The document provides a comprehensive overview of the human digestive system, detailing its structure and functions including ingestion, digestion, absorption, and excretion. It describes the roles of different organs such as the mouth, stomach, small and large intestines, as well as accessory organs like the liver and pancreas. Additionally, it discusses various digestive disorders and their implications for homeostasis.

1. THE DIGESTIVE SYSTEM
By
ANIRBAN GHOSH

2. NUTRITION
TYPES :
* Ingestion
* Digestion
Types of Nutrients :
* Micronutrients
* Macronutrients
FUNCTION OF DIGESTIVE
SYSTEM *
Acquires nutrients from
environment
* Anabolism (synthesize)
* Catabolism
(Decomposes & energy)

3. Actions of Digestive (GI)
Tract & parts
• Ingestion
• Secretion
• Mechanical process –
crushing / shearing
• Adsorption
• Digestion – Chemical
breakdown
• Excretion

4. MOUTH
– Mechanical digestion
• teeth
– breaking up food
– Chemical digestion
• saliva
– amylase
» enzyme digests
starch
– mucin
» slippery protein
(mucus)
» protects soft lining
of digestive system
» lubricates food for
easier swallowing
– buffers
» neutralizes acid to
prevent tooth decay
– anti-bacterial chemicals
» kill bacteria that
enter mouth with
food
Muscles of Mastication
1) Close the jaws
2) Slide or rock lower jaw from side to side
3) Chewing involves mandibular
* Elevation and depression
* Protraction and retraction
* Medial and lateral movement
• Function of Mouth
1. Break up food and digest starch .
2. Kills germs and moisten food .

5. Teeth & Tongue
1. Bitter
2. Sour
3. Salty
4. Sweet
Function :
Taste &
Mastication
* Upper surface -
Epithelium
* Dorsal surface -
Keratinised

6. ESOPHAGUS
Functions :
1.Secrete mucus
2.Moves food from the throat to the stomach
using muscle movement called peristalsis
• If acid from the stomach gets in here that’s
heartburn.
Peristalsis is series of involuntary wave-like
muscle contractions which move food along
the digestive tract . It is also called deglutition ,
it can be initiated voluntarily . This proceeds
automatically and is divided into three phases
i) Buccal phase , ii) Pharyngeal phase , iii)
Esophageal phase .
• A hollow muscular tube .
• About 25 cm (10 in.) long and 2 cm (0.80 in.) wide .
• Conveys solid food and liquids to the stomach .
• Begins posterior to cricoids cartilage .
• Is innervated by fibers from the esophageal plexus .

7. STOMACH
* Stomach is a J-shaped muscular bag that stores the food we eat ,
breaks it down into tiny pieces.
* The stomach takes around 4 hours to do it’s
job on the food, depending on what kinds of
food are digested.
* Regions of the Stomach
- Cardia
- Fundus
- Body
- Pylorus
Constantly being replaced, covered thick mucus, same simple columnar
epithelium
* Pyloric Sphincter – regulates gastric emptying
* Gastric Glands -
In fundus and body of stomach
Extend deep into underlying lamina propria
Each gastric pit communicates with several
gastric glands
Parietal cells
Chief cells
* Parietal Cells –
Secrete intrinsic factor and hydrochloric acid
* Chief Cells -
Secrete hydrochloric acid (HCl)
Are most abundant near base of gastric gland
Secrete pepsinogen (inactive proenzyme)
* Pepsinogen -
Is converted by HCl in the gastric lumen
To pepsin (active proteolytic enzyme)

8. •Functions of STOMACH
– food storage - can stretch to fit ~2L food
– disinfect food – (HCl = pH 2) , kills bacteria
– chemical digestion
• Pepsin
• enzyme breaks down proteins
• NOTES : Still, the epithelium is continually
eroded, and the epithelium is completely
replaced by mitosis every three days.
• Gastric ulcers, lesions in the stomach
lining, are caused by the acid-tolerant
bacterium Heliobacter pylori.
– Ulcers are often treated with antibiotics.
• Pepsin is secreted in an inactive form, called
pepsinogen by specialized chief cells in
gastric pits.
– Parietal cells, secrete hydrochloric acid
converts pepsinogen to the active pepsin only
when both reach the lumen of the stomach,
minimizing self-digestion.
– Also, in a positive-feedback system, activated
pepsin can activate more pepsinogen
molecules.

