Notes on digestive system for medical students.

1. THE DIGESTIVE
SYSTEM
By
Hermann Tchakounte
2nd Year GM
UdM, Bgte

2. OBJECTIVES
By the end of this lecture the student should be able to:
•Define the digestive system and state its components
•Describe digestion along the digestive tract
•Describe the function of the accessory glands in digestion

3. PLAN
Introduction
1. Definition and components
2. The GIT
• Review
• Digestion in the different segments
3. The accessory organs

4. INTRODUCTION

5. 1. DEFINITION

6. DEFINITION
The digestive system is a group of organs
which function together ensuring the complete
breakdown and absorption of food substances
for production of energy and matter (synthesis
of tissues)
It is made up of:
•The gut or gastrointestinal tract or alimentary
canal
•The accessory organs

7. IN GENERAL…
The 6 processes in food digestion include
•Ingestion: taking in of food through the mouth
•Propulsion: movement of food through the tract like swallowing and
peristalsis
•Mechanical digestion: includes chewing, mixing of food in mouth,
churning in stomach and segmentation in intestines
•Chemical digestion: involves enzymes
•Absorption: passage of digested end products from the lumen into
blood (small int)
•Defecation: elimination of indigestible substances via the anus

9. 2. THE
GASTROINTESTINAL
TRACT (GIT)

10. It is the tube-like structure running from the mouth to the anus
It is 9m long in the cadaver and shorter in the living being

11. REVIEW

12. EMBRYOLOGY
It is formed in the 4th week of gestation from the endoderm
The primitive gut is made up of
•Foregut
•Midgut
•Hindgut

13. Foregut Midgut Hindgut
Organs Oesophagus
Stomach
Duodenum (1 and 2)
Accessory organs
Duodenum (3 to 4)
Small intestine
Ascending colon
Proximal 2/3rd of
transverse colon
Distal 1/3rd of
transverse colon
Descending colon
Rectum
Vascularisation Coeliac trunk Superior mesenteric
artery
Inferior mesenteric
artery

14. HISTOLOGY
Its wall is made up of 4 concentric layers:
1. Mucosa:
Consisting of
•an epithelium, secrete mucus which lubricate food and prevent
autodigestion.
•a lamina propria basement membrane of the epith layer, contains
blood and lymph vessels, some portions contain glands.
•an external muscularis mucosa thin layer of smooth muscles.

15. 2. Submucosa:
Layer of connective tissue containing
•blood and lymphatic vessels,
•glands and
•a nerve plexus: submucousal or Meissner’s plexus is located between
the submucosa and the muscularis.

16. 3. Muscularis:
Made up of
•An inner circular layer and
•Outer longitudinal layer
In between lies a nerve plexus(myenteric or Auerbach’s plexus)
4. Serosa:
Outermost covering composed of loose fibrous connective tissue.
The outer surface of the gut is covered by the peritoneum, and forms the
mesenteries which suspend and support the stomach and intestines from
the body wall.

18. INNERVATION
1. Enteric nervous system
•Myenteric plexus: motility
•Submucousal plexus: secretion
2. Autonomous nervous system
•Sympathetic: inhibits
•Parasympathetic: stimulates

19. I. THE MOUTH
(ORAL CAVITY)

20. Boundaries:
Anterior: lips
Lateral: cheeks
Superior: palate
Inferior: tongue
Its walls are lined with nonkeratinized stratified squamous epithelium
Mechanical and chemical digestion occurs here

22. MECHANICAL DIGESTION
Mastication done by muscles of mastication and the teeth
Use:
•Increase the surface area for enzyme action
•Ease swallowing

24. CHEMICAL DIGESTION
Done by enzymes found in saliva
Salivary glands
There are three paired principal salivary glands
Type of secretion Duct
Parotid gland Serous (water, salt and
enzymes)
Stenson’s
Submandibular Mixed (more serous) Wharton’s
Sublingual Mixed (more mucous) Rivinus’

