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The HUMAN DIGESTIVE SYSTEM

Eat anything good lately?
Do you know what happens to all that yummy food after it's swallowed?

Well, by the end of this page I hope you can answer "yes" to at least one of those questions.

DIGESTION (di-'jes-chen): the process of making food absorbable by changing it
and breaking it down into simpler chemical compounds

Yeah, all the usable parts of food (called nutrients) can't be used by your body until they are absorbed into the cells & tissues of your body. They can't be absorbed unless they are small & soluble --- soluble means that they can be dissolved.  So the proteins, carbohydrates, & fats in a turkey sub have to be digested, i.e. changed into a form that can be absorbed and used by the body.

Now, I hate to bring-up haunting material from the past, but I have to. Digestion involves the chemical process of HYDROLYSIS (SCREAM NOW).  And the main nutrients in food exist as large proteins, fats, & carbohydrates; which are converted into small amino acids, simple sugars (monosaccharides), and fatty acids.  Yes, it's that biochemistry stuff again (SCREAM AGAIN). Without re-hashing all that chemistry stuff, let me attempt to summarize the main chemical nutrients, their functions, & their small subunits. To make it less painful, let's think about a Turkey sub with lettuce, tomato, peppers, oil, and provolone cheese while we do this ..................

 

For a more extensive review or reinforcement
of the "Chemistry of Life" material,
look at the Biochem Page.

Organs of the Digestive System - Identification

OK, let's get down to it. 
Step #1 - know what each organ looks like.

THE HUMAN DIGESTIVE SYSTEM looks like this:

 

Organs of the Digestive System

anus appendix esophagus gall bladder large intestine (colon) liver mouth (teeth) pancreas pharynx (throat) rectum salivary glands small intestine stomach tongue

See if you can match the numbers with the correct names for each organ.  This is the first step to learning about this system, and, to be honest, it's the easiest (straight memorization).

Click here to see the answers.
 

 

Feeling confident that you have completed step #1?

If "no", study that diagram some more until you can name most of those things pretty easily.
If "yes" ..... ON TO STEP #2 !

Organs of the Digestive System - Their Functions

You can think of the digestive system as two sets of structures.

The first set, called the alimentary canal, consists of the organs that food passes through directly. It is a one-way tube with two openings; the mouth (entrance) and anus (exit). Because scientists LOVE to name the same thing more than once, the alimentary canal is also referred to as the gastrointestinal (GI) tract.

The second set of structures are called the accessory organs. These organs are outside of, but connected to, the alimentary canal. For the most part these organs produce chemicals (digestive enzymes) that are used during the process of digestion.  Food that is being digested DOES NOT directly enter any of the accessory organs.  Instead, the accessory organs/glands produce their chemicals & then the chemicals are released into the alimentary canal where they do their job.
 

ORGANS OF the ALIMENTARY CANAL
Note: The structures are listed in the order in which food passes through them.
 

Organ	Enzymes Present	What Happens Here
mouth	amylase (in saliva)	mechanical digestion of all food (teeth chewing & grinding) chemical digestion of carbohydrates begins
esophagus	none	food passes from mouth through the esophagus to the stomach
stomach	pepsin gastric juice hydrochloric acid (HCl )*	mechanical digestion of food (churning of stomach walls) chemical digestion of proteins begins
small intestine	pancreatic juice intestinal juice bile*	chemical digestion of carbohydrates, proteins, & lipids continues & is completed the small, soluble nutrients (sugars, amino acids, fatty acids) are absorbed
large intestine	none	water is absorbed from unusable, indigestible wastes (feces)
rectum	none	feces are temporarily stored
anus	none	feces are eliminated (egested) from the body

Notes:
 

·  As you can see from the chart, there are two types of digestion, chemical & mechanical.

·         When you read "mechanical digestion", think chewing, churning or grinding. These are physical processes that break food into smaller pieces, but DO NOT change them chemically. The importance of mechanical digestion is that by breaking food into smaller pieces it increases the surface area on which digestive enzymes can react during chemical digestion.

·         Chemical digestion is straight forward: it involves chemicals (digestive enzymes) and the process of hydrolysis. Large chemical nutrients are chemically converted to their smaller subunits. (This was mentioned earlier --- don't remember? Peek back.)

·  The chemicals marked with a "*" are technically NOT enzymes, but they are chemicals that help to break down food.  HCl is hydrochloric acid, which is a fairly strong acid (pH of 3). Bile is a liquid that assists in the digestion of lipids (fats). In fancy terms we say that "bile emulsifies fats", which translates as: bile breaks big globs of fat into smaller globs of fat. Enzymes in the small intestine have an easier time digesting small globs of fat.

·  Notice that digestion only occurs in three organs: the mouth, stomach, and small intestine.

·  The inside of the small intestine is well-adapted for absorbing nutrients.  Hanging from the walls of the small intestine (which is 6.5 meters long!) are thousands of projections called VILLI.

