1. The Digestive System
Part 2

2. The Digestive System Cont’d…
Learning Goal: To continue exploring the digestive system of humans.
Success Criteria:
I know I am succeeding when I can…
explain that most digestion takes place in the duodenum in the small intestine through the action of bile, hormones, and many pancreatic and liver enzyme
explain that villi and microvilli greatly increase the surface area of the small intestine for absorption
explain the roles of the jejunum, ileum and colon, rectum, and anus

3. The Small Intestine
most digestion and absorption of nutrients takes place in the small intestine
can be up to 7 m in length
lipids and carbohydrates, as well as any remaining proteins, are digested in the small intestine

4. The Small Intestine Cont’d…
three sections of the small intestine:
duodenum  where most enzymes are added and digestion takes place
jejunum  digestion continues and some nutrients are absorbed
Ileum majority of nutrients are absorbed

5. The Small Intestine Cont’d…
the inner surface of the small intestine is adapted to provide the maximum surface area for efficient nutrient absorption
the inner layer of the small intestine is folded into ridges and has many small finger-like projections called villi (singular: villus)
each of the epithelial cells that make up the villi has even smaller, microscopic projections of the cell membrane called microvilli

6. The Small Intestine Cont’d…
within each villus is a network of tiny blood vessels called capillaries
all nutrients, except digested fats, enter the bloodstream through the capillaries
digested fats are transported through small vessels called lacteals, lymphatic system, and from there into the bloodstream

7. The Small Intestine Cont’d…
the pyloric sphincter periodically relaxes to release small portions of chyme into the duodenum
slow and steady release of chyme into the small intestine prevents overloading and allows time for thorough digestion
most of the enzymes required for digestion are added in the duodenum and require the accessory organs

8. The Pancreas
the pancreas is a long, flat gland between the stomach and the duodenum
secretes:
digestive enzymes
hormones that regulate the absorption and storage of glucose from the blood

9. The Pancreas Cont’d…
digestion of carbohydrates (i.e. starch) continues in the duodenum
the enzyme amylase (found in saliva) is also secreted by the pancreas

10. The Pancreas Cont’d…
a hormone called cholecystokinin (CCK) is secreted by duodenum and released into the bloodstream
signals the pancreas to secrete a variety of substances, including ones that control the pH of the intestine and enzymes that are needed for lipid, carbohydrate, and protein digestion
these secretions enter the duodenum through the pancreatic duct
CCK also signals the stomach to slow down the speed of digestion so that the small intestine can effectively digest the fats

11. The Pancreas Cont’d… chyme is acidic (pH of about 2.5)
when chyme enters the small intestine, it converts prosecretin into its active form, secretin
Sercretin stimulates ….
the liver to make more bile
the pancreas to secrete lipid and protein enzymes
the pancreas to release bicarbonate ions (HCO3-) to neutralize the chyme (pepsin is inactivated which protects the small intestine from stomach acids)
secretin also prevents more food from entering the duodenum

12. The Pancreas Cont’d…
pancreas releases trypsinogen and when it reaches the duodenum, an enzyme called enterokinase converts it into active trypsin
trypsin breaks down any partially digested proteins that remain
other protein-digesting enzymes help break the short protein chains into single amino acid molecules

13. The Pancreas Cont’d…
the pancreas also secretes lipases which is a group of enzymes secreted by the pancreas that digest lipids (fats) in the small intestine
lipases break the lipid chains into shorter chains and into individual fatty acid molecules
BUT fats in chyme are present as large globules and for lipases to work they need the fat to be in small globules

14. The Liver
the liver is the largest internal organ of the body, located just underneath the diaphragm

15. The Liver Cont’d… functions
produces and secretes bile (emulsifies fats, breaking them into tiny droplets called micelles)
produces and stores various nutrients (i.e. glycogen and fat soluble vitamins)
removes and breakdowns toxins in our blood, such as alcohol, that have been absorbed by the digestive system

16. The Gall bladder
bile is stored in the gall bladder until food enters the duodenum
lipids entering the duodenum stimulate the gall bladder to contract, which causes bile to be squeezed out from the gall bladder into the duodenum through the bile duct

17. Absorption in the Small Intestine
once proteins, carbohydrates, and lipids are broken down, they are absorbed in the jejunum and ileum
vitamins, minerals, and water are also absorbed in the small intestine
nutrients pass through into the bloodstream, where they are then transported by capillaries to the tissues of the body

18. The Large Intestine
undigested food (the waste) is passed through the large intestine and water, vitamins and ions are absorbed
the large intestine is only 1.5 m in length but is two to three times larger in diameter than the small intestine

19. The Large Intestine Cont’d…
the large intestine consists of the following:
cecum: pouch that receives processed material from the small intestine
colon: the longest part of the large intestine and has four segments (the ascending colon, the transverse colon, the descending colon, and the sigmoid colon)
rectum: holds the waste products of digestion
anus: external opening that eliminates waste
The small intestine does not simply continue on and become the large intestine.

20. Egestion
the absorption of water in the large intestine changes the liquid material in the colon into a soft solid called feces
nerves in the wall of the large intestine detect the movement of feces into the rectum which initiates the defecation reflex
the anus is surrounded by two sphincter muscles one of which is under voluntary control
feces are eliminated through the anus when both sphincters are relaxed
The small intestine does not simply continue on and become the large intestine.

21. The Digestive System Cont’d…
Learning Goal: To continue exploring the digestive system of humans.
Success Criteria:
I know I am succeeding when I can…
explain that most digestion takes place in the duodenum in the small intestine through the action of bile, hormones, and many pancreatic and liver enzyme
explain that villi and microvilli greatly increase the surface area of the small intestine for absorption
explain the roles of the jejunum, ileum and colon, rectum, and anus

22. Homework
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