7.6 The Digestive System

Learning Objectives

• List the functions of the digestive system.
• Explain the role of enzymes in digestion.
• Describe the digestive organs and their functions.
• Explain the roles of helpful bacteria in the digestive system.

Introduction

How do the nutrients in food get to your body’s cells?

Nutrients in the food you eat are needed by the cells of your body. What organs and processes break down the foods and make the nutrients available to cells? The organs are those of the digestive system. The processes are digestion and absorption.

Guided Learning

What does the digestive system do?

The digestive system is the body system that breaks down food and absorbs nutrients. It also gets rid of solid food waste. The main organs of the digestive system are shown in Figure below.

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This drawing shows the major organs of the digestive system. Trace the path of food through the organs of the digestive system as you read about them in this lesson.

Digestion is the process of breaking down food into nutrients. There are two types of digestion, mechanical and chemical. In mechanical digestion, large chunks of food are broken down into small pieces. This is a physical process. In chemical digestion, large food molecules are broken down into small nutrient molecules. This is a chemical process.

Absorption is the process that allows substances you eat to be taken up by the blood. After food is broken down into small nutrient molecules, the molecules are absorbed by the blood. After absorption, the nutrient molecules travel in the bloodstream to cells throughout the body.

Some substances in food cannot be broken down into nutrients. They remain behind in the digestive system after the nutrients are absorbed. Any substances in food that cannot be digested and absorbed pass out of the body as solid waste. The process of passing solid food waste out of the body is called elimination.

The Role of Enzymes in Digestion

Chemical digestion could not take place without the help of digestive enzymes. An enzyme is a protein that speeds up chemical reactions in the body. Digestive enzymes speed up chemical reactions that break down large food molecules into small molecules.

Did you ever use a wrench to tighten a bolt? You could tighten a bolt with your fingers, but it would be difficult and slow. If you use a wrench, you can tighten a bolt much more easily and quickly. Enzymes are like wrenches. They make it much easier and quicker for chemical reactions to take place. Like a wrench, enzymes can also be used over and over again. But you need the appropriate size and shape of the wrench to efficiently tighten the bolt, just like each enzyme is specific for the reaction it helps.

Digestive enzymes are released, or secreted, by the organs of the digestive system. Examples of digestive enzymes are:

• Amylase, produced in the mouth. It helps break down large starches molecules into smaller sugar molecules.
• Pepsin, produced in the stomach. Pepsin helps break down proteins into amino acids.
• Trypsin, produced in the pancreas. Trypsin also breaks down proteins.
• Pancreatic lipase, produced in the pancreas. It is used to break apart fats.
• Deoxyribonuclease and ribonuclease, produced in the pancreas. They are enzymes that break bonds in nucleic acids like DNA and RNA.

Bile salts are bile acids that help to break down fat. Bile acids are made in the liver. When you eat a meal, bile is secreted into the intestine, where it breaks down the fats. Bile acids also help to remove cholesterol from the body.

Hormones and Digestion

If you are a typical teenager, you like to eat. For your body to break down, absorb and spread the nutrients throughout your body, your digestive system and endocrine system need to work together. The endocrine system sends hormones around your body to communicate between cells like chemical messengers.

Digestive hormones are made by cells lining the stomach and small intestine. These hormones cross into the blood where they can affect other parts of the digestive system. Some of these hormones are listed below.

• Gastrin, which signals the secretion of gastric acid.
• Cholecystokinin, which signals the secretion of pancreatic enzymes.
• Secretin, which signals secretion of water and bicarbonate from the pancreas.
• Ghrelin, which signals when you are hungry.
• Gastric inhibitory polypeptide, which stops or decreases gastric secretion. It also causes the release of insulin in response to high blood glucose levels.

Digestive Organs and their Roles

The mouth and stomach are just two of the organs of the digestive system. Other digestive system organs are the esophagus, small intestine, and large intestine. The digestive organs form a long tube. In adults, this tube is about 30 feet long! At one end of the tube is the mouth. At the other end is the anus. Food enters the mouth and then passes through the rest of the digestive system. Food waste leaves the body through the anus.

The organs of the digestive system are lined with muscles. The muscles contract, or tighten, to push food through the system. This is shown in Figure below. The muscles contract in waves. The waves pass through the digestive system like waves through a slinky. Without this movement, food would not be able to move through the digestive system. This is an involuntary process, which means that it occurs without your conscious control.

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This diagram shows how muscles push food through the digestive system. Muscle contractions travel through the system in waves, pushing the food ahead of them. This is called peristalsis.

The liver, gallbladder, and pancreas are also organs of the digestive system. They are shown in Figure below. Food does not pass through these three organs. However, these organs are important for digestion. They secrete or store enzymes or other chemicals that are needed to help digest food chemically.

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This drawing shows the liver, gallbladder, and pancreas. These organs are part of the digestive system. Food does not pass through them, but they secrete substances needed for chemical digestion.

Mouth, Esophagus, and Stomach

The mouth is the first organ that food enters. But digestion may start even before you put the first bite of food into your mouth. Just seeing or smelling food can cause the release of saliva and digestive enzymes in your mouth.

Once you start eating, saliva wets the food making it easier to break up and swallow. Digestive enzymes, including amylase, start breaking down starches into sugars. Your tongue helps mix the food with the saliva and enzymes.