 

·  Inside the villi are two types of vessels, capillaries & lacteals. (The capillaries look like a "web" around the lacteal.) The end products of protein & carbohydrate digestion (namely amino acids & simple sugars) are absorbed into the capillaries. The end products of lipid digestion (fatty acids & glycerol) are absorbed into the lacteals, which are part of the lymph system. The nutrients are then circulated to body cells where they are used for growth, energy, etc.

·  The organs of the alimentary canal are made of smooth muscles, which contract & keep food moving through the long digestive tube. These "waves of contraction" are called peristalsis.

Accessory Organs of the Digestive System  

Accessory Organ	What it does
tongue	helps mix food with saliva, assists in swallowing
salivary glands	produces saliva (which contain the enxymes amylase) amylase chemically digests carbohydrates
liver	produces bile bile EMULSIFIES fats (lipids)
gall bladder	stores bile, then releases into small intestine
pancreas	produces pancreatic juice, which is a mixture of digestive enzymes pancreatic juice ends up in the small intestine where it assists in completing the digestion of proteins, lipids, & carbohydrates

Notes:

·  Another neat structure that is part of the digestive system is the epiglottis. This is a little flap of flesh at the top of your esophagus that covers your windpipe when you swallow, so food goes down the esophagus to the stomach rather than getting lodged in your windpipe & causing you to choke.

·  There is one more flap of flesh to mention.  The appendix is a little deflated-balloon-looking thing that hangs from the large intestine, right around the spot where the small & large intestines meet. It has no function, but scientists think it used to. It's an example of a "vestigial organ", the remains of a once functioning organ from our evolutionary past. You will never pay attention to your appendix unless it becomes infected (appendicitis), in which case you better pay attention to it or it could rupture inside you & cause lots of trouble (even kill you). May your appendix always remain infection-free.

MALFUNCTIONS OF THE DIGESTIVE SYSTEM 

 

Well that's all the nitty-gritty details, time for some Q&A. Do your best.

1. Write the number & name of the organ where the chemical digestion of protein begins.
2. Write the name of the organ where the hydrolysis of carbohydrates begins.
3. Write the number & name of the organ where the absorption of nutrients occurs.
4. Write the number & name of the organ where the absorption of water occurs.
5. In which organ would an ulcer most likely occur ?
6. Write the number & name of the glands that produce amylase.
7. In which organ would you find an acidic pH ?
8. Which organ is lined with villi ?
9. Gallstones effects which organ ?
10. Where is bile produced ?
11. Which two organs produce enzymes that complete the digestion of carbohydrates, lipids, & proteins ?

12. Small lymph vessels that extend into villi are called: a) veins b) lacteals c) capillaries d) gallstones
13. In which organ does peristalsis occur ? a) pancreas b) liver c) oral cavity d) esophagus
14. The importance of mechanical digestion is a) the chemical conversion of food to nutrients b) the synthesis of energy c) the production of enzymes d) the production of more surface area in the food mass
15. The structure in the throat that helps prevent choking is the: a) pharynx b) epiglottis c) trachea d) larynx

16. Name two organs where mechanical digestion occurs.

17. The end products of the chemical digestion of carbohydrates are: a) simple sugars which are used for growth b) amino acids which are used for growth c) simple sugars which are used for energy d) amino acids which are used for energy

18. Which lists digestive structures in the correct order ? a) mouth, esophagus, stomach, small intestine, large intestine, anus b) mouth, stomach, esophagus, small intestine, large intestine, anus c) mouth, esophagus, stomach, large intestine, small intestine, anus d) mouth, esophagus, small intestine, stomach, large intestine
19. Digestion is the process of: a) respiration b) dehydration synthesis c) hydrolysis d) assimilation
20. Compared to ingested food, the end products of digestion are: a) smaller and more soluble b) smaller and less soluble c) larger and more soluble d) larger and less soluble

Check you AnSwErS>
 
 

 

 

ANSWERS

"Organs of the Digestive System"- Identification

1. pharynx (throat)
2. mouth (teeth)
3. tongue
4. stomach
5. pancreas
6. small intestine
7. rectum
8. salivary glands
9. esophagus
10. liver
11. gall bladder
12. large intestine
13. appendix
14. anus

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Digestive System Q & A

1. #4-stomach
2. mouth
3. #6-small intestine
4. #12-large intestine
5. #4-stomach
6. #8-salivary glands
7. #4-stomach
8. #6-small intestine
9. #11-gall bladder
10. #10-liver
11. pancreas (pancreatic juice) & small intestine (intestinal juice)
12. b-lacteals
13. d-esophagus
14. d-increases surface area
15. b-epiglottis
16. mouth (chewing) & stomach (churning)
17. c-simple sugars which are used for energy
18. a-mouth, esophagus, stomach, small intestine, large intestine, anus
19. c-hydrolysis
20. a-smaller and more soluble

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