1. The Digestive System
Part 2

2. What has happened to the food so far?
Food has been ingested
Mechanical digestion has occurred in the mouth increasing the surface area and forming a food bolus
Salivary amylase has been secreted in the saliva and carbohydrates have been broken down into smaller chains
Lingual lipase has been added to the food bolus
Peristaltic motion of the esophagus has moved the bolus to the cardiac sphincter of the stomach

3. The Stomach Mechanically breaks up food particles Liquefies the food
Begins the chemical digestion of proteins and a small amount of fat
Produces CHYME—pasty mixture of semidigested food that is passed into the small intestine

4. Four Regions of the Stomach
Cardiac Region—anterior section (nearest the heart); receives food from esophagus
Fundic Region—Dome shape portion superior to the esophageal attachment
Body Region—Greatest part of the stomach
Pyloric Region—posterior section; where food moves into the small intestine

5. The Wall of the Stomach
Has a layer of mucosa on the inside of the stomach
The gastric mucosa contains depressions called gastric pits
Cells in bottom of pits replace epithelial cells that are sloughed off into chyme
Gastric pits also contain glands that are made of various cell types

6. The Role of Hydrochloric Acid in the Stomach
Gastric juice has high concentration of HCl and can have a pH as low as 0.8
Functions of acidic environment:
Activates enzymes pepsin and lingual lipase
Breaks up connective tissues & plant cells walls, helps to liquefy food & form chyme
Converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+) to be used in hemoglobin production

7. Types of Cells found in Gastric Pits
Mucous Cells—predominate cardiac & pyloric glands; secrete mucous
Regenerative (stem) Cells—produce a continual supply of new cells to replaced sloughed off cells
Parietal Cells—secrete HCl & intrinsic factor; found mostly in gastric glands
Chief Cells—Most numerous cell type, secrete chymosin & lipase in infancy and pepsinogen throughout life
Enteroendocrine Cells—Secrete hormones & other messengers that regulate digestion

8. Digestive Enzymes in the Stomach
Pepsinogen is converted to PEPSIN, an enzyme that breaks down proteins to small amino acid chains
In infancy, gastric lipase digests the butterfat of milk
In infancy, chymosin (renin) curdles milk
Intrinsic Factor—Essential for absorption of vitamin B12 in the small intestine

9. Gastric Motility
The medulla swallowing center signals the stomach to relax when we swallow; this prepares the stomach to receive food
Arriving food stretches the stomach
The stomach begins peristaltic contractions governed by pacemaker cells in the stomach wall
Contractions occur every 20 seconds and get stronger as they move throughout the stomach
After 30 minutes the contractions are strong
About 3 mL of chyme at a time is squirted into the small intestine
Typical meal is emptied from the stomach in 4 hours

10. Why does the stomach not digest itself?
It has a mucous coat that is highly alkaline (basic)
Epithelial cells live only 3 to 6 days and then are sloughed off into the chyme and replaced
Epithelial cells are joined by tight junctions that prevent gastric juice from seeping between them

11. Accessory Organs
Liver—Secretes bile into small intestine; home to the gall bladder; after a meal the liver removes glucose, amino acids, vitamins, and other nutrients for its (liver’s) metabolism & storage
Gall Bladder—Sac on the underside of the liver that stores & concentrates bile
Bile—Yellow-green fluid containing minerals, cholesterol, fats, phospholipids, bile pigments & bile acids; principal pigment is bilirubin derived from the breakdown of hemoglobin; bile acids aid in fat digestion & absorption

12. Accessory Organs
Pancreas—both an endocrine (insulin & glucagon) and exocrine organ; secretes about 1,200 to 1,500 mL of pancreatic juice per day; pancreatic juice is alkaline (basic) and buffers chyme from stomach in the small intestine; pancreatic enzymes include trypsinogen, chymotrypsinogen, and procarboxypeptidase; pancreatic amylase, pancreatic lipase, ribonuclease, and deoxyribonuclease

13. What has happened to the food so far?
The bolus from the esophagus has moved into the cardiac sphincter of the stomach
The stomach mechanically digests the food
The churning mixes the food with pepsin that breaks down protein into smaller amino acid chains
And, lingual lipase is activated beginning to break down lipid (fat) molecules
The food material becomes a pasty substance called CHYME
The chyme is released into the small intestine through the pyloric sphincter about 3 mL at a time.

14. The Small Intestine The site of nearly all of the chemical digestion
The site of nearly all nutrient absorption into the bloodstream
Has a large surface area exposed to chyme because of the presence of villi and microvilli—folds of the mucous membranes of the small intestine
6 to 7 m (18 – 22 ft) in cadaver; 2 m long (6.5 ft) in living person because of muscle tone
Named “small” intestine because diameter is smaller than the “large” intestine

15. Three Regions of the Small Intestine
Duodenum—First 25 cm beginning at the pyloric valve; receives chyme from stomach and the pancreatic juice & bile; stomach acid is neutralized here; fats are broken up by bile acids, pepsin is inactivated by high pH, and pancreatic enzymes take over chemical digestion
Jejunum—Next 2.5 m
Ileium—Last 3.6 m

