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The Digestive System 


                     If your cells could use the food you eat

                   directly, there would be no need for a

digestive system. Your digestive system changes the

food into a form that can be used by your cells. It

enables the nutrients to get into the blood so they

can be transported throughout your body to your cells.

   The digestive system consists of one long tube and the organs that attach to it. The organs produce digestive chemicals (enzymes and acids) that break down the nutrients into simpler forms so they can be absorbed through the intestinal walls and into blood stream.

Read below to find out about your digestive system.


Learning Objectives
By the end of this lesson, students should be able to:

  • Distinguish between mechanical and chemical digestion.

  • Name the organs of the digestive system and state the function of each.

  • Describe the movement of food along the digestive tract.



amylase—an enzyme that begins the process of chemical digestion; produced by salivary glands, amylase begins the breakdown of carbohydrates in the mouth.

anus—the opening at the end of the digestive canal through which solid wastes leave the body.

bile—a chemical produced by the liver and stored in the gall bladder which helps digest fats.

chemical digestion—breaks down large molecules of food into smaller molecules that can be absorbed by cells.

enzyme— a protein that can start certain chemical reactions


epiglottis— a flap-like valve that closes off when you swallow and prevents food from entering the windpipe and lungs.


esophagus—a tube-like structure that connects the throat to the stomach.

gall bladder—an organ that stores bile produced by the liver.


large intestine—the part of the digestive tract that absorbs water from digested materials and produces solid wastes.


liver—an organ that produces bile, removes toxic materials from the body, and stores glycogen.

mechanical digestion—digestive process when food is physically broken into smaller pieces by the mouth and churned in the stomach.


pancreas—a gland near the stomach that secretes digestive fluids into the intestine; also secretes the hormone insulin.


peristalsis—contractions that move food through the digestive system.


pharnyx—a tube-like passage way in the throat for both food and air.


rectum—a holding area for solid wastes.


saliva—the watery substance produced in the mouth that begins the process of chemical digestion.


salivary glands—glands that produce saliva.


small intestine—the chief site of digestion of food; the small intestine secretes enzymes to digest food into small molecules which are absorbed into the body.


sphincter— a ring of smooth muscle that closes off an opening in the body.


stomach—a muscular sac which holds food; the stomach secretes acids that help digest food.


Mechanical and Chemical Digestion

   There are two kinds of digestion: mechanical and chemical. Mechanical digestion physically breaks the food into smaller pieces. Mechanical digestion begins in the mouth as the food is chewed.


   Chemical digestion uses chemicals produced by your body to break down the food into simpler nutrients that can be used by your cells. Chemical digestion also begins in the mouth when food mixes with saliva. Saliva contains an enzyme (amylase) that begins the breakdown of carbohydrates (sugars and starches). (An enzyme is a protein that can start certain chemical reactions.)


The Digestive Process


   Food is chewed into smaller pieces. Adults have 32 specialized teeth that grind, chew, and tear different kinds of food. The tongue is an organ consisting of skeletal (voluntary) muscles that move food around the mouth to allow for efficient mechanical digestion. Salivary glands secrete saliva that allows for easier swallowing of food and the beginning of chemical digestion.



   Swallowing forces the chewed food through the pharynx to the esophagus (food tube). As food is swallowed, a flap-like valve, the epiglottis, closes over the trachea (windpipe) to prevent food from entering the windpipe and causing choking.  
















   The esophagus connects the pharynx with the stomach. Contractions of the esophagus push the food through a sphincter (a ring of smooth muscle that closes off an opening in the body) and into the stomach.

   The digestive system moves food along by way of peristalsis, a wavelike contraction of smooth (involuntary) muscle.

   Other than the chemical digestion of carbohydrates in the mouth, all digestion to this point has been mechanical.



   The stomach is a muscular and stretchable sac with three important functions:

1)    It mixes and stores food until it can be further digested.
2)    It secretes chemicals that help break the food into more digestible forms.
3)    It controls the passage of food into the small intestine.


   The stomach starts chemical digestion of protein. Secretions from the stomach are very acidic, and they help kill bacteria and other pathogens that may have been ingested.

   A thick mucus also produced by the stomach usually keeps the acids from damaging the stomach lining. If not enough mucus is produced or if too much acid is produced, ulcers form. Heredity, stress, smoking, and excessive alcohol intake can make the ulcers worse. The condition can worsen and bleeding ulcers can result. 


   Food stays in the stomach for approximately 3-4 hours and moves through another sphincter muscle to pass into the small intestine.


Small Intestine

   The small intestine is almost 7 meters long! It is folded and curled around inside the abdominal cavity. The inside surfaces of the intestine are covered with finger-like projections called villi. These structures work to absorb food molecules that have been broken down by chemical digestion.


   Most chemical digestion takes place in the small intestine by chemicals secreted by the liver, pancreas, and small intestine.


Large Intestine

   The large intestine receives the material left over from chemical digestion that is basically nutrient-free. Only water, cellulose, and undigestible materials are left. The main job of the large intestine is to remove water from the undigested material. Water is quickly removed from the material through villi and returns to the bloodstream.  



   The last part of the digestive tract is the rectum, a "holding area” for the undigested material. Waste leaves the body from this area through the anus.


Other Organs of the Digestive System

 The liver has quite a few important functions.

It produces bile (an important chemical in

digestion), it metabolizes fats, proteins and

carbohydrates, and it stores glycogen, vitamins

and minerals.                                  Image:  A human liver.


   The pancreas has three important functions

that help the digestive system change food into

a form that can be used by the cells:

1)    It produces enzymes that help break down

proteins, lipids, and carbohydrates.

2)    It produces the hormone, insulin, which

helps regulate blood glucose levels.

3)    It produces sodium bicarbonate, which helps to neutralize stomach acids.


Digestive Fast Facts:

   The length of your digestive system from your mouth to your anus is about 30 feet long.


   Your small intestine is about 22 feet long while your large intestine is about 5 feet long.


Check for Understanding:

1. What is the function of the digestive system?

2. What is the function of the following parts/organs of the digestive system?

a. mouth—

b. teeth—

c. esophagus—

d. stomach—

e) small intestine—

f) large intestine—

g) rectum—

h) pancreas—

i) liver—

j) gallbladder—


3. What is the difference between mechanical and chemical digestion?


4. Label the following diagram:


































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image of digestive system organs from esophagus to the rectum
diagram of the organs and glands of the digestive system
photo of a liver on a table
diagram of the location of the pancreas showing the surrounding organs
diagram of the disgestive system organs and glands-unlabeled
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