What is Digestion? - Definition & Process

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  • 0:00 Parts of the Digestive System
  • 1:31 Digestion Process Overview
  • 2:59 Digestion: Mouth to Stomach
  • 4:53 Digestion: Small to…
  • 5:35 The Role of Solid…
  • 7:27 Lesson Summary
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Lesson Transcript
Instructor: Nadine James

Nadine has taught nursing for 12 years and has a PhD in Nursing research

This lesson will define the process of digestion. All parts of the gastrointestinal system will be discussed. Examples will be given when possible, and you'll have an opportunity to test your knowledge.

Parts of the Digestive System

Every day, we eat food and the body carries out the process of digestion. Food is our body's fuel source. The nutrients in food give the body's cells the energy and other substances they need to operate, and digestion is the process of breaking down the food and drink into smaller molecules like carbohydrates, proteins, fats, and vitamins.

There are many parts of the digestive system, including the gastrointestinal (GI) tract, also called the digestive tract, the liver, the pancreas, and the gallbladder. The gastrointestinal tract is a long, twisting tube of hollow organs that start at the mouth and end at the anus. Have you ever gone to a water amusement park and ridden down one of the tube slides into the water? Well, that could represent the hollow organs of the GI tract. Just picture the food going down the slide.

Hollow organs in the GI tract include the mouth, esophagus, stomach, small intestine, large intestine, including the rectum, and anus. Solid organs of the digestive system are the liver, pancreas, and gallbladder.

However, the hollow and solid organs are not alone in the process of digesting food. There are other areas of the body that assist with digestion. They include bacteria in the GI tract and parts of the nervous and circulatory systems.

Digestion Process Overview

Food enters the mouth and passes to the anus through the hollow organs of the GI tract. Digestion happens when food moves through the GI tract. Muscles in the hollow organs contract and relax, moving like a wave that travels through the ocean. This moves food along. Peristalsis is the process of the movement of food through the GI tract.

So, digestion begins in the mouth when you chew and ends in the large intestine 18 to 20 hours later. As food passes through the GI tract, it mixes with digestive juices, causing large molecules of food to break down into smaller molecules. Think of a can of tomato soup. When you open the can, it is a thick red blob. You have to add water to make the blob turn into the soup - this is similar to how the digestive juices break down the molecules into nutrients.

Absorption of the smaller molecules, including carbohydrates, proteins, fats, vitamins, and minerals, happen through the walls of the small intestine and go into the bloodstream. Then, the circulatory system, specifically the blood, works to distribute these nutrients to the rest of the body.

You eat until you're satisfied, and then you do not think about digestion again. But for the next 18 to 20 hours, your digestive system is doing its job. The food you ate travels throughout your body.

Digestion: Mouth to Stomach

Even before food enters your mouth, the digestive process begins. Think of your favorite food. You can almost taste it, right? Now you have the food placed in front of you, and what happens? You begin to salivate, or produce saliva in your mouth. This happens because the brain sends impulses through the nerves that control the salivary glands, telling them to prepare for a meal - so your body releases saliva.

Then, you begin to eat the food. Your teeth tear and chop the food while the saliva moistens it for easier swallowing. Amylase is an important digestive enzyme in your saliva. It starts to break down some of the carbohydrates in the food even before it leaves your mouth.

Swallowing then moves the food into your throat, also called the pharynx. From the throat, food travels down a muscular tube called the esophagus. Peristalsis then forces the food down through the esophagus to the stomach.

At the end of the esophagus, there is a muscular ring, called a sphincter, that opens for food and then shuts. When the sphincter is closed, food or fluid cannot flow back up into the esophagus.

The stomach muscles churn and mix the food with digestive juices, breaking it into much smaller, more digestible pieces. All of this takes place without you even being aware of the muscles of the esophagus and stomach moving.

Most substances in the food we eat require further digestion and must travel into the intestine before being absorbed. By the time food is ready to leave the stomach, it has been processed into chyme, a thick liquid consisting of gastric juices and partially digested food. This is then squirted down into the small intestine, where digestion of food continues so the body can absorb the nutrients into the bloodstream.

Digestion: Small to Large Intestines

Inside the walls of your small intestine are millions of microscopic projections called villi. Villi are the vehicles that allow nutrients to be absorbed into your bloodstream. If you think of a dry sponge and how it soaks up liquid, you might get a clearer picture of how the villi absorb the nutrients.

Finally, food that your body can't use is passed out of the body through the large intestine as feces. The large intestine's main function is to remove water from the undigested matter and form solid waste that can be excreted. Bacteria in the large intestine, specifically the colon, help to break down the remaining food to form the solid waste.

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