2. Objectives
At the completion of this unit,the learners will be able to:
1. Define the digestive system and list its functions
2. Identify the various organs of digestive system
3. Describe the anatomy & physiology of digestive organs
4. Discuss the role of accessory organs in digestion
5. Discuss digestion of food in
--Mouth
--Stomach
--Small intestines
--Large intestines
6. Discuss the absorption of nutrients in the digestive system
7. Discuss the process of defecation.
3. The digestive system includes
many organs,and each organ has
many various regions.The main
tract through the digestive system
(from the mouth,through the
stomach and intestines) is called
the gastrointestinal tract (GI)
or the alimentary canal.
4. Functions of Digestive System
⦁ Ingestion— taking food and liquid
⦁ Digestion— mechanical and chemical
⦁ Absorption— into blood and lymph
⦁ Defecation— elimination of feces
5. Digestive System
Digestive system has two subdivisions:
Digestive tract
Up to 9 meters (30 feet) long
It consists of mouth,Pharynx,Esophagus,Stomach,
Small intestine,and Large intestine.
Accessory organs
It includesTeeth,Tongue,Salivary glands,Liver
,
Gallbladder
,and Pancreas.
9. Salivary Glands
⦁ Divided into major and minor glands
Major Salivary Glands
⦁ Parotid --- paired and the largest
⦁ Submandibular --- one on each side of the face under the
angle of jaw
⦁ Sublingual---single and the smallest
Minor salivary glands
⦁ There are about 450 minor salivary glands situated in the
mucosa of lips,cheeks, palate,floor of mouth, and
retromolar region.
10. Composition of Saliva
⦁ T
otal amount is 1500 ml/day
⦁ 99 % water and 1 % dissolved substances (organic
and inorganic).
⦁ Ptyaline or salivary amylase,secreted from parotid
⦁ Lingual lipase acting on triglycerides
⦁ Mucin,a glycoprotein is secreted by sublingual and
submandibular gland.
3
⦁ Inorganic substances include Na+, K+, Ca++,HCO -
and Cl-
11. Functions of Saliva
⦁ Lubrication and speech
⦁ Appreciation of taste—only saliva soluble food can stimulate
the taste buds.
⦁ Digestion---- starch is converted into maltose
⦁ Defense--- Lysozyme and IgA
ClinicalTerms:
⦁ Sialorrhea (hypersalivation)
⦁ Xerostomia...dry mouth
⦁ Sjogren's (SHOW-grins) syndrome is an autoimmune disorder
characterized by two most common symptoms — dry eyes
and dry mouth.
12. General Anatomy of Digestive Tract
⦁ Most of the digestive tract consists of four layers,in order
from inner to outer surface:
1. Mucosa
⦁ Is a mucous membrane lining the lumen.
⦁ It has three layers as
i.Epithelium :coming in direct contact with food
ii.Lamina propria:Loose connective tissue layer
iii.Muscularis mucosa:smooth muscle thin layer
(inner circular and outer longitudinal layer)
13. Layers of GI Tract cont..
⦁ The lamina propria (lamina=thin plate; propria = own) is
areolar/loose connective tissue having blood and lymphatic
vessels for absorption.
⦁ It overlies mucosa
14. GI layers cont…
2.Submucosa
⦁ Formed by areolar connective tissue that binds mucosa to
muscularis externa.
⦁ It has a network of (auto) neurons called submucosal plexus
(Meissner Plexus ),glands and lymphatic tissues.
3.Muscularis Externa
⦁ Consists of two layers of smooth muscles as inner circular
and outer longitudinal layers.
⦁ In between the two layers is a myenteric Plexus/Auerbach
plexus
⦁ The involuntary contraction of circular and longitudinal
smooth muscles help break down and propulsion of food
along the GI tract.
15. GI Layers cont…
4.Serosa
⦁ Serosa means serous membrane,the outermost layer
.
⦁ It is also called the visceral peritoneum.
⦁ It begins in the lower 3 to 4 cm of the esophagus and ends
with the sigmoid colon.
⦁ The oral cavity
,pharynx,most of the esophagus and the
rectum are surrounded by a connective tissue layer called
adventitia.
⦁ The esophagus, stomach, and intestines haveanervous
network called the enteric nervous system which regulates
digestive tract motility
,secretion,and blood flow.
16. Neurons of Enteric Nervous System (ENS)
⦁ Neurons of the ENS are arranged into two groups.
⦁ A.Submucosal Plexus or Meissner Plexus
⦁ B.Myenteric Plexus orAuerbach plexus found between
circular and longitudinal smooth muscles of muscularis
externa.
19. Gastric secretions
⦁ Gastric secretion is a colorless,watery
,acidic,produced in the
stomach .
⦁ Physical properties;
⦁ It is a watery fluid,that has a pale color
,pH is 1-3.
⦁ the volume secreted per day is 2-3 L .
