Anatomy of the Digestive System

Picture of the digestive system

• Digestion is the process by which food is broken down into smaller pieces so the body can use them to build and nourish cells and to provide energy.
• Digestion involves the mixing of food, its movement through the digestive tract (also known as the alimentary canal), and the chemical breakdown of larger molecules into smaller molecules.
• Every piece of food eaten has to be broken down into smaller nutrients that the body can absorb, which is why it takes hours to fully digest food.
• The digestive system is made up of the digestive tract. This consists of a long tube of organs that runs from the mouth to the anus and includes the esophagus, stomach, small intestine, and large intestine, together with the liver, gallbladder, and pancreas, which produce important secretions for digestion that drain into the small intestine.
• The digestive tract in an adult is about 30 feet long.

Digestion begins in the mouth, where chemical and mechanical digestion occurs. Saliva or spit, produced by the salivary glands (located under the tongue and near the lower jaw), is released into the mouth. Saliva begins to break down the food, moistening it and making it easier to swallow. A digestive enzyme (amylase) in the saliva begins to break down the carbohydrates (starches and sugars). One of the most important functions of the mouth is chewing. Chewing allows food to be mashed into a soft mass that is easier to swallow and digest later.

Movements by the tongue and the mouth push the food to the back of the throat for it to be swallowed. A flexible flap called the epiglottis closes over the trachea (windpipe) to ensure that food enters the esophagus and not the windpipe to prevent choking.

Illustration of the digestive system.

Once food is swallowed, it enters the esophagus, a muscular tube that is about 10 inches long. The esophagus is located between the throat and the stomach. Muscular wavelike contractions known as peristalsis push the food down through the esophagus to the stomach. A muscular ring (cardiac sphincter) at the end of the esophagus allows food to enter the stomach, and, then, it squeezes shut to prevent food and fluid from going back up the esophagus.

The stomach is a J-shaped organ that lies between the esophagus and the small intestine in the upper abdomen. The stomach has 3 main functions: to store the swallowed food and liquid; to mix up the food, liquid, and digestive juices produced by the sto mach; and to slowly empty its contents into the small intestine.

Only a few substances, such as water and alcohol, can be absorbed directly from the stomach. Any other food substances must undergo the digestive processes of the stomach. The stomach's strong muscular walls mix and churn the food with acids and enzymes (gastric juice), breaking it into smaller pieces. About three quarts of the gastric juice is produced by glands in the stomach every day.

The food is processed into a semiliquid form called chyme. After eating a meal, the chyme is slowly released a little at a time through the pyloric sphincter, a thickened muscular ring between the stomach and the first part of the small intestine called the duodenum. Most food leaves the stomach by four hours after eating.

Most digestion and absorption of food occurs in the small intestine. The small intestine is a narrow, twisting tube that occupies most of the lower abdomen between the stomach and the beginning of the large intestine. It extends about 20 feet in length. The small intestine consists of three parts: the duodenum (the C-shaped part), the jejunum (the coiled midsection), and the ileum (the last section).

The small intestine has two important functions.

1. The digestive process is completed here by enzymes and other substances made by intestinal cells, the pancreas, and the liver. Glands in the intestine walls secrete enzymes that breakdown starches and sugars. The pancreas secretes enzymes into the small intestine that help breakdown carbohydrates, fats, and proteins. The liver produces bile, which is stored in the gallbladder. Bile helps to make fat molecules (which otherwise are not soluble in water) soluble, so they can be absorbed by the body.
2. The small intestine absorbs the nutrients from the digestive process. The inner wall of the small intestine is covered by millions of tiny fingerlike projections called villi. The villi are covered with even tinier projections called microvilli. The combination of villi and microvilli increase the surface area of the small intestine greatly, allowing absorption of nutrients to occur. Undigested material travels next to the large intestine.

The large intestine forms an upside down U over the coiled small intestine. It begins at the lower right-hand side of the body and ends on the lower left-hand side. The large intestine is about 5-6 feet long. It has three parts: the cecum, the colon, and the rectum. The cecum is a pouch at the beginning of the large intestine. This area allows food to pass from the small intestine to the large intestine. The colon is where fluids and salts are absorbed and extends from the cecum to the rectum. The last part of the large intestine is the rectum, which is where feces (waste material) is stored before leaving the body through the anus.

The main job of the large intestine is to remove water and salts (electrolytes) from the undigested material and to form solid waste that can be excreted. Bacteria in the large intestine help to break down the undigested materials. The remaining contents of the large intestine are moved toward the rectum, where feces are stored until they leave the body through the anus as a bowel movement.