1. The Human Digestive System

2. The Digestive System
The digestive system: is a group of organs that take in food and change it into a form the body can use.
Digestion: is the changing large molecules into smaller ones capable of being used by the cells in the body.
Cells need food for energy, growth and repair.
Can be compared to a factory – materials are transported to different locations and are changed at every stop.
The final product is quite different than the original.

3. Nutritional Requirements
Humans need to take in food for:
fuel = chemical energy
raw materials = carbon source
essential nutrients = animals cannot make
elements (N, P, K, Fe, Na, K, Ca...), NAD, FAD, etc.

4. Getting & Using Food Ingest taking in food Digest Physical digestion
breaking up food into smaller pieces
chemical digestion
breaking down food into molecules small enough to be absorbed into cells
enzymes
Absorb
absorb across cell membrane
diffusion
active transport
Eliminate
undigested extracellular material passes out of digestive system

5. Physical vs. Chemical Change
Food gets broken down as it passes through the digestive system in 2 ways
1. Physical Change:
Large pieces are broken down into smaller ones.
The food is still in the same form (only the size and shape of the food particles change).
Occurs through chewing, grinding and mixing.
2. Chemical Change:
This occurs when food changes form so the body may use it.
Body chemicals are added to food to make this happen.
These chemicals are called enzymes.

6. Digestion Digestion Chemical Change Physical Change
Food is broken down into small pieces
Food is not in final form
Chemical Change
Food is changed to a new form
Food is in final form for cells use

7. Digestive System
Water, vitamins, and minerals can be used by cells so do not need to be changed
Fats, proteins, and carbohydrates cannot be used so they need to be broken down by enzymes
Enzymes: are chemicals that speed up the rate of change
There are different kinds of enzymes for different nutrients.

8. Human digestive system Alimentary Canal
After chewing and swallowing, it takes 5 to 10 seconds for food to pass down the esophagus to the stomach, where it spends 2 to 6 hours being partially digested.
Final digestion and nutrient absorption occur in the small intestine over a period of 5 to 6 hours.
In 12 to 24 hours, any undigested material passes through the large intestine, and feces are expelled through the anus.

9. Digestive System Main organ passageway:
1. Mouth (includes salivary glands)
2. Esophagus
3. Stomach
4. Small intestine
5. Large intestine
6. Anus
Additional organs necessary:
Pancreas, liver, and gallbladder all secrete enzymes into the small intestine.

10. Common processes & structures
Movement & Control
peristalsis
push food along by rhythmic waves of smooth muscle contraction in walls of digestive system
sphincters
muscular ring-like valves, regulate the passage of material between sections of digestive system
Accessory glands
salivary glands, pancreas, liver & gall bladder
secrete digestive juices (enzymes & fluid)

11. Swallowing (& not choking)
Epiglottis
problem: breathe & swallow through same orifice
flap of cartilage
closes trachea (windpipe) when swallowing
food travels down esophagus
Esophagus
move food along to stomach by peristalsis (pushing of food along by waves of smooth muscle contractions)

12. Ingestion Mouth physical digestion chemical digestion teeth saliva
breaking up food
chemical digestion
saliva
amylase
enzyme digests starch
mucin
slippery protein (mucus)
protects soft lining of digestive system
lubricates food for easier swallowing
buffers
neutralizes acid to prevent tooth decay
anti-bacterial chemicals
kill bacteria that enter mouth with food

13. mouth
break up food
moisten food
digest starch
kill germs

14. Stomach Functions food storage disinfect food chemical digestion
can stretch to fit ~2L food
disinfect food
HCl = pH 2 so kills bacteria
Has a mucus layer to protect itself from enzymes and HCl
chemical digestion
pepsin
enzyme breaks down proteins
After the stomach completes its part in digestion the food is now in a liquid form called chyme.
Still, the epithelium is continually eroded, and the epithelium is completely replaced by mitosis every three days.
Gastric ulcers, lesions in the stomach lining, are caused by the acid-tolerant bacterium Heliobacter pylori.
Ulcers are often treated with antibiotics.
Pepsin is secreted in an inactive form, called pepsinogen by specialized chief cells in gastric pits.
Parietal cells, also in the pits, secrete hydrochloric acid which converts pepsinogen to the active pepsin only when both reach the lumen of the stomach, minimizing self-digestion.
Also, in a positive-feedback system, activated pepsin can activate more pepsinogen molecules.
But the stomach is made out of protein!
What stops the stomach from digesting itself?
mucus secreted by stomach cells protects stomach lining

15. Stomach
A large muscular bag that has gastric juice (juices given off by the lining of the stomach) that mixes and digests protein. The gastric juice is made of an emzyme (pepsin) and HCl that digests protein.
The walls are muscular and thick that churns and grinds up the food. Once the food is mixed with the digestive juices, it starts to look like a milk shake.
The partially digested food is squeezed out of the stomach in spurts and moves to the small intestine.
Food usually stays in the stomach for 4 – 7 hours.

16. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
store food
break up food
digest proteins
cardiac sphincter
pyloric sphincter

17. Small Intestine Function major organ of digestion & absorption
chemical digestion
digestive enzymes
absorption through lining by diffusion
over 6 meters in length!
small intestine has huge surface area = 300m2 (~size of tennis court)
Structure
A hollow tube with finger-like projections called villi.
Each villus contains blood vessels that carry digested food.
From the blood vessels the food is carried to body cells.
About every 20 seconds, the stomach contents are mixed by the churning action of smooth muscles.
As a result of mixing and enzyme action, what begins in the stomach as a recently swallowed meal becomes a nutrient-rich broth known as acid chyme.
At the opening from the stomach to the small intestine is the pyloric sphincter, which helps regulate the passage of chyme into the intestine.
A squirt at a time, it takes about 2 to 6 hours after a meal for the stomach to empty.

18. Duodenum 1st section of small intestines
acid food from stomach mixes with digestive juices from accessory glands:
pancreas
liver
gall bladder

19. Pancreas Makes enzymes to digest: Fats Proteins Carbohydrates
These enzymes pass from the pancreas to the small intestine through a small tube.
Pancreas also produces insulin to regulate sugars.

20. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
break up food
digest proteins
store food
pancreas
produces enzymes to
digest proteins, starch &
fat

21. Liver produces bile Digestive System Functions Stores unused sugar
stored in gallbladder until needed
breaks up fats
Stores unused sugar
Circulatory System Connection
bile contains colors from old red blood cells collected in liver =
iron in RBC rusts & makes feces brown

22. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
break up food
digest proteins
store food
liver
produces bile
- stored in gall bladder
break up fats
pancreas
produces enzymes to
digest proteins, starch &
fats

23. Digestive Enzymes Carbohydrate Digestion: Protein Digestion:
Mouth
Salivary glands
Small Intestine
Pancreas
Protein Digestion:
Stomach
Enzymes and HCl
Fat Digestion:
Liver
Bile

24. Digestive enzymes

25. Absorption by Small Intestines
Absorption through villi & microvilli
finger-like projections
increase surface area for absorption

26. Absorption of Nutrients
Occurs through diffusion.
Final forms of nutrients:
Carbohydrates (starches and sugars) = glucose
Protein = amino acids
Fats = glycerine and fatty acids

27. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
break up food
digest proteins
store food
liver
produces bile
- stored in gall bladder
break up fats
small intestines
breakdown all foods
- proteins
- starch
- fats
- nucleic acids
absorb nutrients
pancreas
produces enzymes to
digest proteins & starch

28. Large intestines (colon)
Function
re-absorb water
use ~9 liters of water every day in digestive juices
> 90% of water reabsorbed
not enough water absorbed back to body
diarrhea
too much water absorbed back to body
Constipation
No nutrients are absorbed here

29. Rectum salts masses of bacteria
Last section of colon (large intestines)
eliminate feces
undigested materials
extracellular waste
mainly cellulose from plants
roughage or fiber
salts
masses of bacteria
The study of the rabbit is fascinating, and from periods of quiet observation we learn some of the peculiarities of its life and habits. One of the most interesting of these is coprophagy. The word comes from the Greek kopros (dung) and phago (eating). This dung eating is not quite so revolting as it sounds at first, for the rabbit makes a special form of pellet which it takes directly from its anus. Coprophagy plays an important part in the digestive/nutritional process.
This practice involves ingestion of special soft fecal pellets which are excreted in the early morning hours. This is a significant practice in that the bacterial synthesis of certain B vitamins in the cecum are excreted at this time and if rabbits are prevented from this practice they will die from vitamin B deficiency within a rather short period of time.
The special soft pellets are produced at night or during periods of rest and are often called "nocturnal pellets" to distinguish them from the fecal pellets excreted at other times. The process has a distinct analogy with the chewing of the cud by ruminants.
Like the cow, rabbits are herbivorous and their diet contains a high proportion of crude fiber. The cellulose of the fiber has to be broken down before complete digestion and absorption can take place. The rabbit has a comparatively large caecum and colon to facilitate this. In order to obtain the maximum nutriment from its food the rabbit has developed the habit of coprophagy, passing certain of its intestinal contents through the system twice.
In addition to the improved nutrition, it is possible that the soft pellets fulfill a need to give greater bulk to the stomach contents. The rabbit's stomach and intestines are geared to bulk supplies and under some conditions the diet may lack bulk. The stomach has a comparatively poor muscular action and relies to a great extent on the pressure of successive meals to push the mass of food along the digestive tract.
The composition of the two types of pellets is interesting, the soft pellets having much more protein and less crude fiber. The process is controlled by adrenal glands.

30. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
break up food
digest proteins
store food
liver
produces bile
- stored in gall bladder
break up fats
small intestines
breakdown food
- proteins
- starch
- fats
absorb nutrients
pancreas
produces enzymes to
digest proteins & carbs
large intestines
absorb water

31. Appendix
Vestigial organ

32. Problems with the Digestive System
Ulcers
A sore or hole inside either the stomach or small intestine.
Caused by the stomach lining being digested or “eaten away” by enzymes and stomach acids.
The mucus layer helps prevent this but this can be damaged by a bacterial infection called Helicobacter pylori thus resulting in an ulcer.
Heartburn
Caused by stomach acids moving into the esophagus.
This causes a burning feeling.
The esophagus is lies behind the heart so it feels like the heart is burning.
Eating too much at once or too much acidic or rich foods can cause acid to back up.

33. Digestive System - Glands
Salivary glands – secrete amylase for preliminary starch digestion.
Glands in the stomach lining – secrete mucus and gastric juice (contains pepsin and hydrochloric acid) for initial protein digestion.
Liver – secretes bile to emulsify fats.
Pancreas – secretes sodium bicarbonate to neutralize stomach acids and enzymes to act on proteins (proteases), carbohydrates (amylase) and fats (lipases).
Walls of the small intestine – secrete juices that complete the digestion of carbohydrates, fats and proteins.

34. Lab Activity Stations

35. Activity 1: How long is the Digestive System?
Materials: 4 different balls of yarn Description: Students are to cut pieces of different coloured yarn for each of the following measurements. Then they are to tie them together to show how long each piece of the digestive system is in reference to each other. Esophagus 25 cm Stomach 20 cm Small Intestine 700 cm Large Intestine 150 cm

36. Activity 2: Digestion Materials: Description:
Sugar Cubes
Granulated Sugar
Clear cups filled with water
Description:
Place a sugar cube in a cup of water. Place about a teaspoon full of granulated sugar in the other cup of water. Observe.

37. Activity 3: Carbohydrate Digestion
Materials: Unsalted crackers Description: Students chew 2 unsalted crackers for two minutes without swallowing.

38. Activity 4: Hands on Digestion
Materials:
Meatball size of hamburger
One plastic bag
1 M HCL
Digestive juice A (pepsin, trypsin, and water)
Digestive juice B (bile salts, pancreatin enzyme and water)
Description: Place hamburger, 3 eyedroppers full of HCL, 1 tbsp of Digestive juice A and 2 Tbsp of Digestive juice B in the plastic bag. Knead with your hands for 10 minutes (stimulates the stomach) and it will have been reduced to liquid with a definite odor.

39. Activity 5: How do villi aid the Small Intestine in absorption?
Materials:
Paper towels
4 cups of equal amounts of water
Graduated cylinder
Description: Compare how 1,2,3, then 4 paper towels absorb. Dip each paper towel into a cup of water (same amount of water in each cup). Record the volume of water left in each cup (using the graduated cylinder).

40. Activity 6: The Digestive System Story
Description: Assign each student as a part of the digestive system and one student as food. As the food student moves past each digestive system part, the student labelled as that part must describe what they are doing to the food.

41. Questions:
What system in your body is the same length as the completed piece of yarn?
In Activity 2, what can you conclude must be done to food before digestion occurs?
What physical and chemical changes occurred to the soda crackers?
What caused the physical and chemical changes to the crackers?
Did you notice a taste change in the crackers?
How is physical digestion simulated in Activity 4?

42. Questions continued
What evidence was there that chemical digestion occurred in the hamburger?
Which paper towel had the largest surface area?
What relationship can you see between the largest surface area and how villi aid in absorption?
Using Activity 6, create a diagram to demonstrate the passing of food through the digestive system.