Digestive System

The specialty of gastroenterology focuses on the digestive system. The major organs of the digestive system are the esophagus, the stomach, the small intestine, the large intestine, and the rectum. Other parts of this body system include the liver, the gallbladder and the pancreas.

The digestive system has several names. It is called the alimentary canal, the gastrointestinal system, the GI tract, or it can also be referred to as the gut. This body system begins at the mouth and becomes a tube about 30 feet long that eventually exits the body at the end of the large bowel, an area called the rectum.

The digestive system is like a food-processing machine. It has five main functions:

  • Processes and stores food. Food is changed so that it can be absorbed to provide nutrition for health.
  • Manufactures enzymes, hydrochloric acid, mucus, hormones and Vitamin K.
  • Absorbs nutrients from the food into the blood stream.
  • Reabsorbs water, minerals, vitamins.
  • Eliminates formed waste products.

There are two types of digestion:

  • Mechanical, which is the physical breakdown of food (for example, chewing)
  • Chemical, which uses enzymes, acids and water. There are many enzymes. The most common are proteases, which break down proteins; lipases, which break down fats; and amylases, which break down carbohydrates.

What happens as food moves through the digestive system?

When smelling or thinking about food, the glands in the tongue, cheek, and jaw, start to produce saliva. From 1 to 1.5 liters of saliva are produced per day. Saliva and teeth start to “break down” or digest the food. The process of changing food to fuel and energy has begun! This is called the digestive process.

Most of the digestive system is covered by a thick layer of mucous. A mucous membrane covers the tongue.  The tongue senses the temperature and texture of food, mixes the food with saliva, and moves food into the back area of the throat. Food that is mixed with saliva is referred to as a bolus (a small round mass).

When the bolus of food reaches the bottom of the throat, and when swallowing, a sphincter or valve, called the upper esophageal sphincter (UES) opens.  From here on control of the movement of food through the GI tract is no longer possible. Different types of muscles contract to help move the food along.  This movement is called peristalsis.  The food, or bolus, enters a hollow muscular tube that is about 10 inches long and one inch in diameter. It is called the esophagus (esof-a-gus). This tube passes through the diaphragm (dia-fram), a muscular, membrane-like partition that separates the chest cavity or area that contains the lungs, from the area that is the abdomen.

At the end of the esophagus is another valve called the lower esophageal sphincter (LES or gastroesophageal sphincter (gastro-esofa-geeal)). When the LES is working properly, it stops the acid in the stomach from flowing back up (reflux) into the esophagus. If there is damage or weakness to this valve, stomach contents, including hydrochloric acid, flow up into the esophagus and cause injury to the lining of the esophagus.  This causes pain, commonly called “heartburn,” and can cause a chronic condition called gastroesophageal reflux disease (GERD).  The stomach has a very thick mucosal layer; the esophagus is merely a passageway and is not protected by a thick layer of mucous, which could cause cell damage or changes if there is ongoing exposure to acid.

Most of the digestive system has a thick lining of mucous surrounded by a system of circular, longitudinal and oblique muscle fibers. Just below the mucosal lining lay blood vessels and nerve fibers. The mucosa itself contains glands that produce the mucus lining which lubricates the food. The upper esophagus is made of striated (with grooves) muscle. In the area nearer to the stomach, the esophagus is made up of smooth muscle.

As the bolus of food goes down the esophagus, it nudges the LES to open. The food is now in the stomach. The stomach is a very interesting machine. It is a muscular, collapsible sac. It is in the upper left side of your abdominal cavity. It is 10-12″ long and about 4-6″ at its widest. It is divided into three areas. The top most part is called the fundus, the mid portion is called the body of the stomach and the bottom third is called the antrum.

The stomach is very strong. Along with the other layers surrounding the GI tract, the stomach has three layers of smooth muscle: a circular layer, a longitudinal layer and an oblique layer. These muscles, working together, cause the stomach to expand and contract which churns the food. As food enters the stomach it begins to be churned with stomach acid and gastric juices and is turned into a semi-liquid form, called chyme (kime).

Injury to the stomach may or may not produce pain. Medications can cause irritation, inflammation, redness, and erosions to the mucosal lining. Just underneath the mucosal lining is an area where the blood vessels lie. When injury is deep into the tissue, it produces an ulcer. A deep ulcer can develop at one of these blood vessels causing a bleeding ulcer. Ulcers can be common in the stomach, called peptic ulcer, or in the area just beyond the stomach, called a duodenal ulcer.

When food passes out of the stomach it is only about 1 mm in size. It passes into the duodenum, the first portion of the small bowel (small intestine). Here it is mixed with bile from the biliary system (the liver and the pancreas).

The biliary system, although not a part of the GI tract per se, consists of the gallbladder, and the hepatic, cystic and common bile ducts. A duct is a small tube that allows liquid to pass through it. The hepatic duct drains from the liver, the cystic duct from the gallbladder. These two ducts combine to form the common bile duct. The duct from the pancreas joins the common bile duct and these then empty out into the duodenum.

The liquid that is passed through the common bile duct into the duodenum is made up of secretions produced by the pancreas, liver and gallbladder. Bile is secreted by the liver and is stored in the gallbladder. When the food passes into the duodenum, it sets off hormones that release the bile from the gallbladder. Bile is greenish yellow and is made of water, bile salts, fatty acids, lipids, inorganic electrolytes, and bilirubin. Bile breaks down undigested fats, helps to absorb fat-soluble vitamins and helps to neutralize the acid from the stomach.

A common problem occurs when stones from the gallbladder come down the duct and cause the duct to be blocked. This may cause pain, or may make the skin, or the area around the eyes, turn yellow (jaundice). This condition may respond to medication that dissolves the stones. It may require surgery. It is most often treated using a procedure performed by a gastroenterologist called endoscopic retrograde cholangiopancreatogram (ERCP).

The small intestine is approximately 22 feet long and about ¾ to 1 ½” in diameter. It is divided into 3 sections: duodenum, jejunum and ileum. Most of our food is absorbed in the small intestine. The small intestine receives about 8 liters of fluid per day and passes only about ½ to 1 liter to the large intestine. It does so because of the structure of this organ. It has a muscular layer, submucosal layer and a mucus layer of simple columnar epithelium, connective tissue and smooth muscles. The food (chyme) is in the small intestine for 1-6 hours.

Unique to the small intestine is 4 – 5 million small villi, fingerlike projections. Each of these is covered by microvilli. Each of these millions of villi is lined with columnar epithelial cells. Because of the microvilli, the surface area of the small intestine is 600-fold, giving it a greatly expanded area to absorb nutrients from the food. The microvilli are supported by blood capillaries. It is these blood capillaries that carry nutrients to the liver for further processing. Malabsorption disorders are usually caused by improper function or changes in the structure of the small intestine.

At the end of the small intestine is the ileo-cecal valve. This valve prevents contents of the large intestine (also called large bowel or colon) from backing up into the small intestine. The cecum, a part of the large bowel, receives the contents from the small intestine, mostly water and waste material. The large intestine is 4-5 feet long and about 2½ inches in diameter. Its main function is water reabsorption.

From the cecum (the appendix is located here) the waste material moves through the ascending colon, usually called the right colon. It passes through the transverse colon and into the descending (left) colon and on into the sigmoid colon. Stool passes through the rectum and then exits the body through the anus. There are several sphincters in the rectum and many muscles in the “pelvic floor” that give the signal to have a bowel movement.

Food is changed into nutrients by the digestive system. After chewing and swallowing food, the digestive system moves the food and absorbs the nutrients.

Disorders of the GI system are common. Check with your physician if you have any concerns or questions.