This document provides an overview of the physiology of the gastrointestinal system. It begins with an introduction and then covers the functions and contents of various parts of the GI tract, including the mouth, stomach, small intestine, large intestine, liver, gallbladder and pancreas. Specific topics discussed include the role of saliva, gastric juice, pancreatic juice, and bile in digestion and absorption of nutrients. The functions of digestive enzymes and how carbohydrates, proteins and fats are broken down are also explained.

1. By: SATHISH Rajamani
Asst Professor
VNC - Panipat
PHYSIOLOGY
OF
GI SYSTEM
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2. CONTENTS
 Functions and Organs of Digestive Tract
 Movements of Alimentary Tract
 Digestion in
 Mouth
 Stomach
 Small Intestines
 Large Intestines
 Absorption of Food
 Functions of Liver, Gall Bladder and Pancreas
 Metabolism of Carbohydrates, Proteins & Fats

3. INTRODUCTION TO DIGESTIVE SYSTEM
 Digestion is defined as the process by which food is broken down into simple chemical substances that can
be absorbed and used as nutrients by the body.
 A normal young adult consumes 1 kg of solid diet and 1–2 Liter of fluid per day.
 Functions of digestive system includes
 Ingestion or consumptions of food substances.
 Breaking them into small particles.
 Transport of small particles to different areas of the digestive tract.
 Secretion of necessary enzymes and other substances for digestion.
 Digestion of the food particles.
 Absorption of the digestive products.
 Removal of unwanted substances from the body.

4. FUNCTIONS OF THE MOUTH
 Primary functions of the mouth is eating and it has few other important functions
also.
 Ingestion of food materials
 Chewing of food and mixing it with saliva
 Appreciation of taste of the food
 Transfer of the food (bolous) to the Esophagus by swallowing
 Role in speech
 Social functions such as smile and other expressions

5. PROPERTIES AND COMPOSITION
OF SALIVA
 Volume – 1000 ml to 1500 ml per day is secreted.
 Reaction – Slightly acidic with pH, of 6.35 to 6.85.
 Specific gravity ranges from 1.002 to 1. 012.
 Saliva is hypotonic to plasma.
Contribution of Secretion of saliva by each major salivary gland is
 Parotid Gland – 25 %
 Sub maxillary Gland – 70 %
 Sublingual Gland – 5 %

7. FUNCTIONS OF SALIVA
Saliva is a very essential digestive juice, since it has many functions, its absence leads to many functions.
Preparation of food for swallowing
Appreciation of taste
Digestive function
Cleansing and protective functions
Role in speech
Excretory function
Regulation of body temperature
Regulation of water balance

8. FUNCTIONS OF SALIVA
 Preparation of food for swallowing
• When food is taken in mouth, it is moistened and dissolved by saliva. The
mucus membrane of mouth is also moistened by saliva.
• It facilitates chewing by the movement of tongue, the moistened and
masticated food is rolled into a bolus.
• Mucin of saliva lubricates the bolus and facilitates the bolous and
swallowing.
 Appreciation of Taste
• Taste is a chemical sensation.
• By its solvent action, saliva dissolves the solid food substances, so that the dissolved substances can
stimulate the taste buds.
• The stimulated taste buds recognize the taste.

9. FUNCTIONS OF SALIVA
 Digestive Functions
• Saliva has 3 digestive enzymes, namely salivary amylase, maltase and lingual lipase.
Salivary Amylase – it is a carbohydrate – digestive enzyme. It acts on cooked or boiled starch and
converts into dextrin and maltose.
Optimum pH, necessary for the activation of salivary amylase is 6. salivary amylase cannot act on
cellulose.
Maltase – It is present only in traces in human saliva and it converts maltose into glucose.
Lingual Lipase – lingual lipase is a lipid digesting (lipolytic) enzyme.
It is secreted from serous glands situated on the posterior aspect of the tongue.
It digests milk fats. It hydrolyses triglycerides into fatty acids and diacylglycerol.

10. FUNCTIONS OF SALIVA
 Cleansing and Protective Functions
• The mouth and teeth are rinsed and kept free off food debris due to the constant secretions of
saliva.
• Enzyme lysozyme of saliva kills some bacteria such as staphylococcus, streptococcus and
brucella.
• Proline rich saliva possess antimicrobial property and neutralizes the toxic substances such as
tannins. Tannins are present in fruits.
• Lactoferrin of saliva also has antimicrobial property.
• Immunoglobulin IgA in saliva also has antibacterial and antiviral actions.
• Mucin present in the saliva protects the mouth by lubricating the mucus membrane of mouth.

