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 break down
Chemical food break down
Absorption
Defecation

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

4. Mechanical Breakdown Mechanical breakdown happens:
In the mouth by the tongue
In the stomach by churning
In the small intestine by segmentation

5. Chemical Breakdown
The process of which large molecules are broken down to their building blocks:
Building blocks of the 3 macromolecules:
Carbohydrate : Monosaccharide
The only carbohydrates that our digestive system is able to digest are sucrose (table sugar), lactose (milk sugar), maltose and starch
Although we eat foods containing other polysaccharides, such as cellulose (found in vegetable), we do not have enzymes that break them down, thus they have no nutritional value, but help move the foodstuffs along the GI tract

6. Chemical Breakdown
2. Proteins: Amino Acid 3. Lipids: Fatty acid and glycerol

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

8. 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

9. 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

10. Activities occurring in the mouth, pharynx and esophagus
Food ingestion and breakdown:
First the food is physically broken down into smaller particles by chewing
As the food is mixed with saliva, salivary amylase begins the chemical digestion of starch, breaking it down into maltose
No food absorption occurs in the mouth
The pharynx and esophagus have no digestive function

11. Activities occurring in the mouth, pharynx and esophagus
Food propulsion:
Deglutition, or swallowing, is a complicated process, it has 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

12. Activities of the Stomach
Gastric juice: secreted by the stomach glands, it is regulated by both neural and hormonal factors:
Gastrin: the presence of food and falling pH in the stomach stimulates the stomach to produce the hormone gastrin to help the stomach glands to produce more protein digesting enzymes
Hydrochloric Acid: makes the stomach contents very acid, activates pepsinogen to pepsin, the active protein digesting enzyme

13. Stomach related diseases
Ulcers: The digestion of the stomach itself caused by insufficient mucus production
Heartburn: The inflammation of the esophagus, caused by the back flow of the gastric juice into the esophagus
Hiatal Hernia: A structural abnormality in which the superior part of the stomach protrudes slightly above the diaphragm

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

15. 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 (via the action of trypsin, chymotrypsin, carboxypeptidase and others)
Are totally responsible for fat digestion since the pancreas is the only source of lipases
Digest nucleic acids (nucleases)

16. 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
cholecystokinin (CCK): causes the gallbladder to contract and release stored bile into the bile duct so that bile and pancreatic juice enter the small intestine together

17. Activities of the Small Intestine
Most substances:
are absorbed by active transport
then they enter the capillary bed in the villus to be transported in the blood to the liver via the hepatic portal vein
Lipids:
Are absorbed by diffusion
Then they enter both the capillary bed and the lacteal in the villus and are carried to the liver by both blood and lymphatic fluid

18. 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 vitamines ( vitamin K and B)

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

20. Activities of the Large Intestine
The rectum is generally empty, but when feces are forced into it by mass movements and its wall is stretched, the defecation reflex is initiated:
It is a spinal reflex that causes the walls of the sigmoid colon the rectum to contract and the anal sphincters to relax