How the Digestive System Works

When you eat things like bread, meat, and vegetables, they are not in a form that the body can use as nourishment. They must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion allows your body to get the nutrients and energy it needs from the food you eat.

Parts of the Digestive System

The digestive system consists of the digestive tract and other organs that assist in the digestion process. The digestive tract is made up of organs joined together in a long tube that runs from the mouth to the anus. The digestive tract includes the following:

  • Mouth

  • Esophagus

  • Stomach

  • Small intestine

  • Large intestine

  • Rectum

  • Anus

Other organs aid in the digestion process but are not part of the digestive tract. They include:

  • Tongue

  • Glands in the mouth that make saliva

  • Pancreas

  • Liver

  • Gallbladder

Other organ systems, such as the nervous system, are also involved in processing food.

Starting the Process

It’s time for breakfast. You get your bowl of cereal and take a bite. You chew and swallow. What happens from the moment the food goes in your mouth until it leaves your body? When you see, smell, taste, or even imagine a tasty snack, your salivary glands, which are located under the tongue and near the lower jaw, begin producing a digestive juice called saliva.

Your teeth break the food into smaller pieces and the digestive enzymes in your saliva start to break down some of the carbohydrates. When you’re ready to swallow, the tongue pushes a tiny bit of chewed-up food called a bolus toward the back of your throat and into the opening of your esophagus, the second part of the digestive tract.

You swallow by choice, but once the swallow begins, the digestion process becomes involuntary and moves along under the control of the nerves. The esophagus is like a stretchy pipe that’s about 10 inches (or 25 centimeters) long. It moves food from the back of your throat to your stomach.

Muscles in the walls of the esophagus move in a wavy way to slowly squeeze the food through the esophagus. Think of it like squeezing toothpaste out of a tube. It would work even if you were standing on your head! A muscular ring called a sphincter allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus.

The Stomach’s Job

Digestion continues when food reaches your stomach. The stomach has a stretchy wall covered with special cells. Some of the cells make digestive juice to continue breaking down food. Some things you drink or eat like water, salt, sugars, and alcohol can be absorbed directly through the stomach wall. The rest needs the help of the stomach before passing down to the small intestine.

Fact

An adult’s stomach has a volume of just 1.6 fluid ounces when it is empty, but it can expand to hold more than 64 fluid ounces of food after a large meal.

Mucus coats the food and the wall of the stomach and helps protect the stomach wall from acid. Acid plays an important role in helping to kill germs in the food and soften the food. The stomach stores the swallowed food and liquid, which requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material. Acting like a mixer, the lower part of the stomach mixes up the food, liquid, and digestive juices produced by the stomach by muscle action. Every twenty seconds, the muscles in the stomach wall squeeze and turn your dinner into a soupy liquid.

Next, the stomach empties its contents into the small intestine to convert food into absorbable nutrients and energy. Food moves, a teaspoon at a time, out of the stomach and into the small intestine. The amount of time it takes for the stomach to empty depends on the type of food being digested and the amount of fluid in it. The more fluid you have, the faster the stomach empties.

The Small Intestine and Liver

Essential

The intestine has a big job. More than a gallon of water containing over an ounce of salt is absorbed from the intestine every 24 hours.

From the stomach, food enters the small intestine. The 22-foot-long small intestine is a narrow tube that loops around your stomach several times and is the longest part of your digestive system. It does the bulk of the digestive work. It takes one to four hours for the small intestine to break food down into nutrients and absorb it into the blood.

A number of important digestive hormones and digestive enzymes help to regulate digestion, especially in the upper gastrointestinal tract. The small intestine is a coiled tube made up of three sections—the duodenum, jejunum, and ileum. It is covered with billions of microscopic, finger-like bumps called villi. The walls of the villi are very thin and covered with tiny blood vessels. The nutrients go through the walls of the villi and into your blood.

Blood carries absorbed nutrients through the bloodstream to the liver. The liver cleans the blood, straining out nutrients when they arrive. Some nutrients are stored in the liver and stay there until the body needs them. Other nutrients leave quickly and are distributed to other parts of the body. After every useful, digestible ingredient other than water has been wrung out of the remaining material, called chyme, the remaining waste passes into the large intestine.

Bile from the gallbladder (hidden just below the liver) and liver (located under the rib cage in the right upper part of the abdomen) emulsify fat and enhance the absorption of fatty acids. The enzymes in the pancreas cause chemical changes that, with the help of bile, break down proteins, fats, and carbohydrates. An enzyme is a protein that can cause chemical changes in organic substances like food.

Getting Rid of Waste

Fact

The large intestine is home to the appendix, the small finger-like pouch that can become inflamed and be extremely painful. Some scientists suggest that the appendix—the slender two- to four-inch pouch located near where the large and small intestine meet, once thought to be worthless—may actually produce and protect good germs for your body.

The undigested material from the small intestine is vegetable roughage (cellulose), meat connective tissue, some digested but unabsorbed nutrients, and large amounts of water. What’s left over in the small intestine gets passed to the body’s waste-processing plant, the five-foot-long large intestine. The intestine forms a big loop that runs up your right side, across your middle and down your left side.

The walls of the large intestine are smooth on the inside with large colonies of bacteria. The bacteria act on the undigested waste and convert it into gases, acids, and vitamins. There are three parts of the large intestine: the cecum, the colon, and the rectum.

The cecum is a pouch at the beginning of the large intestine that joins the small intestine to the large intestine. This transition area expands in diameter, allowing food to travel from the small intestine to the large.

How long does it take for food to go through your digestive system?

It takes about 24 hours for a meal to move through your entire digestive system. The exact amount of time depends on the size of your meal and on the kinds of food you ate. In other words, soup moves a lot faster than a steak dinner!

The colon extends from the cecum up the right side of the abdomen, across the upper abdomen, and then down the left side of the abdomen, finally connecting to the rectum. The colon has three parts: the ascending colon and the transverse colon, which absorb fluids and salts, and the descending colon, which holds the resulting waste. Billions of bacteria live in the colon and help to ferment and absorb substances like fiber.

The rectum is the last twelve inches of bowel above the anus. Here, your feces wait to be excreted through the anus in your next bowel movement. The anus is held closed by a ring of muscles. When you have to go to the bathroom, you relax those muscles and feces are released from your body.

The Digestive System

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