11. FUNCTIONS OF SALIVA
 Role in Speech
By moistening and lubricating soft parts of mouth and lips, saliva helps in speech. If the mouth
becomes dry articulation and pronunciation becomes difficult.
 Excretory Function
• Both organic and inorganic waste substances are excreted in saliva. Substanceslike mercury
potassium, iodine and lead.
• Saliva also excretes some viruses like rabies and mumps.
• During abnormal pathological conditions like nephritis urea is excreted. Calcium is excreted
during hyperparathyroidism.

12. FUNCTIONS OF SALIVA
 Regulation of Body Temperature
Saliva does not play any role in body temperature regulation in human beings.
 Regulation of Water Balance
• Saliva secretion decrease when body water content decreases. This causes dryness of mouth and induces
thirst. When water is taken, it quenches the thirst and restores the body water content.

13. FUNCTIONS OF STOMACH
 Mechanical Function
 Digestive Function
 Protective Function
 Hemopoietic Function
 Excretory Function

14. FUNCTIONS OF STOMACH
 Mechanical Function
Storage Function – food is stored in the stomach for a long period.
i.e. for 3 to 4 Hrs and emptied into the intestine slowly.
The maximum capacity of stomach is upto 1.5L slow emptying of
stomach provides enough time for proper digestion and absorption of
food substances in the small intestine.

15. FUNCTIONS OF STOMACH
 Mechanical Function
Formation of Chyme – peristaltic movements of stomach mix the
bolus with gastric juice and convert it into the semisolid material
known as chyme.

16. FUNCTIONS OF STOMACH
 Digestive Function
Gastric juice present in stomach mainly acts on the proteins.
Proteolytic enzymes of the gastric juice are pepsin and renin.
Gastric juice also contains some other enzymes like gastric lipase,
gelatinase, urase and gastric amylase.

17. FUNCTIONS OF STOMACH
 Digestive Function
Pepsin – it is secreted as inactive pepsinogen. Pepsinogen is converted
Into pepsin by HCL. Optimum pH, for activation of pepsinogen is
below 6.
Action of Pepsin – it converts proteins into proteoses, peptones, and
polypeptides, pepsin also causes curdling and digestion of milk (Casein).
Gastric Lipase – it is a weak lipolytic enzyme when compared to pancreatic
lipase. It is active only when the pH, is between 4 and 5 and becomes inactive
at pH, below 2.5.

18. FUNCTIONS OF STOMACH
 Protective Function
Mucus is a mucoprotein, secreted by mucus neck cells of the gastric glands and
surface mucus cells in fundus, body and other parts of stomach. It protects
the gastric wall by the following ways.
Mucus – Protects the stomach wall from irritation of mechanical injury.
Prevents the digestive action of pepsin on the wall of the stomach, particularly
gastric mucosa.
Protects the gastric mucosa from hydrochloric acid of gastric juice because of
its alkaline nature.

19. FUNCTIONS OF STOMACH
 Hemopoietic Function
Intrinsic factor of Castle, secreted by parietal cells of gastric glands [lays an
important role in erythropoiesis.
It is necessary for the absorption of Vitamin B12 (Extrinsic factor) from GI
tract from GI tract into the blood.
Vitamin B12 is an important maturation factor during erythropoiesis.
Absence of intrinsic factor in gastric juice causes deficiency of vitamin B12
leading to pernicious anemia.

20. FUNCTIONS OF STOMACH
 Excretory Function
Many substances like toxins, alkaloids and metals are excreted through gastric
juice.

21. GASTRIC JUICE
• Properties and Composition of Gastric Juice
Volume: 1200 ml / day to 1500 ml / day.
Reaction: Gastric juice is highly acidic with a pH, of 0.9 to 1.2
(Acidity of juice is due to presence of HCL)
Specific gravity: 1.002 to 1.004

22. PANCREATIC JUICE
• Properties of Pancreatic Juice
• Volume: 500 to 800 ml / day
• Reaction: Highly Alkaline with a pH of 8 to
8.3
• Specific gravity: 1.010 to 1.018

23. PANCREATIC JUICE
FUNCTIONS
• Two functions of pancreatic juice are Digestive & Neutralizing
function.
• Digestive function
– Proteins
– Carbohydrates
– Lipids
Digestion of Proteins – Major proteolytic enzymes are Trypsin and
Chymotrypsin. Others are Carboxypeptidases, Nuclease, Elastase and
Collegenase.