9. Small Intestines are roughly 7 meters long .
Lining of intestine walls has finger-like projections called villi, to increase
surface area.
The villi are covered in microvilli which further increases surface area for
absorption.
Nutrients from the food pass into the
bloodstream through the small
intestine walls. Simple sugars and
proteins are absorbed into the inner
lining. Fatty acids and glycerol go to
lymphatic system.
Absorbs:
80% ingested water ,Vitamins ,
Minerals , Carbohydrates , Proteins
Lipids
* Secretes digestive enzymes
Function
* chemical digestion - major organ of
digestion & absorption .
* absorption through lining over 6
meters! small intestine has huge surface
area = 300m2 (~size of tennis court) .
Structure : duodenum = most digestion
jejunum = absorption of nutrients & water
ileum = absorption of nutrients & water

10. • The Duodenum
– The segment of small intestine
closest to stomach , is 25 cm (10 in.)
long .
– “Mixing bowl” that receives chyme
from stomach and digestive
secretions from pancreas and liver .
Functions of the duodenum :
1)To receive chyme from stomach .
2) To neutralize acids before they can
damage the absorptive surfaces of
the small intestine .
* About every 20 seconds, the stomach contents are mixed by the churning action of smooth muscles . A
squirt at a time, it takes about 2 to 6 hours after a meal for the stomach to empty.
The Jejunum
- Is the middle segment of small intestine about
2.5 meters (8.2 ft) long .
- Is the location of most Chemical digestion , Nutrient
absorption .
- Has few plicae circulares .
The Ileum
- The final segment of small intestine
3.5 meters (11.48 ft) long
- Ends at the ileocecal valve, a sphincter that controls
flow of material from the ileum into the large intestine
Brush Border Enzymes
Integral membrane proteins
On surfaces of intestinal microvilli
Break down materials in contact with brush border.
Brush border enzymes break nucleotides into Sugars,
Phosphates , Nitrogenous bases .
Intestinal Glands
Enteropeptidase
A brush border enzyme
Activates pancreatic proenzyme trypsinogen
Enteroendocrine cells
Produce intestinal hormones such as gastrin,
cholecystokinin, and secretin
Duodenal Glands
Also called submucosal glands or Brunner glands
Produce copious quantities of mucus with chyme

11. LARGE INTESTINE• Is horseshoe shaped
• Extends from end of ileum
to anus
• Lies inferior to stomach and
liver
• Frames the small intestine
• Also called large bowel
• Is about 1.5 meters (4.9 ft)
long and 7.5 cm (3 in.) wide
Living in the large intestine is a community of
helpful bacteria that ferments carbohydrates
and breaks up proteins .
Escherichia coli (E. coli)
produce vitamins
vitamin K ; B vitamins
generate gases that are by-product
of bacterial metabolism methane,
hydrogen sulfide, Indole & skatole.
Bacteria convert bilirubin to
urobilinogens and stercobilinogens
FUNCTION :
• Re-absorption of water
use ~ 9 liters of water every day in digestive juices >
90% of water reabsorbed If not enough water
absorbed = diarrhea ; If too much water absorbed
= constipation
• Absorption of important vitamins produced by
bacteria; Vitamins K ( fat soluble ) Required by liver for
synthesizing four clotting factors, including prothrombin
and B (Pantothenic acid B5) manufacture of steroid
hormones and some neurotransmitters , Biotins ( Water
soluble ) Important in glucose metabolismare
reabsorbed with the water.

