The document provides a comprehensive overview of the human digestive system, explaining the necessity of digestion for energy use and the processes involved from ingestion to defecation. It details the roles of various organs and substances, such as the mouth, stomach, pancreas, liver, and small and large intestines, in breaking down food into usable nutrients. Additionally, it highlights the specific enzymes and stages involved in digestion, as well as the absorption of essential vitamins and the final formation of feces.

1. DIGESTIVE SYSTEM
Prof. Amol B Deore
Department of Physiology
MVPs Institute of Pharmaceutical Sciences, Nashik

2. Why Is Digestion of Food Necessary?
• The food we eat, for example, rice, bread, vegetables and
fruits, cannot be directly used by our body to get energy.
• The pieces of food are too big to enter the tiny cells of our
body. The food must therefore be broken down into small,
simple pieces.

4. Digestion
•The breakdown of complex food (carbohydrates,
proteins, and fats) into simple molecules that can then
be absorbed into the blood to be transported to cells.

5. Flow chart of Digestion
Complex food Simple form
Blood
circulation
Metabolism Energy

6. Digestion
•Digestion allows the body’s cells to convert food into
the high-energy ATP molecules that run the cell’s
machinery.
•The major organs and accessory structures that
perform this function are collectively referred to as
the digestive system.

7. Digestive System
• Digestion takes place in a
number of steps.
• There are several organs in the
body that help digestion.
Together, they make up our
digestive system.

9. Stages of digestion
Ingestion Mastication Deglutition Propulsion
Chemical
digestion
Defecation

10. Divisions of the Digestive System
Alimentary
canal
oral cavity, pharynx,
esophagus, stomach,
small intestine, and
large intestine
Accessory
organs
the teeth, tongue,
salivary glands, liver,
gallbladder, and
pancreas

11. MOUTH CAVITY
• Digestion starts in the mouth. The food must be first broken
down into smaller pieces by our teeth when we chew it.

12. Functions of mouth cavity
• Chewing i.e. Mastication
• Formation of Bolus
• Deglutition
The sharp teeth in front, bite and cut the food into smaller pieces.
The back teeth are flat and help to grind the food into a paste.

13. TEETH
• The teeth break up food by chewing.
• A complete set of permanent teeth consists of 32 teeth
Dental formula
• Incisors (8 total)
• Canines (4 total)
• Premolars (8 total)
• Molars (12 total)

14. Functions of teeth
• The incisor and canine teeth are the cutting teeth and are
used for biting off pieces of food,
• whereas the premolar and molar teeth used for grinding
or chewing food.

15. SALIVARY GLAND
• The saliva, a digestive juice made by
the salivary glands, mixes well with
the food while chewing.
• This softens the food, so that it can be
swallowed easily. Saliva also starts
breaking down starches into simpler
soluble substances.

16. Saliva
•Secretion of saliva is
continuous, but the amount
varies in different situations.
•The presence of food, sight or
smell of food in the mouth
increases secretion of saliva.
COMPOSITION OF THE SALIVA
Daily salivary secretion:1400 mL
PH = 5.8 – 7.4
1. Water
2. Mucus
3. Mineral salts: Na+, K+, Mg+2,
Ca+2, HCO3, SO4-2 etc.
1. Salivary amylase (Ptyline)
2. Lingual lipase
3. Lysozyme
4. Antibodies
5. Clotting factors
6. Urea, uric acid
7. Serum albumin, globulin

17. Functions of saliva
• Digestion of carbohydrates
• Digestion of fats
• Lubrication of the food
• Cleansing
• Defense mechanism
• Taste sensation

18. Function of saliva
• Digestion of carbohydrates
Cooked and uncooked
Starch
(Polysaccharide)
Maltose and
α-Dextrin
(Disaccharide)
SALIVARY AMYLASE
Salivary amylase initiates the breakdown of complex sugars (polysaccharides) like
starch, reducing them into disaccharide maltose.

