1. Anatomy and Function of the Digestive System
Partnership for Environmental Education and Rural Health
Texas A&M University
peer.tamu.edu

2. Nutrition versus Digestion
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed.
Recall the definition of nutrition.
Using these definitions, describe the relationship between nutrition and digestion and then share your answer with a neighbor.
Nutrition is the process by which an animal takes in and utilizes food substances through various chemical reactions and physiological processes.
Therefore digestion is the method or process through which animals obtain nutrition.

3. The Digestive System
How many structures of the digestive system can you name?
Images -
Mouth, teeth, tongue, esophagus, stomach, small intestine, pancreas, liver, large intestine, cecum, colon, rectum

4. Functions
Digestion - breaking down food by mechanical and enzymatic action in the stomach and intestines into substances that can be used by the body.
Absorption
Protection from microorganisms & noxious substances
Remove undigested material & waste
Immunological function
Metabolism
Endocrine functions

5. Types of Digestion
Mechanical Digestion- the physical break down of food into smaller pieces without changing its chemical structure.
Increases the surface area of food particles
Chemical Digestion- food is chemically broken down; its chemical structure is changed.
Enzymes and acids
Can you think of places where this might happen?
Answer: mechanical - teeth grinding, tongue turning, muscles in esophagus, stomach, and intestines squeezing.
chemical – stomach, small intestine, pancreas, liver - acid and enzymes

6. Break It Down
What types of digestion are occurring in the following images?
Puppy – mechanical by chewing and chemical with saliva
Stomach – mechanical by churning and chemical with acid and enzymes

7. Digestion Begins Prehensile organs (lips, tongue) Mouth
Move food to the mouth
Mouth
Taste – accept or reject food
Mechanical digestion – mastication (chewing)
Mix digesta (food) with saliva

8. Saliva Lubricates food with mucoproteins Initiates enzymatic digestion
Salivary amylase – breaks down carbohydrates
Salivary lipase – breaks down lipids
Buffers stomach – prevents pH from rapidly changing
Salivary glands

9. Esophagus Muscular tube from pharynx to stomach
Peristalic (wave-like) contractions propel food down to stomach
Made of striated and smooth muscle
Ruminants have striated muscle throughout in order to regurgitate food.

10. Simple Stomach
Muscular bag that mixes food by peristalic contractions & holds for gradual release to small intestine
Digests proteins
Denatures (breaks apart) with hydrochloric acid
Enzymatic digestion with pepsin
Produces mucus to protect from acids
pH of stomach is about 2
With out mucus develop ulcers
Absorption of alcohol and volatile fatty acids

11. Photo from: http://www.rivm.nl/interspeciesinfo/intra/human/stomach/
Cardia region – produces mucus
Body region – produces mucus enzymes and hydrochloric acid

12. Ruminants What makes the ruminant stomach unique?
Identify the ruminant animals from the following images.
The ruminant stomach has 4 compartments: the reticulum, rumen, omasum, and abomasum
Ram and cow are ruminants

13. Ruminant Stomach Occupies nearly ¾ of the abdominal cavity 4 chambers
Reticulum – formation of bolus (cud) and regurgitation
Rumen - microbial digestion of feed and production of volatile fatty acids
Omasum – water and inorganic mineral absorption
Abomasum – “true stomach”, secretes acids and enzymes
The function of the omasum is poorly understood and this is currently the most popular theory on its function
The abomasum must process large quantities of bacteria and so secretes lysozyme which breaks down bacterial cell walls.

14. Pictures from: http://www. wisconsincentral

15. Review Break #1: What are the functions of the digestive system?
Give an example of the two types of digestion that occur in the digestive system.
What is the function of saliva?
How does food move down the esophagus?
Which part of the ruminant stomach is most like a simple stomach? Why?

16. Folds Upon Folds What organ do you think is shown here?
What might be the purpose of the villi?
Pictures from:
Small intestine villi
Villi contain extensive circulatory and lymphatic system components to transport nutrients following absorption
Microvilli are the microscopic projections from the cells of the villi

17. Small Intestine
Primary site for enzymatic digestion and nutrient absorption
Divided into 3 regions:
Duodenum – most digestive enzymes enter here
Jejunum
Ileum
Folded and lined with villi and microvilli – increases the surface area (therefore absorption efficiency) 600 fold

18. Pancreas, Liver and Gallbladder
Pancreas – adds enzymes and buffers to the duodenum
Enzymes digest starches, fats, and proteins
Produces insulin and glucagon
Liver – produces bile
Emulsifies large fat globules into a suspension of fine droplets for enhanced digestion
Gallbladder – stores bile
Horses have no gallbladder, bile is secreted directly into the stomach
Buffers keep the pH of the small intestine around 5 – 7; if the pH is too low digestability decreases
Insulin regulates glucose metabolism and glucagon opposes insulin
Fine droplets of fat are soluble in aqueous solutions

19. click here for more information on the liver and pancreas

20. Why is the term “large” used to describe the large intestine?
Image from
Although the large intestine is much shorter than the small intestine its diameter is much greater.

21. Cecum The start of the large intestine Large in herbivores
Contains bacteria that aid in the enzymatic digestion of cellulose
cecum

22. Large Intestine Composed of the cecum, colon, and rectum
Bulks up and stores material
Absorbs water, volatile fatty acids, and some minerals
Site of microbial digestion
Digestion of cellulose producing volatile fatty acids – particularly important in non-ruminants
Synthesis of vitamin K and certain B vitamins
Animals cells do not produce the enzyme cellulase and so are unable to digest cellulose (a major component of plants) on their own. Bacteria in the large intestine produce cellulase allowing herbivores to gain energy from plant material.
The horse has a significant bacterial population in its large intestine (hindgut) providing as much as 60-75% of its nutrient requirements from hindgut fermentation (if fed high amounts of fiber).

23. Review Break #2:
Where does the majority of digestion and absorption of nutrients occur?
What are the functions of the liver, pancreas, and gallbladder?
What are villi and what is their purpose?
Why is the large intestine so important to non-ruminants?

24. Fun Digestive Facts:
The domesticated dog’s digestive system is similar to a human's; except, the dog's intestinal tract is shorter, due to the amount of protein dogs consume.
Cows produce about 100 liters of saliva a day, humans produce about 1.5
Sheep and goats produce gas at the rate of 5 liters per hour

25. Summarize it!
Take time to summarize the structure and function of the digestive system.
With a partner, go through each organ of the digestive system and tell its location and function. Be sure to say whether mechanical, chemical, or both types of digestion occur there.
Use the diagram in your notes or your flow chart to help!

26. References Digestive Physiology of Herbivores Applied Animal Nutrition
Feeding the Endurance Horse