12. PARTS OF LARGE INTESTINEThe Cecum
•Is an expanded pouch
•Receives material arriving from the ileum
•Stores materials and begins compaction
Appendix
•Also called vermiform appendix
•Is a slender, hollow appendage about 9 cm
(3.6 in.) long
•Is dominated by lymphoid nodules (a
lymphoid organ)
The Colon
•Has a larger diameter and thinner
wall than small intestine
•The wall of the colon forms a series
of pouches (haustra)
•Haustra permit expansion and
elongation of colon

13. PARTS OF COLON
• Ascending Colon
– Begins at superior border of cecum
– Ascends along right lateral and posterior wall of peritoneal cavity to inferior surface of
the liver and bends at right colic flexure (hepatic flexure)
• Transverse Colon
– Crosses abdomen from right to left ; turns at left colic flexure (splenic flexure)
– Is supported by transverse mesocolon
– Is separated from anterior abdominal wall by greater omentum
• The Descending Colon
– Proceeds inferiorly along left side to the iliac fossa (inner surface of left ilium)
– Is retroperitoneal, firmly attached to abdominal wall
• The Sigmoid Colon
– Is an S-shaped segment, about 15 cm (6 in.) long
– Starts at sigmoid flexure
– Lies posterior to urinary bladder
– Is suspended from sigmoid mesocolon
– Empties into rectum

14. PARTS OF LARGE INTESTINE
• The Rectum
– Forms last 15 cm (6 in.) of digestive tract
– Is an expandable organ for temporary storage
of feces
– Movement of fecal material into rectum
triggers urge to defecate
• The anal canal is the last portion of the rectum
– Contains small longitudinal folds called anal
columns
• Anus
– Also called anal orifice
– Is exit of the anal canal
– Has keratinized epidermis
like skin

15. Accessory Organs
• Not part of the path of food, but play a critical
role.
• Include: Liver, gall bladder, and pancreas

16. Gall Bladder
• Pouch structure located near the liver
which concentrates and stores bile
• Bile duct – a long tube that carries BILE.
The top half of the common bile duct is
associated with the liver, while the
bottom half of the common bile duct is
associated with the pancreas, through
which it passes on its way to the
intestine.
Bile
Bile emulsifies lipids (physically breaks
apart FATS)
Bile is a bitter, greenish-yellow alkaline
fluid, stored in the gallbladder between
meals and upon eating is discharged into
the duodenum where it aids the process
of digestion.
Pancreas
An organ which secretes both digestive enzymes (exocrine) and hormones (endocrine).
Lies posterior to stomach from duodenum toward spleen is bound to posterior wall of
abdominal cavity . It is wrapped in thin, connective tissue capsule
Functions of the Pancreas
1)Endocrine cells of the pancreatic islets : Secrete insulin and glucagon into
bloodstream .
2) Exocrine cells : Acinar cells and epithelial cells of duct system secrete pancreatic
juice .
Pancreatic Acini : It is a Blind pockets , they are lined with simple cuboidal epithelium ,
Contain scattered pancreatic islets (Endocrine tissues of pancreas , contains Scattered
1% of pancreatic cells) .

17. • Pancreatic Enzymes
– Pancreatic alpha-amylase
• A carbohydrase .
• Breaks down starches .
• Similar to salivary amylase ( i.e ; helps in chemical
digestion ) .
– Pancreatic lipase
• Breaks down complex lipids .
• Releases products (e.g., fatty acids) that are easily
absorbed .
– Nucleases
• Break down nucleic acids .
– Proteolytic enzymes
• Break certain proteins apart .
• Proteases break large protein complexes .
• Peptidases break small peptides into amino acids .
• 70% of all pancreatic enzyme production .
• Secreted as inactive proenzymes .
• Activated after reaching small intestine .

18. LIVER
•Is the largest visceral organ (1.5 kg; 3.3 lb)
•Lies in right hypochondriac and epigastric
regions
•Extends to left hypochondriac and umbilical
regions
•Performs essential metabolic and synthetic
functions
* Anatomy of the Liver
Is wrapped in tough fibrous capsule
Is covered by visceral peritoneum
Is divided into lobes
* Physiology of the Liver
1. Metabolic regulation
2. Hematological regulation
3. Bile production
Metabolic Regulation
The liver regulates:
1. Composition of circulating
blood
2. Nutrient metabolism
(carbohydrate, lipid & amino
acid)
3. Waste product removal
4. Vitamin Storage (A, D, E & K)
5. Nutrient storage (iron)
6. Drug inactivation

