The digestive system is responsible for the digestion, absorption, and assimilation of fluids, macro- and micro-nutrients (vitamins and minerals) and the elimination of waste from the gastrointestinal tract. Food and drink that we ingest is broken down by our digestive system into smaller simple particles (molecules) before it is absorbed by the small intestine and transported into the blood stream that carries the nutrients, vitamins, and minerals to cells throughout the body. In the cells the molecules provide energy and nourishment to the body.

Anatomy

The digestive system is a series of hollow organs joined in a long twisting tube from the mouth to the anus. Inside this tube is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest the food. The liver and the pancreas are solid organs that produce digestive juices that flow through small tubes (ducts) into the upper small intestine. They also play a vital role in controlling the metabolic functions of the body (see Table 1). In a healthy person a large volume of food and fluid flows through the hollow tubes of the digestive system. The small intestinal mucosal cells have many special systems that ensure the absorption of carbohydrates, protein, fats, vitamins, water, and salt (see Table 2). In the colon (also called the large intestine), the cells are arranged to absorb water from the intestinal contents, so that fecal elimination can occur at a convenient time and in a convenient form.

Regulation and Coordination

While the anatomy of the digestive system is simple, its function and interaction with other systems is complex and life sustaining. The walls of the hollow organs are composed of muscles arranged in layers that propel the contents by peristalsis in waves away from the mouth, out of the stomach, through the small intestine and colon. This propulsion of food and liquid by peristalsis is regulated and coordinated with the secretion of digestive juices from the salivary glands, stomach, liver, pancreas, and small intestine by hormones (see Table 3) and nervous system input (see Table 4).

Common Symptoms and Diseases

Symptoms and complaints related to the digestive tract are some of the most common reasons we take over-the-counter medications, prescription medication, or seek the advice of health care providers. Each month 44 percent of adults take antacids or other medicines to treat heartburn. Irritable Bowel Syndrome (IBS) is a common disorder of the intestines that leads to crampy pain, gassiness, bloating, and changes in bowel habits, and is one of the most common reasons for persons to visit their primary care doctor and miss work. Viral Hepatitis is caused by a number of viruses and results in inflammation of the liver. Five million people in the U.S. are infected with Hepatitis C virus that frequently leads to chronic hepatitis, cirrhosis, and liver cancer. Many people (15 percent) develop gallstones and about half of these people develop symptoms of episodic upper abdominal pain and will require treatment, usually removal of the gallbladder and the stones. Inflammatory Bowel Disease (Crohn’s disease and ulcerative colitis) is a chronic inflammatory condition of the intestines of unknown cause that frequently infects adolescents and adults. Usually present in people with IBD are long standing and variable symptoms that may include chronic diarrhea, bloody stools, abdominal pains, weight loss, fatigue, fever, and occasionally problems outside of the intestines, such as mouth ulcers, joint pain, skin problems, and kidney stones.

Gastroenterologists

Gastroenterologists are physicians who have completed special training in the diagnosis and treatment of symptoms and diseases of the gastrointestinal tract and liver. Physicians may be trained in the treatment of adults (Internal Medicine and Gastroenterology) or children and adolescents (Pediatrics and Pediatric Gastroenterology). Gastroenterologists use a variety of tests, including X-rays and endoscopy, to study the intestines and liver of patients who visit them.

Table 1: Digestive Juices and Their Functions
Salivary Glands
Bicarbonate fluid	Lubricates food during chewing
Salivary Lipase	Prepares fat for digestion
Gastric Secretions
Acid	Initiates digestion of protein and kills bacteria
Pepsin	Aids in initiation of protein digestion
Gastric Lipase	Initiates digestion of dietary fat
Mucous	Lubricates and protects lining of stomach
Intrinsic Factor	Aids in absorption of Vitamin B12 by small intestine
Liver Secretions
Bile Acids	Detergent that helps dissolve dietary fat
Phospholipids	Promotes absorption of fats
Cholesterol	Excreted in bile
Immunoglobulins	Protect from bacteria and other harmful organisms
Mucus	Protects from bacteria
Pancreatic Secretions
Bicarbonate	Neutralizes acid and protects digestive enzymes
Water and electrolytes	Fluid delivery system for digestive enzymes
Amylase	Digests dietary starch and glycogen
Lipases	Digest dietary fat
Proteases	Digest dietary protein

Table 3: Hormonal Regulation and Coordination
Endocrine- hormones arrive by blood stream, secreted from a distant site
Insulin- controls glucose metabolism
Gastrin- stimulates gastric acid secretion and promotes growth of gastric and intestinal cells
Secretin- stimulates pancreatic secretion of fluid and bicarbonate, the liver to produce bile, and the stomach to produce pepsin
Motilin- regulates peristalsis and secretion between meals
Paracrine- hormones released locally
Substance P- influences secretion, absorption, blood flow, motility, and immunology
Hormones that can act as endocrine or paracrine substances
CCK- signal for gallbladder contraction, pancreatic secretion (and growth), & satiety
Somatostatin- inhibits secretion by gut cells, nerves, and hormone secreting cells
Neurotensin- increases blood flow & stimulates secretion