24. Digestion of Proteins
• Trypsin – it is a polypeptide, it contains 229 amino acids. It is secreted as inactive trypsinogen, which
is converted into active trypsin by enterokinase and it is secreted by the brush – bordered cells of
duodenal mucous membrane.
– Actions of Trypsin - Most powerful proteolytic enzymes
Curdling of Milk (Converts caseinogen in the milk to casein
Accelerates blood clotting
• Chymotrypsin - It is secreted in an inactive form chymotrpsinogen which is converted into an
active form Chymotrypsin by trypsin.
Action of Chymotrypsin – Converts proteins into polypeptides
It digests caseinogen faster than trypsin
No action on Blood Clooting

25. Digestion of Lipids
• Pancreatic Lipase is a powerful lipolytic enzyme.
• It digests triglycerides into monoglycerides and fattyacids.
• Activity of pancreatic lipase is accelerated in the presence of bile.
• Digestion of fat by pancreatic lipase requires two more factors/
– Bile Salts: It is responsible for the emulsification of fat, prior to the digestion.
– Colipase: It is a coenzyme necessary for the pancreatic lipase to digest the dietary lipids.
About 80 % of the fat is digested by pancreatic lipase. Deficiency or absence of this enzyme leads to
excretion of undigested fats in faeces (Steatorrhea)

26. Digestion of Carbohydrates
• Pancreatic amylase is the amylotic enzyme present in pancreatic juice.
• Like pancreatic amylase also converts starch into dextrin and maltose.

27. BILE
• PROPERTIES OF BILE
 Volume – 800 – 1200 ml / day
 Reaction – Alkaline
 pH – 8 to 8.6
 Specific gravity – 1.010 – 1.011
 Colour – Golden Yellow or Green

28. BILE
• Composition of Bile
– Water – 97.6 %
– Salts – 2.4 % (Organic and Inorganic)

29. BILE
• Secretion of Bile
• Secreted by hepatocytes
• It contains bile acids, bile pigments,
cholesterol, lecithin and fatty acids.
• Most of the bile are stored in the gall bladder.

30. BILE
• Bile Salts are the sodium and potassium salts of bile acids.
• Functions of Bile Salts are
1. Emulsification of fats
2. Absorption of fats
3. Choleretic Action
4. Cholagogue Action
5. Laxative Action
6. Prevention of Gallstone Formation

31. BILE
Emulsification of fats – bile salts emulsify the fats by
reducing the surface tension due to their detergent
action.
Absorption of fats – bile salts combine with fats and
makes complexes of fat called micelles. The fat in the
form of micelles can absorbed easily/
Choleretic Action – Bile salts stimulates the secretion of
bile from the liver. This action is called choleretic
action.

32. BILE
Cholagogue Action– Cholagogue is an agent which
causes contraction of gall bladder and release of bile
into the intestine. Bile salts acts indirectly by
stimulating the secretion of hormone Cholecystokinin.
This hormone causes contraction of gallbladder,
resulting in the release of Bile.
Laxative Action – Laxative is an agent which induces
defecation. Bile acts as laxatives by stimulating
peristaltic movements of the intestine.

33. BILE
Prevention of Gallstone Formation – Bile salts prevent
the formation of gallstone by keeping the cholesterol
and lecithin in solution. In the absence of bile salts,
cholesterol precipitates along with lecithin and forms
gallstone.

34. BILE PIGMENTS
• Bile pigments are the excretory products of bile.
• Bilirubin and Biliverdin are the two bile pigments
and bilirubin is the major bile pigment in human
beings.

35. BILE PIGMENTS
• Bile pigments are formed during the breakdown of haemoglobin,
which is released from the destroyed RBCs in the reticuloendothelial
system.
• Normal plasma levels of Bilirubin – Normal bilirubin (Total Bilirubin)
content in plasma is 0.5 – 1.5 Mg/dl. This condition is called
hyperbilirubinemia. When it exceds 2 mg/dl Jaundice occurs.

36. BILE FUNCTIONS
• Most of the functions of the bile are due to the bile salts
1. Digestive function
2. Absorptive function
3. Excretory function
4. Laxative function
5. Antiseptic action
6. Choleretic action
7. Maintenance of pH in GI tract
8. Prevention of gallstone formation
9. Lubrication function
10. Cholagogue action

37. BILE FUNCTIONS
1. Digestive function – Refer bile salts function
2. Absorptive function - Refer bile salts function
3. Excretory function – bile pigments are the major excretory products of the bile.
Other substances excreted in the bile are
1. Heavy metals like copper and iron
2. Some bacteria like typhoid bacteria
3. Some toxins
4. Cholesterol
5. Lecithin
6. Alkaline phosphatase
4. Laxative action – Bile salts acts as laxatives
5. Antiseptic action – Bile salts inhibits the growth of certain bacteria in the lumen of
intestine by its natural detergent action.

