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13.37: Digestive System Organs

Difficulty Level: At Grade Created by: CK-12
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Specifically, our energy comes from what?

The respiratory and circulatory systems work together to provide cells with the oxygen they need for cellular respiration. Cells also need glucose for cellular respiration. Glucose is a simple sugar that comes from the food we eat. To get glucose from food, digestion must occur. This process is carried out by the digestive system.

Overview of the Digestive System

The digestive system consists of organs that break down food and absorb nutrients such as glucose. Organs of the digestive system are shown in Figure below. Most of the organs make up the gastrointestinal tract. The rest of the organs are called accessory organs.

The digestive system includes organs from the mouth to the anus.

The following interactive animation demonstrates the flow of food through the gastrointestinal (GI) system.

The Gastrointestinal Tract

The gastrointestinal (GI) tract is a long tube that connects the mouth with the anus. It is more than 9 meters (30 feet) long in adults and includes the esophagus, stomach, and small and large intestines. Food enters the mouth, passes through the other organs of the GI tract, and then leaves the body through the anus.

At the link below, you can watch an animation that shows what happens to food as it passes through the GI tract. http://www.youtube.com/watch?v=QtDgQjOGPJM.

The organs of the GI tract are lined with mucous membranes that secrete digestive enzymes and absorb nutrients. The organs are also covered by layers of muscle that enable peristalsis. Peristalsis is an involuntary muscle contraction that moves rapidly along an organ like a wave (see Figure below). You can watch an animation of peristalsis at this link: http://en.wikipedia.org/wiki/File:Peristalsis.gif.

Peristalsis pushes food through the GI tract.

Accessory Organs of Digestion

Other organs involved in digestion include the liver, gall bladder, and pancreas. They are called accessory organs because food does not pass through them. Instead, they secrete or store substances needed for digestion.

Functions of the Digestive System

The digestive system has three main functions: digestion of food, absorption of nutrients, and elimination of solid food waste. Digestion is the process of breaking down food into components the body can absorb. It consists of two types of processes: mechanical digestion and chemical digestion.

  • Mechanical digestion is the physical breakdown of chunks of food into smaller pieces. This type of digestion takes place mainly in the mouth and stomach.
  • Chemical digestion is the chemical breakdown of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by the blood. This type of digestion begins in the mouth and stomach but occurs mainly in the small intestine.

After food is digested, the resulting nutrients are absorbed. Absorption is the process in which substances pass into the bloodstream, where they can circulate throughout the body. Absorption of nutrients occurs mainly in the small intestine. Any remaining matter from food that cannot be digested and absorbed passes into the large intestine as waste. The waste later passes out of the body through the anus in the process of elimination.


  • The digestive system consists of organs that break down food, absorb nutrients, and eliminate waste.
  • The breakdown of food occurs in the process of digestion.

Practice I

Use this resource to answer the questions that follow.

  • http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: The Human Digestive System
  1. Where does the digestion of food begin?
  2. What is saliva?
  3. List the organs involved in the journey of food.
  4. Describe the structures of the stomach and small intestine.

Practice II


1. What organs make up the gastrointestinal tract? What are the accessory organs of digestion?

2. Describe peristalsis and its role in digestion.

3. Define mechanical and chemical digestion.

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Process in which substances such as nutrients pass into the blood stream.

accessory organ

Organ that secretes or stores substances needed for digestion.

chemical digestion

Chemical breakdown of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by the blood.


Process of breaking down food into nutrients that can be absorbed by the blood.

digestive system

Organ system that breaks down food, absorbs nutrients, and eliminates any remaining waste.


Process in which waste passes out of the body.

gastrointestinal tract

The part of the anatomy involved with the process of digestion; refers to the stomach and intestine; may refer to all the structures from the mouth to the anus.

mechanical digestion

Physical breakdown of chunks of food into smaller pieces by organs of the digestive system.


Rapid, involuntary, wave-like contraction of muscles that pushes food through the GI tract and urine through the ureters.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 28, 2016
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