Week 11 - Healthy Human Body Systems

3 The Digestive System

Lesson 11

Science in Action 8
Pages 126-130

ScienceFocus 8
Pages 149-150

Lesson 11: The Digestive System

Like all living organisms we need energy to carry on our day to day activities. At peak times of activity, like these women playing rugby, large amounts of energy are needed by our cells over a short period of time. So it is important that we eat a well balanced diet that includes proteins, carbohydrates and lipids.

But the food we eat is much too large to be useful to our cells. The purpose of the digestive system is to break down the food we eat into smaller pieces that can be carried by the circulation system and used by our cells.

Background Information
There are two types of digestion; mechanical digestion and chemical digestion .

Mechanical digestion involves actions that physically break food apart. Activities such as chewing and the muscular movement of your stomach are considered to be forms of mechanical digestion. As food is torn apart into smaller pieces the surface area is increased. This allows digestive juices to work faster.

Chemical digestion involves the use of enzymes and acid to chemically break down large molecules into smaller ones. For example, chemical digestion starts in the mouth and as your saliva begins the breakdown of starch in to sugar.

Now its time to watch an animated video on digestion.
First log in using: User Id: argyllbp
Password: brainpop
Then type in the word "digestion" and select proper video.

Try This!
Find a cracker and scrap off all the salt. If its an unsalted cracker all the better. Chew the cracker but do not swallow it.

After one minute how does it taste?

After five minutes
how does it taste?

What do you think happened to the cracker as you chewed it over the five-minute period?


Examine the diagram below. By the end of this lesson you should be able to identify the major parts of the digestive system.

(Its on your unit test.)

Both your trachea and your esophagus begin at the back of your throat. Swallowing food into your trachea would not be a good idea. Why? (Think back to the lesson on respiration.) There is a small flap of skin, called the epiglottis , which closes over the trachea as you swallow.

The food is then directed to the esophagus. What do you think causes the food to move down into the stomach? Perhaps its by the force of gravity. But astronauts live in the gravity free environment on the space shuttle and they can still eat, so it must be something else. Your digestive system uses the muscles along the digestive pathway to push the food along in waves. This pushing action is called peristalsis .

Both chemical and mechanical digestion occur in the stomach . While muscles squeeze and churn the food, enzymes and acid are added. This combination of actions causes the food to turn to the consistency of a milkshake. But if you have ever gotten sick to your stomach, you know that it neither looks nor tastes like a milkshake! The strong acid breaks down proteins like meats and milk products.

But the stomach is made of proteins! How does it keep from being eaten away? Hmmmm, what did you learn about the stomach in the movie that prevented this problem?

After 2 to 3 hours this ground-up food now enters the small intestine . Immediately, it is mixed with enzymes from the pancreas and gallbladder and any fats that you ate are further broken down.

The inside of the small intestine is covered with very small finger-like projections called villi . These villi increase the surface area for absorbing food. By the time the food particles reach the villi in the small intestine they are small enough to be able to be absorbed through the thin walls of the villi and into the blood stream. This process is called absorption should sound familiar, as it is very similar to gas exchange in the lungs.

Any food particles that aren't absorbed in the small intestine are passed along to the large intestine . Here, any remaining water, minerals and vitamins are absorbed and the remaining material compacted and stored in the rectum as feces until you go to the bathroom.

Exercise 3.3: The Digestive System

1. Explain the major purpose of the parts of the digestion system by following a sandwich down your digestive tract. Use the following vocabulary below to guide your journey: 
teeth, tongue, salivary glands, epiglottis, peristalsis, esophagus, stomach, small intestine, villi, large intestine and rectum.
2. What is the relationship between the digestive system and circulatory system?
3. Describe the similarities and differences between the processes that take place in the alveoli and villi.
4. What do you think might happen if your epiglottis didn't work fast enough while you were swallowing?
5. What would happen if peristalsis was reversed and moved from the stomach to your mouth?
6. What keeps the stomach from being eaten away by acid?

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