The document provides an overview of the digestive system, including: 1. It outlines the functional structures of the gastrointestinal tract and their roles in digestion. 2. It describes the secretions produced in the mouth, stomach, pancreas, liver, and intestines that aid in digestion of carbohydrates, proteins, and fats. 3. It explains how nutrients are absorbed and how metabolism of carbohydrates, proteins, and lipids provides energy for the body.

1. DIGESTIVE SYSTEM
1
Nigist Belay

2. Outline
• Objectives
• General overview of the GIT
• Functional Structures of the GIT
• Salivary Secretion
• Gastric secretion
• Pancreatic secretion
• Intestinal secretion
2

3. • Bile secretion
• Digestive & absorptive function of GIT
• Metabolism and energy
• Pathophysiology of GIT
3

4. Objectives
• At the end of this system, you will be expected to:
Describe functional structures of the GIT
Identify secretory functions of the GIT
Explain digestive and absorptive functions of the GIT
Describe energy and metabolism
Distinguish the clinical correlates of the GIT
4

5. General overview of the GIT
• Human digestive system is a complex series of organs and glands that
processes food
• Includes the entire alimentary canal plus all of the accessory organs
• Some of the important roles of the GIS include:
1. In homeostasis
2. In microbial defense
5

6. In homeostasis
• Day-to-day intake and excretion of food substances
guarantees perpetuation and survival of life
6

7. A. Secretion:
~ 7 L of fluid are secreted daily
~ 2 L of fluid is drunk daily
= 9 L of fluid is added into GIT/day
B. Absorption:
~ 8.5 L of fluid is absorbed by SI
~ 400 ml is absorbed by the LI
C. Excretion: only ~ 50 -100 ml of fluid is excreted daily
7

8. In microbial defense
a. Mouth: saliva
b. Stomach: HCl
c. Small intestine: Payer's patches
d. Macrophages: developed from monocytes
8

9. Functional Structures of the GIT
1. Mouth
2. Pharynx
3. Esophagus
4. Stomach
5. SI
6. LI
7. Rectum & Anus
1. Salivary
glands
2. Pancreas
3. Liver
4. Gallbladder
Alimentary canal Accessories
9

10. Mouth
• Ingests food
• Teeth- masticates food
• Tongue- mixes food
- rolls food
• Saliva- moisten and soften food
10

11. Esophagus
• Secretes mucus
• Moves food from the throat to the stomach
11

12. • What happens during swallowing and breathing?
12

13. Stomach
• Stores the food you eat
• Mixes food with digestive juices
• Acid in the stomach kills bacteria
• Chyme
13

14. Small Intestine
• Lining of intestine walls- villi
• The villi are covered in microvilli
• The majority of chemical digestion occurs duodenum
• Chyme- acidic
- triggers conversion of prosecretin  secretin
14

15. Large Intestine
• Accepts what small intestines don’t absorb
• Rectum
15

16. Anus
• External opening of the rectum
• Controlled by sphincture muscles
• Feces- defecation
16

17. Salivary Glands
• Parotid glands- serous, watery
• Submaxillary glands- serous and mucus
• Sublingual glands- saliva
17

18. Liver
1. Regulation of blood glucose concentration
2. Bile secretion
3. Fe storage
4. Protein synthesis
5. Deamination of amino acids
18

19. Gall Bladder
• Stores bile
• Secretes bile to the duodenum
 Bile- contains bile salts, pigments, cholesterol & phospholipids
- an emulsifier
19

20. Pancreas
• Releases chemicals
• S-cells located in the walls of the duodenum
• Secretin reaches and binds to receptors
• Duct cells release HCO-
3
20

21. Secretory function of the GIT
Reading assignment
21

22. Digestive and absorptive function of GIT
22

23. Digestion of Carbohydrates
Digestion of Carbohydrates in the Mouth and Stomach
• Ptyalin hydrolyzes starch into maltose and other small polymers of glucose
not more than 5% of all the starches will have become hydrolyzed
• Starch digestion sometimes continues in the body and fundus of the stomach
for as long as 1 hour
30 - 40 % of starches will have been hydrolyzed mainly to form maltose
• Activity of the salivary amylase is blocked by acid of the gastric secretions

24. Digestion of Carbohydrates in the Small Intestine
Digestion by Pancreatic Amylase
• 15 - 30 minutes after the chyme empties from the stomach into the duodenum
and mixes with pancreatic juice
• Carbohydrates are almost totally converted into maltose and/or other very
small glucose polymers

25. Hydrolysis of Disaccharides and Small Glucose Polymers into
Monosaccharides by Intestinal Epithelial Enzymes
• Enterocytes lining the villi of small intestine contain four enzymes (lactase,
sucrase, maltase, and a-dextrinase)
disaccharides are digested as they come in contact with these enterocytes

27. Digestion of Proteins in the Stomach
• Pepsin, the important peptic enzyme of the stomach
for this enzyme to cause digestive action on protein, the stomach juices
must be acidic
• One of the important features of pepsin digestion is its ability to digest the
protein collagen