19. Digestion of fats
• Lingual lipase initiates the breakdown of milk fats (triglycerides)
into free fatty acids and monoglycerides and diglycerides.
Fats and
Triglycerides
free fatty acids,
monoglycerides,
diglycerides
Lingual lipase

20. Tongue
•Our tongue helps mix the food with the saliva, and
to push the food into the food pipe.

21. ESOPHAGUS
• The food goes to the stomach
through the esophagus (food
pipe).
• The stomach muscles churn the
food with more digestive juices.
• It does not produce any
digestive enzymes and it does
not absorb food.

22. STOMACH
• Stomach juice break down proteins and other substances into simple
soluble substances.
• The food remains in the stomach for upto three hours.

24. Anatomy of stomach
•Stomach is ‘J’ shaped hollow, muscular organ
•acts as temporary reservoir of food bolus
•Stomach is connected to the esophagus by esophageal
sphincter and to the duodenum by pyloric sphincter.
•Stomach border consist of the two curvatures:
•Lesser curvature
•Greater curvature.

25. • The stomach is divided in a four
portions: the cardia, the fundus, the
body, and the pylorus.
• The fundus and body are mainly
storage areas, whereas most digestion
takes place in the pylorus.

26. Wall of the stomach
• The wall of the stomach made up of four layers: mucosa, submucosa,
muscularis and serosa.
Mucosa:
• The innermost mucosa contains tube-like openings called gastric pits.
• The inner membrane consists of folds called rugae. The rugae increase
the surface area and help in mixing the contents of the stomach.
• The gastric glands consist of mucous secreting cells, parietal cells, chief
cells and G-cells.

27. Mucosa
• Mucous cells secrete
mucus;
• Chief cells secrete the
gastric enzyme pepsinogen
and gastric lipase;
• Parietal cells secrete
hydrochloric acid and
intrinsic factor;
• G-cells secrete gastrin.

28. Chyme
• The combination of all of the
stomach secretions is known
as gastric juice.
• Food enters the stomach and
combines with gastric juice to
form a pasty substance called
chyme.
COMPOSITION OF GASTRIC JUICE
PH : acidic < 4
Daily secretion: 1000-2000 mL per day
1. Water
2. Mineral salts
Na+,K+, Mg+2, Ca+2, HCO3
-, SO4
-2 etc.
1. Mucus
2. Pepsinogen
3. Gastric lipase
4. Hydrochloric acid
5. Intrinsic factor (glycoprotein)

29. Function of the stomach and gastric juice
• Mechanical digestion:
• Protein digestion:
• Digestion of fats
• absorption of vitamin B12
• lubricantion
• Absorption of water and acidic drugs
• Formation of chyme

30. Mechanical digestion:
•In the churning activity, the gastric juice is properly
mixed with the food bolus to convert it into the
chyme; this chyme is ready for the digestion.

31. Protein digestion:
• Pepsinogen is converted into active pepsin by hydrochloric acid.
• Pepsin has proteolytic activity which promotes protein
digestion.

32. Digestion of fats
•Gastric lipase initiates the breakdown of milk fats
(triglycerides) into free fatty acids and
monoglycerides and diglycerides.
Triglycerides Gastric lipase free fatty acids, monoglycerides, diglycerides

33. Absorption of vitamin B12
• The intrinsic factor is necessary for the absorption of vitamin B12 in
the small intestine.
• Vitamin B12 (cyanocobalamine) is essential for erythropoiesis
(production of the RBC) in the red bone marrow.
intrinsic factor + vitamin B12 Absorption of vitamin B12 in small intestine

34. PANCREAS
Pancreas is found beneath
the great curvature of the
stomach and is connected
by a duct to the duodenum
of the small intestine.

35. Pancreatic gland
•Pancreas is made up of masses of cells are called the
acini which The are the exocrine glands of the organ.
•The acini release a mixture of digestive enzymes
called the pancreatic juice into the pancreatic duct
which is connected to the duodenum in
hepatopancreatic ampulla.