16. Specialized Structures of the Small Intestine
Villi—Fingerlike projections of the mucosa that increase SA
Covered with absorptive cells (to move nutrients to blood stream)
Core contains an arteriole, a capillary network (for nutrient absorption), a venule, and a lymphatic capillary called a lacteal (absorbs fat)

17. Specialized Structures of the Small Intestine
Microvilli—Tiny projections from each villus; called a brush border; further increases SA; contains brush border enzymes; chyme must come in contact with these enzymes for digestion to occur
Intestinal Crypts—Similar to gastric pits; dividing stem cells replacing mucosal cells every 3 to 6 days
Paneth Cells—Secrete lysozyme, & other chemicals protecting against bacterial infection
Duodenal Glands—Secrete bicarbonate-rich mucus to neutralize stomach acid

18. Intestinal Secretion
Intestinal crypts secrete 1 to 2 L of intestinal juice per day
Fluid has a pH of 7.4 to 7.8
Contains water & mucus but little enzyme

19. Intestinal Motility Contractions of small intestine serve 3 functions:
Mix chyme with intestinal juice, bile & pancreatic juice for acid neutralization and chyme digestion
Churn chyme and bring it into contact with mucosa for contact digestion & nutrient absorption
Move residue toward the large intestine

20. Intestinal Motility
Segmentation, ringlike constrictions, appear at several places of the small intestine simultaneously
Internal pacemaker cells control contractions
Contractions occur 12 times per minute in duodenum & 8 – 9 times per minute in ileum
When most nutrients have been absorbed, segmentation is replaced with peristalsis which moves remaining chyme toward large intestine

21. Chemical Digestion & Absorption
CARBOHYDRATES
Starch is digested into oligosaccharides (chains of up to 8 glucose molecules), then to disaccharide maltose (2 glucose molecules) and finally to glucose molecules which are absorbed by the small intestine
Salivary amylase breaks starch into oligosaccharides
Pancreatic amylase breaks oligosaccharides into maltose in the small intestine by contact digestion
Maltase hydrolyzes maltose into individual glucose molecules which are absorbed into the bloodstream

22. Chemical Digestion & Absorption
PROTEINS
Proteases (peptidases)—enzymes that digest proteins
Pepsin begins protein digestion in the stomach; 10% to 15% of protein is digested here; pepsin works at pH 1.5 to 3.5 so only works in the stomach
In the small intestine trypsin & chymotrypsin hydrolyze proteins into smaller amino acid chains
Other enzymes remove one amino acid at a time by contact digestion
Individual amino acids are absorbed into the bloodstream.

23. Chemical Digestion & Absorption
LIPIDS—Hydrophobic quality makes their digestion & absorption more complicated
Lipases—enzymes that digest fats
Lingual lipase secreted in saliva is activated by stomach pH to start fat digestion; digests 10% of fat
Pancreatic lipase digests most of fat in the small intestine
Bile emulsifies (breaks down into droplets) fat to make digestion easier (exposing more SA to digestive enzymes)
Average daily intake of fat can be digested in 1 to 2 minutes
The free fatty acids and glycerol and then absorbed into the bloodstream or the lacteal duct

24. Chemical Digestion & Absorption
Nucleic Acids (DNA & RNA)
Present in much smaller quantities than other macromolecules
Nucleases break DNA and RNA into their individual nucleotides
Contact digestion then break nucleotides into sugar, phosphate and nitrogen base
These products are then absorbed into the bloodstream

25. Chemical Digestion & Absorption
WATER
Digestive tract receives about 9 L of water per day, 0.7 L in food, 1.6 L in drink, and 6.7 L in gastrointestinal secretions
8 L of this is absorbed in the small intestine
0.8 L is absorbed by the large intestine
0.2 L voided in feces
Too little water absorption in large intestine is diarrhea
Too much water absorption in large intestine is constipation

26. What has happened to the food so far?
After the chyme reaches the small intestine, it is mixed with pancreatic juice and bile
Segmentation and peristalsis move the chyme through the small intestine
Carbohydrate, protein, lipid, and nucleic acid digestion is completed
Small nutrient molecules are absorbed into the bloodstream
Undigested material is moved into the large intestine

27. The Large Intestine 1.5 m (5 ft) long aka colon
Begins with cecum—blind pouch which has the appendix attached to it
Contains ascending, transverse, descending, and sigmoid regions

28. The Large Intestine There are no villi in the large intestine
There are intestinal crypts that have mucus as their only secretion
Large intestine is heavily populated with normal bacterial flora that ferment cellulose, & other undigested nutrients and synthesize B vitamins & vitamin K
Average person expels 500 mL of flatus (gas) per day
“The hydrogen gas in flatus is combustible & has been known to explode in surgery that used electrical cauterization.”

29. Absorption & Motility
It takes the large intestine about 12 to 24 hours to reduce a meal’s residue to feces
It removes water & electrolytes from the residue and reabsorbs it
Contractions occur every 30 minutes that move residue short distances
MASS MOVEMENTS occur 1 to 3 times per day, lasting about 15 minutes that move residue several cm; they often occur after the stomach fills & within an hour after breakfast

30. Defecation Removal of waste residue from the body
Internal & external anal sphincter is composed of skeletal muscle and is under voluntary control