Your teeth also help digest food. Your front teeth are sharp. They cut and tear food when you bite into it. Your back teeth are broad and flat. They grind food into smaller pieces when you chew. Chewing is part of mechanical digestion. Your tongue pushes the food to the back of your mouth so you can swallow it. When you swallow, the lump of chewed food passes down your throat to your esophagus.

The esophagus is a narrow tube that carries food from the throat to the stomach. At the lower end of the esophagus, a circular muscle controls the opening to the stomach. The muscle relaxes to let food pass into the stomach. Then the muscle contracts again to prevent food from passing back into the esophagus.

Some people think that gravity moves food through the esophagus. If that were true, food would move through the esophagus only when you are sitting or standing upright. In fact, food can move through the esophagus no matter what position you are in — even upside down! Just don’t actually try to swallow food when you’re upside down; you could choke.

The stomach is a sac-like organ at the end of the esophagus. It has thick muscular walls. The muscles contract and relax. This moves the food around and helps break it into smaller pieces. Mixing the food around with the enzyme pepsin and other chemicals helps digest proteins.

Water, salt, and simple sugars can be absorbed into the blood from the stomach. Most other substances are broken down further in the small intestine before they are absorbed. The stomach stores food until the small intestine is ready to receive it. A circular muscle controls the opening between the stomach and small intestine. When the small intestine is empty, the muscle relaxes. This lets food pass from the stomach into the small intestine.

Small Intestine

The small intestine is narrow tube that starts at the stomach and ends at the large intestine (see Figure above). In adults, the small intestine is about 23 feet long. This is where most chemical digestion takes place. Many enzymes and other chemicals are secreted here. This is where most nutrients are absorbed into the blood. The small intestine is lined with tiny “fingers” called villi. A magnified picture of villi is shown in Figure below. Villi contain very tiny blood vessels. Nutrients are absorbed into the blood through these tiny vessels. There are millions of villi, so altogether there is a very large area for absorption to take place. In fact, villi make the inner surface area of the small intestine 1,000 times larger than it would be without them. The entire inner surface area of the small intestine is about as big as a basketball court! From the small intestine, any remaining food waste passes into the large intestine.

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This is what the villi lining the small intestine look like when magnified. Each one is actually only about 1 millimeter long. Villi are just barely visible with the unaided eye.

The small intestine is much longer than the large intestine. So why is it called “small”? If you compare the small and large intestines in Figure above, you will see why. The small intestine is smaller in width than the large intestine.

Large Intestine

The large intestine is a wide tube that connects the small intestine with the anus. In adults, it is about 5 feet long. Waste enters the large intestine from the small intestine in a liquid state. As the waste moves through the large intestine, excess water is absorbed from it. After the excess water is absorbed, the remaining solid waste is called feces.

Circular muscles control the anus. They relax to let the feces pass out of the body through the anus. After feces pass out of the body, they are called stool. Releasing the stool from the body is referred to as a bowel movement.

Liver

The liver has a wide range of functions, a few of which are:

• Removing toxins from the blood.
• Keeping glucose levels stable.
• Creating proteins.
• Producing biochemicals for digestion.

The liver is necessary for survival. You cannot live without a liver. The liver is one of the most important organs in the body when it comes to getting rid of toxins, especially from the gut. The liver filters blood from the intestine. This filtering process can remove microorganisms such as bacteria, fungi, viruses, and parasites from the blood. Almost two quarts of blood pass through the liver every minute. Since the liver also ensures that glucose levels remain stable, people with liver problems are at risk for diabetes.

Bacteria in the Digestive System

Your large intestine is not just made up of cells. It is also an ecosystem, home to trillions of bacteria. But don't worry. Most of these bacteria are helpful. They have several roles in the body. For example, intestinal bacteria:

• Produce vitamins B12 and K.
• Control the growth of harmful bacteria.
• Break down poisons in the large intestine.
• Break down some substances in food that cannot be digested, such as fiber and some starches and sugars.

Summary

• The digestive system breaks down food, absorbs nutrients, and gets rid of food wastes.
• Digestive enzymes speed up the reactions of chemical digestion.
• The main organs of the digestive system are the mouth, esophagus, stomach, small intestine, and large intestine.
• Bacteria in the large intestine produce vitamins and have other roles in the body.

Vocabulary

absorption

The process that allows substances you eat to be taken up by the blood.

chemical digestion

Large food molecules are broken down into small nutrient molecules; this is a chemical process.

digestion

Process of breaking down food into nutrients.

digestive system

The body system that breaks down food and absorbs nutrients; also gets rid of solid food waste.

enzyme

A protein that speeds up chemical reactions in the body; digestive enzymes speed up chemical reactions that break down large food molecules into small molecules.

esophagus

A narrow tube that carries food from the throat to the stomach.

large intestine

A wide tube that connects the small intestine with the anus; waste enters the large intestine from the small intestine in a liquid state and as the waste moves through the large intestine, excess water is absorbed from it.

mechanical digestion

Large chunks of food are broken down into small pieces; this is a physical process.

small intestine

Narrow tube that starts at the stomach and ends at the large intestine; where most nutrients are absorbed into the bloodstream.

stomach

Sac-like organ at the end of the esophagus; it has thick muscular walls that contract and relax, moving the food around and helping break it into smaller pieces.

villi

Tiny “fingers” that contain very tiny blood vessels; nutrients are absorbed into the blood through         these tiny vessels.