⦁ Chemical composition; It is 97-99% water ,it contains inorganic
salts ,and organic components that include,mucin,digestive
enzymes,hormones
20. Gastric Glands and secretions
⦁ The gastric secretion is produced by the epithelial cells of the
gastric glands.
⦁ Goblet cells or mucus cells: mucous cells,which cover the
entire lumenal surface.
These cells secrete a bicarbonate-rich mucus that coats and
lubricates the gastric surface,and serves an important role in
protecting the epithelium from hydrochloric acid and pepsin
and other chemical damage.
21. Gastric Glands and secretions
• Parietal or oxyntic cells: They secret HCl into the
stomach lumen where it establishes an extremely acidic
environment.
• This acid is important for activation of pepsinogen and
inactivation of ingested microorganisms such as bacteria.
• They also secret the intrinsic factor
,a glycoprotein
which is necessary for intestinal absorption of vitamin
B12.
22. Gastric Glands and secretions cont..
• Chief cells: they secrete pepsinogen(zymogen). Once
secreted,pepsinogen is activated by stomach acid into
the active protease pepsin,which initiates digestion of
proteins in the stomach.
• In young animals,chief cells also secrete rennin a
protease that helps coagulate milk allowing it to be
retained more than briefly in the stomach.
⦁ The epithelium cells also produce important hormone
gastrin,a peptide that is important in control of acid
secretion and gastric motility
, and somatostatin.
23. Produce alkaline
mucus that covers
mucosa layer
Synthesize and
secrete the protease
precursor known as
pepsinogen.
Synthesize and
secrete the HCl acid
responsible for the
acidic pH in the
gastric lumen.
Exocrine gland cells of gastric pits
24. Gastric secretion phases
Gastric acid secretion can be divided into three phases:
Cephalic phase
⦁ mediated by the CNS and triggered by sight,smelling,
chewing or even the thought of food.Mediated by the vagus
and accounts for 10-30% of the acid secretion.
Gastric phase
⦁ triggered by the presence of food in the stomach (both
chemical and mechanical sensing are involved).Accounts for
70-90% of acid secretion
Intestinal phase
⦁ Presence of chyme, most probably amino acids, in the
intestine triggers approximately 5% of the gastric acid
secretion.
25. Gastric and Peptic Ulcers
⦁ Peptic ulcers:
⦁ Erosions of the mucous membranes of the stomach or duodenum
produced by action of HCl.
⦁ Zollinger-Ellison syndrome:
⦁ Ulcers of the duodenum are produced by excessive gastric acid
secretions.
⦁ Helicobacter pylori:
⦁ Bacteria that reside in GI tract and may produce ulcers.
⦁ Acute gastritis:
⦁ Histamine released by tissue damage and inflammation stimulate further
acid secretion.
30. Function of the Stomach
⦁ The stomach performs three main functions:
1. Store ingested food.
2. Secretes hydrochloric acid (HCl) and enzymes
that begin protein digestion.
3. The ingested food is pulverized and mixed with
gastric secretions to produce a thick liquid
mixture known as chyme.
31. Functional Anatomy of Liver and Biliary
System
⦁ Liver is the largest gland in the body
⦁ Hepatic Lobes:
⦁ Right lobe
⦁ Left lobe
⦁ Quadrate lobe
⦁ Caudate lobe
⦁ Hepatic Lobules:
⦁ Hepatic Lobule is the structural and functional unit of
liver
.
⦁ There are 50,000 to 100,000 lobules in the liver
32. ⦁ Hepatic lobule is a honeycomb like structure which is
made up of hepatocytes.
⦁ Hepatic Plates:
⦁ Hepatocytes are arranged in columns which form
hepatic plates.
⦁ Each plate is made up of two columns of hepatocytes.
⦁ In between two columns of each plate lies a bile
canaliculus.
⦁ In between the plates,a blood space called sinusoid is
present.
⦁ Sinusoid is lined by the endothelial cells along with
macrophages in between the cells.
38. Gallstone
⦁ Gallstone is a solid crystal deposit that is formed by
cholesterol,calcium ions and bile pigments in the
gallbladder or bile duct.Cholelithiasis is the presence
of gallstones in gallbladder
.Choledocholithiasis is the
presence of gallstones in the bile ducts.
41. Absorption of nutrients
The structure of the small intestine, in place to aid absorption of nutrients, consists of three
levels: on the macro-scale, folds or plicae intrude into the lumen; on the meso-scale villi
cover the surface; and on the micro-scale microvilli cover the villi on a cellular level.
44. References
1. Ross andWilson,Anatomy & Physiology in Health and
Illness,12 Edition
2. Ross &Wilson.(2000)Anatomy & Physiology in Health
& Illness.Edinburgh:Churchill,10th Edition
3. PRAshalatha & G Deep,Anatomy & Physiology for
Nurses 4th Edition