36. COMPOSITION OF
PANCREATIC JUICE
Daily secretion: about 500 to
600mL
PH : about 8
1. Water
2. Mineral salts
3. Pancreatic amylase
4. Pancreatic lipase
Proteolytic enzymes
1. Trypsinogen &
Chymotrypsinogen
2. Carboxypeptidase
3. Ribonuclease
4. Deoxyribonuclease

37. Functions of pancreatic juice
In small intestine, pancreatic amylase promote
breakdown of starch into corresponding
maltose and α-dextrin (diasaccharides).
Cooked & uncooked starch pancreatic amylase maltose and α-dextrin
(Polysaccharide) (Disaccharide)

38. Pancreatic and intestinal proteolytic enzymes promote
digestion of polypeptides in to peptides and amino acids.
Pancreatic enzymes ribonuclease and deoxyribonuclease
promote breakdown of RNA and DNA into nucleotides.
Bile salts and pancreatic lipase promote breakdown fats
into fatty acids and glycerol.

39. GALLBLADDER

40. Gallbladder
•The Gallbladder is a sac about 3 to 4 inches (7.5 to 10 cm)
long located on the undersurface of the liver.
•Bile in the hepatic duct of the liver flows through the
cystic duct into the gallbladder, which stores bile until it is
needed in the small intestine.
•When fatty foods enter the duodenum, the gallbladder
release bile into the cystic duct, then into the common
bile duct, and on into the duodenum.

41. Bile
COMPOSITION OF BILE
Daily secretion: about 500mL
PH :- About 8.0
1. Water
2. Mineral salts
3. Mucus
4. Bile salts: Sodium taurocholate,
Sodium glycocholate,
1. Bile pigment: Bilirubin
2. Stercobillin
3. Cholesterol
Functions of bile
• emulsification of fats
• Excretion of bilirubin through
stools

42. SMALL INTESTINE
Small intestine is continuing from the pyloric sphincter of
the stomach and further connected to the large intestine.
Small intestine is an elongated, tube like structure and
about 5 meter in length and averages 2.5 cm in diameter.
The small intestine surrounded by large intestine.

44. The small
intestine is
divided into
three
portions.
• First is the duodenum, which originates
at the pyloric sphincter and joins the
second portion, the jejunum.
• The jejunum is about 8 feet long and
extends to the third part, the ileum.
• The ileum measures about 12 feet and
links the large intestine at the ileocecal
valve (sphincter).

45. Fig 9.8: anatomy of small intestine, villi & microvilli

46. Wall of small intestine
• The intestine wall is made up of four layers: Mucosa, submucosa,
muscularis & serosa.
• Mucosa: the mucosa contains permanent circular folds (cavities), villi
and microvilli which provide large surface area for digestion and
absorption.
• The villi are tiny finger like projections of mucosa layer of the small
intestine.

47. Villi
• The villi are about 0.5-1 mm long and give velvety
appearance to the mucosa.
• The mucosa contains about 30-40 villi per mm2.
• The walls of villi are made up of columnar epithelium
(enterocytes) and they are covered by tiny brush border
like microvilli (size 1µm).
• The columnar epithelium of villi is made up of goblet cells,
absorptive cells, paneath cells and enteroendocrine cells.

48. Goblet cells secrete mucus
Absorptive cells secrete digest and absorb nutrients
Paneath cells secrete lysozymes for phagocytosis
Enteroendocrine cells secrete hormone: secretin, CCK and GIP.
The combination of all of the small intestine secretions is known as
intestinal juice.

49. • The villi enclose a blood capillary network and lymph capillaries called
lacteals. Water-soluble nutrients are absorbed into the blood in the
capillary networks. Fat-soluble nutrients are absorbed into the lymph
in the lacteals of the villi.
• When chyme passes into small intestine it is mixed with pancreatic
juice, bile and intestinal juice and is in contact with villi.