28. • Collagen is a major constituent of the intercellular connective tissue of meats
• In persons who lack pepsin in the stomach juices, the ingested meats are less
well penetrated by the other digestive enzymes
• Pepsin only initiates the process of protein digestion, usually providing only
10-20% of the total protein digestion
this splitting of proteins occurs as a result of hydrolysis at the peptide
linkages between amino acids

30. Digestion of Proteins by Pancreatic Secretions
• Most protein digestion occurs in the upper small intestine, in the duodenum
and jejunum, under the influence of proteolytic enzymes
trypsin, chymotrypsin, carboxypolypeptidase, and proelastase

31. Digestion of Peptides by Peptidases in the EnterocytesThat Line
the Small IntestinalVilli
• Two types of peptidase enzymes are especially important
aminopolypeptidase
several dipeptidases

32. • Inside the cytosol of the enterocyte are multiple other peptidases that are
specific for the remaining types of linkages between amino acids
within minutes, virtually all the last dipeptides and tripeptides are digested
to the final stage to form single amino acids
• More than 99 % of the final protein digestive products that are absorbed as
individual amino acids

33. Digestion of Fats
Digestion of Fats in the Intestine
• A small amount of triglycerides is digested in the stomach by lingual lipase
< 10 % and generally unimportant
instead, essentially all fat digestion occurs in the small intestine

34. Emulsification of Fat by Bile Acids and Lecithin
• The first step in fat digestion is physically to break the fat globules into very
small sizes
emulsification of the fat
• Most of the emulsification occurs in the duodenum under the influence of bile
• Bile contains a large quantity of bile salts as well as the phospholipid lecithin

35. • The fat-soluble portions dissolve in the surface layer of the fat globules, with
the polar portions projecting
• The polar projections, in turn, are soluble in the surrounding watery fluids
• Major function of the bile salts and lecithin is to make the fat globules readily
fragmentable by agitation with the water in the small bowel
same as that of many detergents

36. Digestion ofTriglycerides by Pancreatic Lipase
• Important enzyme for digestion of the triglycerides is pancreatic lipase enough
to digest within 1 minute all triglycerides
• Enterocytes of the small intestine contain still more lipase, known as enteric
lipase, but this is usually not needed

37. End Products of Fat Digestion
• Most of the triglycerides of the diet are split by pancreatic lipase into free fatty
acids and 2-monoglycerides

38. ABSORTION OF CARBOHYDRATES
• As monosaccharides, viz. glucose, galactose and fructose
Absorption of Glucose
• By means of sodium cotransport
Energy is obtained by the binding process
• Glucose is absorbed into the portal vein by facilitated diffusion

39. Absorption of Galactose
• In the same mechanism as that of glucose
Absorption of Fructose
• By means of facilitated diffusion
• Some are converted into glucose

41. ABSORPTION OF PROTEINS

42. ABSORPTION OF LIPIDS
Reading assignment

43. Vomiting
• Forceful expulsion of stomacheal and upper intestinal contents
Mechanism of vomition reflex:
• Mainly controlled by “a vomition center”
• “chemoreceptor trigger zone, CTZ” located at the root of the 4th ventricle
there is a neural connection between the two centers

45. Importance of vomition:
• Removal of toxic substances before absorption
Consequences of prolonged vomition:
• Excessive Vomition  large loses of fluids and salt  severe dehydration 
decreased plasma volume  circulatory and metabolic imbalances etc.

46. Feces
• Composed of 75% water and 25% solids
• Bulk of fecal solids are bacteria, undigested organic matter and fibers
• Brown color is due to stercobilin and urobilinogen
• Odor results from gases (skatol, inodle, hydrogen sulfide etc)

47. Defecation reflex
• Sacral spinal cord is involved
• Cholinegic PS fibers are important during reflex
• Stimulation of receptors in the rectum  impulses to the sacral spinal cord
 impulses through PSN (pelvic nerves) back to (descending colon,
sigmoid, rectum, and anus)  Powerful peristalsis + Relaxation of internal
sphincter  expulsion of the bowel

48. METABOLISM AND ENERGY

49. Metabolism of Carbohydrates

50. Metabolism of Proteins

51. Metabolism of Lipids

52. Energy production
• Our body normally burns
CHO (~60 %),
fat (~25%), and
protein (~15%)

53. • On average, the daily caloric needs of food approximately include:
 1600 Cal for children
 2000 Cal for adults
 2800 Cal for active men

54. • So, how do you calculate the % of CHO, fat and protein in your normal daily
diet?
A. Assume how many calories you need each day, E.g. 2000 Cal
B. Determine what % of your calories come from CHO, fat and protein
C. Multiply your total calorie by each of the % and then divide the result by
calories per gram obtained from each food

55. Pathophysiology of GIT
• Gastro-esophageal reflux disease (heart burn)
• Achalasia
• Gastritis- Inflammation of gastric mucosa
• Peptic ulcers
• Appendicitis
• Constipation
• Diarrhea
• Lactose intolerance