26. SALIVA
Saliva is a clear hypotonic secretion
pH: 6.7
Volume: 1-1.5l/day
Composition:
•Water (97-99.5%)
•Lysozyme and IgA
•Enzymes: ptyalin (salivary amylase) and lingual lipase
•Electrolytes: Na, K, Cl, PO4,HCO3)
•Mucus

27. Functions:
•Lubrification
•Digestion
•Antimicrobial

29. DEGLUTITION/SWALLOWING
After digestion in the mouth, the tongue rolls the food into a bolus
and send it into the oesophagus.
This occurs in 2 phases
•Buccal phase: voluntary. Food is sent to the pharynx
•Pharyngeal-oesophageal phase: involuntary
Tongue blocks mouth
Soft palate closes nasopharynx
Epiglottis covers respiratory passage
Upper oesophageal sphincter opens

31. II. THE PHARYNX

32. PHARYNX/THROAT
It is the aerodigestif junction
Divided into 3:
•Nasopharynx has no digestive role
•Oropharynx
•Laryngopharynx

33. III. THE OESOPHAGUS

34. Muscular tube-like structure extending from the
pharynx to the stomach
25cm long and involved in peristalsis (wave-like
movement of the muscles)
Lined with non-keratinized stratified squamous
epithelium

36. IV. THE STOMACH

37. ANATOMY
J-shaped pouch-like organ, 15-25cm long, capacity of 4l of food
Located in the LUQ
Made up of 4 parts:
•Cardia
•Fundus
•Body
•Pyloric region

39. HISTOLOGY
Epithelium: simple columnar composed of goblet cells
Gastric glands are made up of
•Mucous secreting cells at the neck of the glands
•Parietal cells/oxyntic cells: HCl and IF
•Chief cells/ zymogen cells: inactive enzymes (pepsinogen, prorenin),
lipase
•Enteroendocrine cells: histamine, serotonin, somatostatin and gastrin

43. GASTRIC SECRETION
3 phases
Cephalic phase (20%): a few minutes long
Triggered by aroma, taste, sight or thought of food
Gastric phase (70%): 3-4hrs long
Triggered by food in the stomach
Intestinal phase (10%): chyme in the intestine

44. V. THE SMALL
INTESTINE

45. It is a convoluted tube extending from the pyloric sphincter to the
ileocaecal valve
Longest part of the gut
2-4 m long in live being and 6-7 m in a cadavre
Divided into 3 parts:
Duodenum
Jejunum
Ileum

47. HISTOLOGY
It is the site of absorption
Its wall has plicae circularis, villi and microvilli that increase the
surface area for absorption
Epithelium: simple columnar epithelium
Between the villi are pits called crypts of Lieberkuhn which secrete
intestinal juice
Volume: 1-2l
pH: 7.4-7.8

49. These glands are composed of:
•Enterocytes: absorbs water and electrolytes
•Goblet cells: secretes mucus
Intestinal juice contains enzymes notably
Peptidase: PP to aa
Sucrase: sucrose to glucose and fructose
Maltase: maltose to glucose
Intestinal lipase: lipids to f.a and glycerol

50. THE DUODENUM
C-shaped structure made of 4 parts (D1-4)
25cm long
Its submucosa has Brunner’s glands which produce an alkaline mucus
Site of production of secretin and cholecytokinin
Secretin is stimulated by acid chyme, and stimulates H2CO3 in
pancreas and liver
CCK stimulated by fats and proteins, and stimulates synthesis of
pancreatic enzymes and secretion of bile.

