The digestive system The digestive tract performs the important function of bringing water and nutrients into the body. It consists of several organs that have specialized functions. Digestion begins as food is chewed and mixed with saliva, which lubricates the food and contains an enzyme that helps break down starches. The food then passes through the esophagus to the stomach via a process known as peristalsis. During peristalsis, muscles surrounding the esophagus contract in a rhythmic wave that pushes the food toward the stomach, much like a ball being pushed into a sock [suggest link to a fluoroscopic video of a barium swallow].
The stomach is an expandable sack that has muscular valves on each end. These valves stay closed to hold food in the stomach over the 1-2 hours it takes to complete initial digestion. Cells in the lining of the stomach secrete an enzyme called pepsin, which begins the process of breaking down protein. Other cells in the stomach produce hydrochloric acid, which creates a very acidic environment that aids the digestive process. Muscles in the wall of the stomach continue to mix the food and other substances in the stomach until it progresses into the small intestine.
The small intestine is a very long muscular tube that lies in numerous loops within the abdomen that are held in place by bands of fibrous tissue. The small intestine consists of three sections called the duodenum, the jejunum, and the ileum. As the contractions of peristalsis move food through these sections, bile is added (to the duodenum) from the gallbladder and digestive enzymes from the pancreas. Bile breaks fat lumps into smaller particles, and digestive enzymes such as trypsin and sucrase continue the process of breaking down fats, proteins, starches, and sugars. Useful nutrients are then absorbed through the lining of the small intestine, while the waste products are sent on to the large intestine.
The large intestine or colon is a larger muscular tube that connects the small intestine to the anus. As peristaltic waves move the waste material through the colon, excess water is absorbed giving the feces its normal consistency. This fecal matter is stored in the colon until muscular valves in the rectum are relaxed (usually a voluntary process), allowing elimination of feces outside the body.
Vomiting is caused by irritation of the stomach or first part of the small intestine. During vomiting, the peristaltic waves reverse direction in an attempt to purge the digestive system of the irritation. The most common cause of vomiting in pets is eating things such as sticks, leaves, bones, trash, or greasy table foods that irritate the lining of the digestive tract. This irritation usually doesn’t last long, resolving after a few hours to a day or two. However, foreign materials can sometimes get stuck within the digestive tract, causing an obstruction. If the first parts of the digestive tract become obstructed, the peristaltic waves meet a dead end and reverse, resulting in severe, persistent, and often projectile vomiting. Vomiting can also be caused by viral or (more rarely) bacterial infection of the digestive tract. Parasites such as roundworms or hookworms are also common causes of vomiting in young pets.
Vomiting can also sometimes be caused by disorders outside the digestive tract. For example, liver and kidney disease can allow toxins to build up in the body and lead to generalized illness and vomiting. In older pets, cancers can invade the digestive tract, or impinge upon it, leading to irritation or obstruction and subsequent vomiting.
The character of the vomit can offer clues as to the problem. For example, the vomit may contain bits of foreign material or white roundworms, which can resemble coiled spaghetti [suggest link to photo of a roundworm]. Vomit that contains red blood suggests bleeding at the very beginning of the digestive tract, while a “coffee-ground” appearance suggests digested blood from further down the tract. If yellow bile is seen in the vomit, it may be because the stomach is empty or the duodenum is irritated.
Irritation farther down the digestive tract in the small or large intestine usually results in diarrhea because muscle spasms speed up peristalsis and cause feces to be eliminated before digestion has been completed or excess water has been absorbed. As with vomiting, irritating objects or foods, parasites, and viral infections are the most common culprits. Obstruction further down the digestive tract is also a serious situation. An obstructed bowel usually results in a very painful abdomen and straining to defecate, without producing feces or producing only small amounts of jellied blood. In some cases, a foreign object can actually puncture the digestive tract, letting intestinal contents leak into the abdominal cavity, which is a situation known as peritonitis. Obstruction or perforation of any area of the digestive tract is an emergency situation, requiring immediate evaluation and treatment by a veterinarian.
As with vomiting, the character of the diarrhea often provides clues about the problem. For example, roundworms, sticks, or trash may be present in the feces. Red blood and mucus usually suggest damage to the colon, while black, tarry feces suggest digested blood from higher up the tract. Soft, but otherwise normal feces often indicate irritation of the colon, in which normal feces were expelled before excess water was removed. Fetid, runny diarrhea often indicates incomplete digestion and absorption within the small intestine.
Many times, a detailed history, physical examination, and fecal examination can rule out serious conditions and allow conservative medical treatment of the digestive upset. However, additional diagnostic evaluation is sometimes necessary. For example, your vet may recommend blood tests to check for problems such as liver or kidney disease, or abdominal x-rays to look for signs of foreign bodies or obstruction. More involved or complicated cases may require additional, more specialized diagnostic tests such as ultrasound, a contrast study, or a CAT scan or MRI. Contrast studies involve giving your pet barium or some other contrast medium that highlights the inside surfaces of the digestive tract on x-ray [link to barium radiograph]. The contrast medium is administered either orally or via an enema, depending on the suspected problem.
Often, digestive upset can be handled with conservative medical therapy. For example, your vet may recommend withholding food or feeding several small, bland meals to rest the digestive tract so that it can heal. A prescription diet or medication to soothe the tract or inhibit vomiting may be prescribed.
More serious or persistent cases may require hospitalization and possibly surgery. Vomiting or diarrhea for even a day or two can lead to dehydration, and your pet may need fluid therapy. Fluids are usually administered intravenously, but are sometimes also given under the skin, where they are absorbed gradually over several hours. Hospitalized pets may also receive treatment with antibiotics, nutrients, or other medications.
Pets with a suspected obstruction or perforation require emergency surgery to remove the foreign material and to check for bowel damage or peritonitis. In an obstruction, blood flow can be cut off to a section of the intestine, causing the tissues of that part of the bowel to die, resulting in serious inflammation and infection. In some cases, one or more sections of dead bowel may need to be removed, with the healthy ends sewn together [possible link to a diagram of anastamosis surgery]. If there are no complications after surgery (eg, infection) and with good follow-up care, most pets are back to normal within a few weeks.