38. BILE FUNCTIONS
6. Choleretic Action – with the help of bile salts, bile secretion is stimulated from the
liver.
7. Maintenance of pH in the GI – tract – As bile is highly alkaline, it neutrakizes
the acid chyme which enters the intestine from stomach. Thus an optimum pH is
maintained for the action of digestive enzymes.
8. Prevention of Gallstone Formation – Bile salts prevents the formation of
gallstone by keeping the cholesterol and lecithin in solution. In the absence of bile salts,
cholesterol precipitates along with lecithin and forms gallstone.

39. BILE FUNCTIONS
9. Lubrication Function – The mucin in bile acts as a lubricant for the chyme in the
intestine.
10. Cholagogue Action – Cholagogue is an agent which causes contraction of
gallbladder and release of bile into the intestine. Bile salts in bile acts as cholagogues
indirectly by stimulating the secretion of hormone cholecystokinin. This hormone
causes contraction of gallbladder resulting in release of bile.

40. FUNCTIONS OF LIVER
Liver is the largest gland and one of the vital organs of the body. It performs
many vital metabolic and homeostatic functions, which are summarized
below
1.Metabolic function – In liver fats, carbohydrates, proteins, vitamins and
many hormones are metabolized.
2.Storage function – Substances like glycogen, amino acids, iron, folic acid
and vitamin A, B 12, and D are stored in liver.
3.Synthetic function – Liver produce glucose by gluconeogenesis. It
synthesize all the plasma proteins and other proteins (except
immunoglobulins) such as clotting factors. It also synthesizes steroids.
4.Secretion of bile – liver secretes bile which contains bile salts, bile pigments,
cholesterol, fatty acids and lecithin.

41. FUNCTIONS OF LIVER
5. Excretory function – liver excretes. Cholesterol, bile pigments, heavy
metals (like lead, arsenic and bismuth) toxins, bacteria and virus (yellow
fever virus) through bile.
6. Heat production – Enormous amount of heat is produced in the liver
because of metabolic reactions . Liver is the organ where maximum heat
is produced.
7. Hemopoietic Function – In fetus liver produces the blood cells, it stores
Vitamin B12 for the erythropoiesis and iron necessary for the synthesis of
hemoglobin. Liver produces thrombopoietin that promotes the
production of thrombocytes.
8. Hemolytic function – The senile RBC’s after the life span of 120 days are
destroyed by reticuloendothelial cells of liver (Kupffer Cells)

42. FUNCTIONS OF LIVER
9. Inactivation of Hormones and Drugs – Liver catabolizes the growth
hormones, parathormone, cortisol, insulin, glucagon and estrogen it also
inactivates the drug. The fat soluble drugs are converted into water
soluble substances, which are excreted through bile or urine.
10. Defensive and Detoxification function – Reticuloendothelial cells
(Kupffer cells) of the liver play an important role in the defence of the
body. Liver is also involved in the detoxification of the foreign bodies.

43. FUNCTIONS OF GALLBLADDER
Bile secreted from liver is stored in gallbladder. The capacity of gallbladder is approximately 50
ml.
Gallbladder is not essential for life and it is removed (Cholecystectomy) in patients suffering from
gallbladder dysfunction.
Major functions of Gallbladder are as follows
 Storage of bile
 Concentration of bile
 Alteration of pH of bile
 Secretion of Mucin
 Maintenance of pressure in biliary system

44. FUNCTIONS OF GALLBLADDER
 Storage of bile – Bile is continuously secreted by liver is stored in the gallbladder, it is
released in the intestine intermittently when it is required.
 Concentration of bile – Substances like bile pigments, cholesterol, lecithin helps to
concentrate bile 5 to 10 times.
 Alteration of pH of bile – The pH of bile is decreased from 8 – 8.6 to 7 – 7.6 and it
becomes less alkaline when it is stored in gallbladder.

45. FUNCTIONS OF GALLBLADDER
 Secretion of Mucin – Gallbladder secretes mucin. Mucin is added to bile, when bile is
released into intestine, mucin acts as lubricant for movement of chyme in the
intestine.
 Maintenance of pressure in biliary system – Due to the concentrating capacity
gallblader maintains pressure of about 7cm H2ONin billiary system. This pressure in
the billiary system is essential for the release of bile into the intestine.