50. INTESTINAL JUICE
COMPOSITION OF INTESTINAL JUICE
Daily secretion: about 2000 mL
PH : 7.8 – 8.0
1. Water,
2. Mucus,
3. Mineral salts
Na+, K+, Mg+2, Ca+2, HCO3
-, SO4
-2 etc
1. Enzymes
- Enterokinase or (enteropeptidase),
- α-Dextrinase, Maltase, Sucrase,
- Lactase, Peptidase, Nucleosidase
Function of small intestine
• Peristalsis
• Absorption of nutrients
• Digestion of
carbohydrates
• Digestion of proteins
• Digestion of nucleic acid
• Digestion of fats/lipids

51. Function of small intestine
Peristalsis:
• The contraction and relaxation of smooth muscles
permits onward movement of chyme in the intestine.
• Peristalsis mixes the chyme with digestive juices and
initiates digestion and absorption of nutrients.

52. Absorption
•In small intestine the digestion of all nutrients is
completed.
•Approximately 80% of all absorption of nutrients
(simple sugars, amino acids, fatty acids, water,
vitamins, and minerals) occurs in the small intestine.

53. Digestion of carbohydrates

54. Digestion of proteins
• Pancreatic and intestinal proteolytic enzymes promote
digestion of polypetides in to peptides and amino acids.

55. Digestion of nucleic acid
• Pancreatic enzymes ribonuclease and deoxyribonuclease
promote breakdown of RNA and DNA into nucleotides.
• Intestinal enzymes nucleosidase promote breakdown of
nucleotides into nitrogenous bases, pentose sugars and
phosphates.

56. Digestion of fats/lipids
•Bile salts and pancreatic lipase promote breakdown
fats into fatty acids and glycerol.

57. THE LARGE INTESTINE
The large intestine is about 5 feet
in length and averages 2.5 inches
in diameter.
It is also referred to as the bowel.
It is divided into four principal
regions: the cecum, the the colon,
the rectum; and the anal canal.

59. Functions of large intestine
•Absorption of water: The colon promotes the
absorption of water, minerals, and vitamins and
the elimination of indigestible material. About
80% of the water that enters the colon is
absorbed (400 to 800 mL per day).

60. Functions of large intestine
• Mass movements are long, slow-moving but powerful contractile
waves that move over large areas of the colon three or four times
daily and force the contents toward the rectum.
• Typically, they occur during or just after eating, when food begins
to fill the stomach and small intestine. Bulk, or fiber, in the diet
increases the strength of colon contractions and softens the tool,
allowing the colon to perform its function more effectively.

61. Intestinal microbial flora
• It consists of the trillions of non-pathogenic bacteria that live in the colon:
Streptococcus fecalis, Clostridium welchili, Enterobacter aerogen, and E.-
coli. They inhibit the growth of pathogens. They also synthesizing certain
vitamins that are also absorbed in the colon:
• Vitamin K needed for clotting.
• Biotin needed for glucose metabolism.
• Vitamin B5 needed to make certain hormones and neurotransmitters.
• Folic acid needed for RBC production in red bone marrow.

62. Defecation:
• Feces consist of cellulose and other indigestible
material, dead and living bacteria, and water.
Defecation is the act of emptying the rectum and is
the final activity of the digestive system. Feces consist
of water, Mucus, undigested fibers, undigested food,
dead or live bacteria, fatty acid, and stercobillin.

63. LIVER

65. • The liver consists of two large lobes, right and left, and fills the upper
right and center of the abdominal cavity, just below the diaphragm.
The structural unit of the liver is the liver lobule, a roughly hexagonal
column of liver cells (hepatocytes). Between adjacent lobules are
branches of the hepatic artery and portal vein. The capillaries of a
lobule are sinusoids, large and very permeable vessels between the
rows of liver cells. The hepatic artery brings oxygenated blood, and
the portal vein brings blood from the digestive organs and spleen.
Each lobule has a central vein. The central veins of all the lobules
unite to form the hepatic veins, which take blood out of the liver to
the inferior vena cava

66. • The cells of the liver have many functions (which are discussed in a
later section), but their only digestive function is the production of
bile. Bile enters the small bile ducts, called bile canaliculi, on the liver
cells, which unite to form larger ducts and finally merge to form the
hepatic duct, which takes bile out of the liver. The hepatic duct unites
with the cystic duct of the gallbladder to form the common bile duct,
which takes bile to the duodenum.