19. HEMATOLOGICAL REGULATION
• Composition of Circulating Blood
– All blood leaving absorptive surfaces of digestive tract
• Enters hepatic portal system
• Flows into the liver
– Liver cells extract nutrients or toxins from blood
• Before they reach systemic circulation through hepatic veins
– Liver removes and stores excess nutrients
• Corrects nutrient deficiencies by mobilizing stored reserves or performing synthetic
activities
• Hematological Regulation : Largest blood reservoir in the body Removes damaged / old red blood cells
and it receives 25% of cardiac output
• Functions of Hematological Regulation
1. Phagocytosis and antigen presentation
2. Synthesis of plasma proteins : Plasma proteins: albumin – contrbutes to
osmotic concentration
3. Removal of circulating hormones : Reabsorped Epinephrine,
norepinephrine, insulin, thyroid & steroid hormones
4. Removal of antibodies : Removes antibodies and converts to amino acids
5. Removal or storage of toxins : Traps some lipid-soluble toxins (DDT) or
breaks down and removes from blood
6. Synthesis and secretion of bile

20. BILE PRODUCTION
1. The Bile Duct System
• Liver secretes bile fluid
• Into a network of narrow channels
(bile canaliculi)
• Between opposing membranes of
adjacent liver cells
2. Right and Left Hepatic Ducts
• Collect bile from all bile ducts of liver lobes
• Unite to form common hepatic duct that
leaves the liver
3. Bile Flow
• From common hepatic duct to either
• The common bile duct, which
empties into duodenal ampulla
• The cystic duct, which leads to the
gall-bladder
4. The Common Bile Duct
•Is formed by union of
- Cystic duct
- Common hepatic duct
•Passes within the lesser omentum toward
stomach
•Penetrates wall of duodenum
•Meets pancreatic duct at duodenal ampulla

21. Coordination of Secretion &
Absorption• Secretin
Is released when chyme arrives in duodenum
Increases secretion of bile and buffers by liver and pancreas
• Cholecystokinin (CCK)
Is secreted in duodenum
When chyme contains lipids and partially digested
proteins
Accelerates pancreatic production and secretion of digestive
enzymes
Relaxes hepatopancreatic sphincter and gallbladder
Ejecting bile and pancreatic juice into duodenum
• Gastric Inhibitory Peptide (GIP)
Is secreted when fats and carbohydrates enter small intestine
• Vasoactive Intestinal Peptide (VIP)
Stimulates secretion of intestinal glands
Dilates regional capillaries
Inhibits acid production in stomach
• Gastrin
Is secreted by G cells in duodenum
When exposed to incompletely digested proteins
Promotes increased stomach motility
Stimulates acids and enzyme production
• Enterocrinin
Is released when chyme enters small intestine
Stimulates mucin production by submucosal glands of
duodenum

22. Digestive Homeostasis Disorders
• ULCERS – erosion of the surface of the
alimentary canal generally associated with
some kind of irritant ; it is generally caused by
HCl secretion for long period of time in the
stomach .

23. Digestive Homeostasis Disorders
CONSTIPATION – a
condition in which the
large intestine is emptied
with difficulty.
Too much water is
reabsorbed and the solid
waste hardens .

24. Digestive Homeostasis Disorders
•DIARRHEA – a gastrointestinal
disturbance characterized by
decreased water absorption and
increased peristaltic activity of the
large intestine.
•This results in increased,
multiple, watery feces.
•This condition may result in
severe dehydration, especially in
infants

25. Digestive Homeostasis Disorders
APPENDICITIS – an
inflammation of the
appendix due to
infection
Common treatment is
removal of the
appendix via surgery

26. Digestive Homeostasis Disorders
• GALLSTONES – an accumulation of hardened
cholesterol and/or calcium deposits in the
gallbladder
• Can either be “passed” (OUCH!!) or surgically
removed

27. Digestive Homeostasis Disorders
• ANOREXIA NERVOSA - a psychological
condition where an individual thinks they
appear overweight and refuses to eat.
• Weighs 85% or less than what is
developmentally expected for age and
height
• Young girls do not begin to menstruate at
the appropriate age.

28. Digestive Homeostasis Disorders

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