bacteria, viruses, fungal and protozoal organisms, as well as intestinal parasites are quite common in both cats and dogs. Different infections often involve isolated portions of the GI tract.
• Inflammation. Various inflammations can develop any where along the GI tract. When inflammation arises in the mouth, it is called stomatitis. Inflammation of the esophagus is esophagitis. Gastritis is inflammation of the stomach. Enteritis is inflammation of the intestine. Colitis is inflammation of the colon. Inflammation of the rectum is proctitis. These types of inflammation can be either acute or chronic.
• Inflammatory bowel disease (IBD) is a microscopic infiltration of the small intestinal wall with inflammatory cells. It is believed to be associated with an abnormal immune response to environmental stimuli that, when continued, creates a self-perpetuating inflammation resulting in the disease.
• Intussusceptions (telescoping of part of the bowel into an adjacent segment of bowel) are seen in both cats and dogs. They are often associated with parasites, foreign bodies, tumors or chronic diarrhea, and usually affect the small intestines of young animals.
• Foreign bodies (rocks, bits of clothing) of the GI tract are common in cats and dogs due to their indiscriminate eating habits. They result in local inflammation, obstruction and sometimes perforation of the GI tract.
• Ulcerative gastroenteritis (interruptions in the lining of the gastrointestinal tract) may develop and be secondary to inflammation, drug administration, neoplasia or foreign bodies.
• Hemorrhagic gastroenteritis (HGE) is a dramatic, potentially fatal disorder causing sudden onset bloody diarrhea in adult dogs. Although the exact cause is unknown, it may be due to some sort of bacterial toxin. HGE has a predilection for small breed dogs.
• Acute gastric dilatation or bloat is a condition seen mainly in large, deep-chested breeds of dogs. For a variety of reasons, the stomach suddenly dilates like a balloon and fails to empty. It may twist upon itself (called a volvulus), and entirely obstruct the opening/exit passages of the stomach. Acute gastric dilatation is a life-threatening emergency that may rapidly result in shock and collapse of the dog.
• Paralysis of portions of the GI tract may occur. Paralysis of the esophagus results in a grossly enlarged esophagus, known as megaesophagus, and regurgitation of food and water. Paralysis of the stomach results in delayed gastric emptying. Paralysis of the intestines is known as ileus. Paralysis of the colon results in enlargement of the colon (megacolon) and constipation.
• Certain abnormalities in the digestion and absorption of nutrients can occur, particularly in the small intestines. These are known as maldigestion and malabsorption disorders. Diseases can also arise that result in the excessive loss of nutrients into the intestines, which then pass out of the body through the feces.
• Trauma can occur along different segments of the GI tract. Trauma to the esophagus most often arises with bite wounds to the neck. The intestines may be injured through either blunt abdominal trauma (e.g. automobile accidents, falling from heights, kicking injuries) or penetrating trauma (e.g. bite wounds, bullet and arrow wounds, falling on sharp objects). Trauma to the pelvis and tail may affect the rectum and anal canal.
• Tumors may develop anywhere along the entire length of the intestinal tract. Different tumors arise in different areas because the types of cells present in each area are unique. Tumors of the gastrointestinal tract may be either benign or malignant. They can grow into the cavity of the tract, and can involve the wall of the tract or the surrounding soft tissues.

What Types of Diagnostic Tests Are Used to Evaluate the Gastrointestinal Tract?

Numerous diagnostic tests are helpful in evaluating the GI tract.

• Baseline tests such as a complete blood count (CBC), biochemical profile, and urinalysis are essential, as changes in these tests may suggest infection, inflammation, electrolyte and acid-base imbalances, and/or other organ involvement.
• Serologic tests for viruses, fungi, protozoal and tick borne infections may be indicated. Measurement of certain circulating nutrients in the body (Vitamin B, folate, etc.) may be done to assess the absorption capabilities of the intestines. Specialized tests that look for ingestion and exposure to toxins, such as lead and botulism, may be helpful. Certain immune tests are indicated when immune-mediated diseases are suspected.
• Other laboratory tests are used to rule out diseases of the liver, pancreas and other abdominal organs as the cause of gastrointestinal symptoms.
• A fecal examination is necessary to rule out parasitism, a common cause of diarrhea in cats and dogs. Sophisticated analysis of the feces for its protein and fat content may also be considered. Microscopic examination and culture of feces for certain bacteria may also be done.
• Thoracic (chest) radiographs (x-rays) are needed to evaluate the size/shape of the esophagus, assess for the presence of a foreign object or growth, and assess the lungs for the possibility of secondary pneumonia, which may develop with esophageal disease.
• Abdominal x-rays are very helpful in assessing the GI tract. They help to identify enlargement, dilation, and twisting of the stomach and intestines, the presence of fluid and gas within the GI, displacement of the GI tract, the presence of a foreign body or foreign material, and may provide clues to the presence of a tumor. Abdominal x-rays also help to rule out other abdominal diseases and other causes of the animal’s clinical signs.
• Abdominal ultrasonography is also helpful in evaluating the GI tract. It is a noninvasive procedure that provides information about the GI tract and all the other abdominal organs. Your veterinarian may refer your dog to a veterinary internal medicine specialist for an abdominal ultrasound.
• Certain x-ray procedures can be performed that are designed to assess the interior of the GI tract. These tests involve the swallowing or administration by tube, of a substance such as barium, that shows up white on x-rays. These x-ray tests are called positive contrast procedures. Examples of positive contrast procedures include an esophagram (barium swallow), an upper GI series, and a barium enema. These tests evaluate the lining of the GI tract and may detect the presence of a stricture (narrowing), dilation, obstruction, foreign body, mass or ulcers.
• Positive contrast studies can also be performed under a type of video x-rays, called fluoroscopy. Fluoroscopy provides a way to watch the movement of the barium material as it passes through the pharynx, the esophagus and stomach. It provides information on the muscular coordination of these structures. Fluoroscopy is only available at certain referral practices and institutions, because it requires specialized, expensive equipment. Your veterinarian may refer your dog to a veterinary internal medicine specialist or a veterinary radiologist for performance of these tests.
• Advanced imaging tests such as CT scan, radio-isotope scans, and MRI may be helpful in assessing the abdominal gastrointestinal tract and nearby organs.
• Endoscopy and biopsy of the upper and/or lower bowel are often performed when GI tract diseases are suspected. Endoscopy involves the passage of a flexible viewing tube into the upper or lower portion of the GI tract. The interior of the esophagus, stomach, duodenum, anal canal, rectum and lower colon can be examined through endoscopy. Biopsies of the lining of these structures can be obtained through the endoscope, and submitted for microscopic evaluation. Sometimes foreign bodies of the esophagus and stomach can be removed via endoscopy. Biopsies can also be taken of masses that are growing into the center or lumen of these structures. General anesthesia is necessary for endoscopy; however, it is considered a relatively low risk procedure compared to abdominal surgery. This procedure may require the expertise of a specialist and specialized equipment.
• Surgical exploration of the abdomen is sometimes needed to examine the GI tract, to establish a diagnosis, and to provide effective treatment of certain GI diseases. The entire abdominal GI tract can be examined by surgically opening the abdomen, and biopsies can be taken from both the inside or from the walls of the GI tract, as well as from surrounding structures.
• Diseases of the GI tract are sometimes difficult to confirm, and may require a combination of many of the above tests in order to reach a diagnosis.