1. Functions of the Digestive System

2. Overview of GI processes and Controls
The major functions of the digestive tract are usually summarized in 2 words: Digestion and Absorption
The essential activities of the GI tract include:
Ingestion
Propulsion
Physical food breakdown
Chemical food breakdown
Absorption
Defecation

3. Ingestion/ Propulsion
Ingestion: food is placed into the mouth, this is voluntary a process
Propulsion:
Peristalsis is involuntary and involves alternating waves of contraction and relaxation of the muscles in the wall of esophagus
In segmentation, single segments of the intestine alternately contract and relax: the food is mixed rather than propelled along the tract

4. *Don’t try this, it’s uncomfortable *
Peristalsis
Because of peristalsis, you can eat upside down against the forces of gravity -- the muscles of the esophagus will propel food down towards stomach
*Don’t try this, it’s uncomfortable *

5. Question → What do you think retroperistalsis is?
reverse of the involuntary smooth muscle contractions of peristalsis.
usually occurs as a precursor to vomiting

6. Mechanical Breakdown In the mouth by the tongue
In the stomach by churning
In the small intestine by segmentation

7. Chemical Breakdown
The process by which large molecules are broken down to their building blocks:
3 macromolecules:
1) Carbohydrates
2) Proteins
3) Lipids

8. Chemical Breakdown to Building Blocks
Carbohydrate : Monosaccharide
We can digest carbs of sucrose
(table sugar), lactose (milk sugar)
maltose and starch
Although we eat foods containing other polysaccharides, such as cellulose (cabbage), we do not have enzymes that break them down, but they help move food along the GI tract and prevent constipation
start here anatomy 2

9. Chemical Breakdown to Building Blocks
2. Proteins: Amino Acid
protein
3. Lipids: Fatty acid and glycerol
start here, anatomy 1

10. Absorption/Defecation
Absorption: The transport of digested end products from the lumen (innermost) of the GI tract to the blood or lymph
The small intestine is the major absorptive site
Defecation: The elimination of indigestible substances from the body in the form of feces

11. Control of the digestive system
Digestive activity is mostly controlled by reflexes via the parasympathetic division of the autonomic nervous system
The sensors involved in these reflexes are located in the walls of the alimentary canal organs and respond to a number of stimuli:
Stretch of the organ by food in the lumen
pH of the contents
Presence of certain breakdown products of digestion

12. Control of the digestive system
When the receptors are activated, they start reflexes that activate or inhibit:
The glands that secrete digestive juices into the lumen or hormones into the blood
The smooth muscles of the muscularis that mix and propel the foods along the tract
What are these processes called again?

13. Mouth, pharynx and esophagus
Food ingestion and breakdown:
Food is physically broken down into smaller pieces by chewing
As food is mixed with saliva, salivary amylase begins the chemical digestion of starch, breaking it down into maltose
What is amylase again?
No food absorption occurs in the mouth
The pharynx and esophagus have no digestive function

14. Mouth, pharynx and esophagus
Food propulsion:
Deglutition, or swallowing, is a complicated process with 2 major phases:
The voluntary buccal phase: (occurs in the mouth) chewing and mixing
The involuntary pharyngeal-esophageal phase: Transport food through the pharynx and esophagus

15. Activities of the Stomach
Gastric juice: secreted by the stomach glands, it is regulated by both neural and hormonal factors:
Gastrin: a hormone that helps the stomach glands to produce more protein digesting enzymes -- the presence of food and falling pH in the stomach stimulates production of gastrin
Hydrochloric Acid: makes stomach contents very acidic, converts pepsinogen (made by stomach walls) to pepsin, the active protein digesting enzyme
What does “pepsin” remind you of?

16. Stomach Related Diseases
Ulcers: The digestion of the stomach itself caused by insufficient mucus production
Can be caused by too much aspirin, poor lifestyle, or
H. pylori bacteria -- treatment includes limited use of drugs, alcohol, highly processed foods, no smoking, low stress

17. Stomach Related Diseases
Heartburn/Acid Reflux: The inflammation of the esophagus
Caused by excess stomach
acid or poor closing of
esophageal sphincter (leakage into esophagus)
start here anatomy 2
Treatment → eating an earlier dinner, avoiding trigger foods (spicy, acidic), taking antacids, raising the head of your bed

19. Stomach Related Diseases
Hiatal Hernia: A structural abnormality where the superior part of the stomach protrudes above the diaphragm
Caused by weakened diaphragm muscle, lifting heavy objects often, persistent coughing/vomiting
Treatment→ medication/surgery

20. Activities of the Small Intestine
The microvilli of small intestine have important enzymes:
brush border enzymes that break down double sugars into simple sugars and complete protein digestion
Food entering the small intestine are flooded with pancreatic enzymes

21. Pancreatic Enzymes Pancreatic juice contains enzymes that:
Along with brush border enzymes, complete the digestion of starch (pancreatic amylase)
Carry out about half of protein digestion by protease enzymes ( trypsin, chymotrypsin, carboxypeptidase)
Help with fat digestion → pancreas creates lipases
Digest nucleic acids (nucleases)
stop here anatomy 1

22. Activities of the Small Intestine
When chyme enters the small intestine, it stimulates the mucosa cells to produce several hormones:
secretin: causes the liver to increase its output of bile (think of “secrete”)
cholecystokinin (CCK): causes the gallbladder to contract & release stored bile into the bile duct. Result → bile & pancreatic juice enter the small intestine together
start here anatomy 1

23. Activities of the Small Intestine
Most substances:
are absorbed by active transport → explain
then they enter the capillary bed in the villi to be transported in the blood
Lipids:
Are absorbed by diffusion → explain
Then they enter both the capillary bed & lacteal in the villi → carried to the liver by blood and lymphatic fluid

24. Activities of the Large Intestine
The colon itself produces no digestive enzymes
The bacteria that live within the lumen:
metabolize some of the remaining nutrients, releasing gases
Produce vitamins ( vitamin K and B)
What type of biological relationship is this?
Symbiosis → Mutualism

25. Activities of the Large Intestine
Two major types of propulsive movements:
Colon peristalsis: sluggish and contribute very little to propulsion
Mass movements: slow-moving but powerful contractile waves that move over large areas of the colon and force the contents toward the rectum

26. Activities of the Large Intestine
The rectum is generally empty, but when feces are forced into it, the wall is stretched → defecation reflex is initiated:
It is a nerve (spinal) reflex that causes the walls of the sigmoid colon & rectum to contract and the anal sphincters to relax