52. JEJUNUM
About 2.5m long
Extends from duodenum to ileum

53. ILEUM
The main site of absorption
About 3.6m long
Absorption of products of digestion, mainly of vitamin B12 and bile
acids

54. VI. THE LARGE
INTESTINE

55. It is 1.5 m long
Mucosa is simple columnar with goblet cells
Site of water reabsorption
Stores residues to eliminate as faeces
Contains many bacteria which ferment some indigestible
carbohydrates producing gases
They also synthesize B complex vitamins and vitamin K (CFs)

59. MACRO-MOLECULE
INGESTED
CARBO-HYDRATES PROTEINS FATS (TRI-GLYCERIDES
OR TG)
NUCLEIC ACIDS
SITE OF DIGESTION 1.MOUTH;
2.DUODENUM
3.DUODENUM
1.STOMACH;
2.DUODENUM
3.DUODENUM
DUODENUM DUODENUM
DIGESTIVE
ENZYME(S)
1. SALIVARY
AMYLASE
2. PANC. AMYLASE,
3. SUCRASE, LACTASE,
MALTASE
1.PEPSIN;
*HCl
2.PANC. PROTEASES;
3. PEPTIDASES;
LIPASES
*BILE
PANCREATIC
NUCLEASES
END-PRODUCT(S) 1 & 2. STARCH AND
GLYCOGEN TO
DISACCS;
3. DISACCS TO MONO-
SACCS
HCl DENATURES
PROTEINS;
1 & 2. PROTEINS TO
PEPTIDES;
3. PEPTIDES TO AMINO
ACIDS
BILE EMULSIFIES TG’S;
LIPASES BREAK TG’S
INTO FATTY ACIDS &
MONOGLY-CERIDES
NUCLEO-TIDES
SITE AND MODE OF
ABSORPTION
DISTAL SM. INTESTINE;
FACILI-TATED
DIFFUSION
DISTAL SM.
INTESTINE;
ACTIVE TRANSPORT
DISTAL SM.
INTESTINE;
SIMPLE DIFFUSION
DISTAL SM. INTESTINE
ABSORBED INTO
BLOOD OR LYMPH
BLOOD BLOOD LYMPH BY LACTEAL BLOOD
REGULA-TION SECRETIN FOR PANC.
AMYLASE
GASTRIN FOR PEPSIN CCK FOR BILE;
SECRETIN FOR PANC.
SECRETIN FOR PANC.
NUCLEASES

61. 3. ACCESSORY
ORGANS

62. I. THE PANCREAS

63. Tail-like organ embedded in the second portion of the duodenum
15cm long
Divided into the head, neck, body and tail
It is a mixed gland
•Endocrine (1%): islets of Langerhans
•Exocrine (99%): acini which produce pancreatic juice (1l/day)

66. Pancreatic juice contains enzymes for:
•Protein digestion: trypsinogen, chymotrypsinogen,
procarboxylpeptidase, elastase
•Lipid digestion: lipase
•Starch digestion: amylase
•Nucleic acid digestion: ribonuclease, deoxyribonuclease

68. REGULATION OF
PANCREATIC JUICE
SECRETION
Nervous:
•Enteric and PSNS stimulate
•SNS inhibits
Hormonal:
Secetin CCK
Duodenal cell S I
Trigger in duodenum Acidic chyme lipids
Effect Production of pancreatic
juice rich in
bicarbonates
Production of pancreatic
juice rich in enzymes
Contraction of gall
bladder=>bile

70. II. THE GALL BLADDER

71. Pocket-shaped organ, about 10cm long
Posteroinferior aspect of the liver
Stores and concetrates bile
Bile: dark green fluid helps in emulsifying fats
Composed of water, bile salts, bile pigments, fats
500-1000ml/day

73. FUNCTIONS OF BILE
Emulsification of fats
Coloration of faeces

74. III. THE LIVER

75. Located at the RUQ, largest gland in the body (1.4kg)
Divided into right and left by the falciform ligament
Functional unit is the lobule made up of liver cells (hepatocytes) and a
central vein
The hepatocytes secrete bile
The macrophages in the liver are called kupffer cells

78. CONCLUSION

79. THANK YOU FOR YOU
PARTICIPATION
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