67. Function of the Liver

68. Carbohydrate metabolism
 Excess glucose and other monosaccharides can be converted and stored in
the form of glycogen by the process of glycogenesis.
Glucose Insulin Glycogen
 During hypoglycemia, fasting or stress situations, glycogen is converted back
to glucose to raise the blood glucose level by the process of glycogenolysis.
Glycogen Glucagon Glucose glycolysis & Krebs cycle ATP
 The conversion of glycerol, amino acid into glucose molecules by the process
of gluconeogenesis.
Glycerol, amino acid & disacch. Gluconeogenesis Glucose

69. Lipid metabolism
 The long chains fatty acids are split into acetyl coenzyme-A by a process of
Beta oxidation. The acetyl coenzyme-A may be used by the liver cells to
produce ATP molecules by Krebs cycle.
Fatty acids beta-oxidation acetyl co-A Krebs cycle energy ATP
 Liver synthesizes lipoproteins LDL, VLDL and HDL for the transport of fatty
acids, triglycerides and cholesterol in the blood and to the body tissues.
 The liver also synthesizes cholesterol and excretes excess cholesterol into
bile to be eliminated in feces.

70. Protein metabolism
• Excess amino acids cannot be stored in liver. Hence the liver cells
promote deamination of amino acids in which -NH2 group is
removed from an amino acid, and the remaining carbon chain may
be converted to a simple carbohydrate molecule or to fat. Thus,
excess amino acids are utilized for energy (ATP) production by Krebs
cycle.
• This process produces ammonia as a waste product, which is
converted into urea (harmless) that is then excreted in urine.
• The liver regulates levels of amino acids by transamination, i.e. the
transfer of an amino group (-NH2) from an amino acid to another to
form new amino acid molecule.

71. Synthesis of plasma proteins
•Hepatocytes synthesize plasma proteins
albumin, alpha globulin, beta globulin, &
glycoproteins.
•Synthesis of anticoagulant heparin as well as
coagulation factors including I (prothrombin), II
(fibrinogen), V, VII, VIII, IX, and X.

72. Secretion of bile
•The liver produces bile salts (sod. taurocholate,
sod. glycocholate) that digest (break down) fats.
These bile salts are transported to the small
intestine for the emulsification of fats, lipids,
cholesterol, phospholipids and lipoproteins.

73. Inactivation of the hormones
•Hepatocytes chemically alter or inactivate
certain hormones including insulin growth
hormone, thyroid hormones, estrogen and
progesterone etc.

74. Detoxification of the drugs
• The hepatocytes of the liver perform the detoxification of drugs &
convert them into inactive and less toxic metabolites so as to be
excreted out of body. This process is called as drug metabolism.
e.g. metabolism of penicillin, erythromycin, sulfa drugs etc.
ACTIVE DRUG
INACTIVE
METABOLITE EXCRETION

75. Metabolism of the alcohol
•Metabolism of alcohol from beverages
& Ayurvedic medicines take place in the
hepatocytes.

76. Synthesis of vitamins A
•Liver promote the synthesis of vitamin A
(Beta-carotene and retinol) from the green
vegetables sources & fruits like carrots,
pumpkins, sweet potatoes etc.

77. Activation of vitamin D
•The liver, skin and kidney convert vitamin D to its
active form (calciferol). Vitamin D is important for
normal bone and teeth development.
Vitamin D Calciferol

78. Storage of vitamins
•The liver stores the fat-soluble vitamins A, D, E,
and K, and the water-soluble vitamin B complex
(including riboflavin, niacin, folic acid, pyridoxine
& cynocobalamine).
•The liver also stores minerals (iron and copper).

79. Phagocytosis
•The fixed macrophages of the liver are
called Kupffer cells.
•Besides destroying old RBCs, Kupffer cells
phagocytize pathogens or other foreign
material that circulate through the liver.

80. Excretion of bilirubin
•The liver contains fixed macrophages (Kupffer
cells) that phagocytize old red blood cells (RBCs).
•Bilirubin is then formed from the heme-portion
of the hemoglobin. The bilirubin excreted into
the bile so as to be eliminated in feces.