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The primary struggle for any life form on Earth is first 'survival of self and then
'survival of its species'
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Nutrition (L
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The components of food when digested, absorbed and assimilated by a living
body help in their growth, development, maintenance of structure, repair and
protection
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Vitamins
and minerals are also required in small quantities
...
It is composed of
fibers of plant origin and connective tissues present in food from an animal
source
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Food eaten by human beings comes majorly from two sources i
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, plants and
animals
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HUMAN DIGESTIVE SYSTEM
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Human beings exhibit a 'tube within a tube' body plan
...
The second inner tube is represented by the digestive tract
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❖ Accessory digestive glands like liver, gallbladder, pancreas, salivary
glands that are located outside the alimentary canal
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Mouth
• Anterior opening or slit of the alimentary canal
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Buccal cavity
It is subdivided into two parts
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❖ Peripheral in location
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Main oral cavity
❖ It is the inner and central
part
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❖ Contains openings of
salivary glands
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Palate: The Roof of Buccal Cavity
• The anterior part of the palate is
hard palate
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• It consists of transverse ridges/folds (palatine rugae)
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• The posterior part of the palate is soft palate and it helps in swallowing
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• The posterior outgrowth of the soft palate is uvula or velum palate
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Tongue
• Tongue is joined to floor of the buccal cavity by frenulum linguae
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• By sulcus terminalis, the upper surface of tongue is divided into two unequal
parts
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Gustatory (taste) receptors
are present on the tongue
surface in the form of taste
buds
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Taste
buds present anteriolaterally sense salty taste
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The tongue helps in
reception of taste, mixing of food with saliva, cleaning of the oral cavity, speech,
and swallowing food
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Teeth
In humans, teeth are anchored to the maxilla and mandible bone
...
Thus, teeth are embedded in alveoli or
sockets
▪ Structure of Tooth
A tooth is differentiated into three parts- Crown, neck, and root
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It is the hardest material in the body of human
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❖ The main part of tooth is dentine
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It contains fine canaliculi passing from pulp cavity to the enamel
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❖ Root: It is the basal part of tooth that is embedded inside the socket of jawbone
(alveoli) and covered by cement and periodontal membrane
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❖ Cement is further covered by a periodontal ligament which fixes the tooth in its
socket
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It consists of blood vessels, nerves,
odontoblasts, and loose connective tissue
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❖ Through apical foramen, blood capillaries and nerve fibers enter into the root
canal of the tooth
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Enamel originates from the ectoderm of the
embryo
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Human Dentition
❖ Milk teeth: Teeth which appear during childhood are called milk
teeth/temporary teeth/primary teeth
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▪

Types of Teeth
In mammals, four types of teeth are found:

▪

Incisors: These are the long, chisel-like teeth with sharp edges for gnawing the
food
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Premolars: These bicuspid teeth are used for chewing and crushing the food
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These are also meant for chewing and crushing
the food
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g
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• Diphyodont: These teeth erupt twice in life e
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, incisors, canines
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• Thecodont: These teeth are attached to the bony sockets of the jaw
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Do You Know ?
Permanent human teeth do not form again because after certain time odontoblast
activity gets over
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• Pharynx has three parts- nasopharynx, oropharynx, and laryngopharynx
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The mouth of this tube is enclosed by
tubal tonsils
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Oropharynx (behind the oral cavity): In oropharynx, two palatine tonsils occur
laterally
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Deglutition Reflex
• Ideally, the masticated food in the oral cavity (bolus) should move into the
esophagus via the oropharynx
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• Physiologically, larynx is placed ventrally, and esophagus is placed dorsally in
the neck
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• During the initial stage of swallowing, the tongue voluntarily exerts upward and
backward pressure against the soft palate
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• In the brain, a deglutition reflex is formed, because of which two responses
occur:
❖ The soft palate including uvula is pulled upwards to close the internal
nares
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❖ The neck muscles contract and pull the trachea up so that glottis emerges
at a higher position than that of gullet
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• The upper surface of epiglottis becomes like a slide or slippery surface over
which the food slides and enters the gullet
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• From gullet food sinks down the esophagus Involuntarily and reaches the
stomach due to peristalsis
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• Bolus: Soft mass of chewed partially digested food mixed with saliva which
moves into the stomach from oral cavity via esophagus
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It
includes lingual, palatine, tubal and nasopharyngeal tonsils and provides lymphocytes
and Immunoglobulin A
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• Originates from lower end of pharynx and runs downward through the thorax,
pierces the diaphragm and finally opens into stomach through the gastroesophageal orifice
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Instead, a white fibrous
peritoneum covering called tunica adventitia is present
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• No digestion takes place in the esophagus as enzyme secreting glands are
absent in its inner epithelial lining (mucosa)
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• It is the broadest part of the alimentary canal
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• Stomach is covered by a peritoneum layer
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Such peritoneum is called omentum
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Right
curved surface of the stomach is covered by the lesser omentum
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❖ Fundus: It is filled with air
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❖ Pylorus: It is the posterior end part of the stomach through which food
enters the duodenum through pyloric sphincter
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• The longitudinal, circular and oblique muscular layers present in the gastric wall
thoroughly mix the bolus with acidic gastric juice of the stomach
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Do You Know ?
Heartburn: Sometimes the lower esophageal sphincter fails to close adequately
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This is called
gastroesophageal reflux disease (GERD) or heartburn
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8) which gives an
optimum functioning condition for gastric enzymes and kills microorganisms
coming along with the swallowed materials
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• Mucus and bicarbonates present in the gastric juice lubricate and protect the
mucosal epithelium from highly concentrated HCL
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25 m long
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• Absorption of digested food also happens in the small intestine
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• It also secretes intestinal digestive enzymes
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Maximum digestion takes place in duodenum
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5m long): It is a long, coiled, middle part of the small intestine
where maximum absorption takes place
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5m long): It is the longest part of the small intestine
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Various modifications are present in the small intestine to increase the surface
area for efficient absorption of digested food, e
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, greater length of the intestine,
presence of permanent deep folds in mucosa called plicae circulares or Valves of
Kerckring (prominent in jejunum), finger like projections called villi of the mucosa
of small intestine (absent over Peyer's patches), finger-like extensions (microvilli)
of the cell membrane of cells present in the villi (enterocytes)
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5m in length
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• Large intestine is differentiated into three parts-Caecum, colon, and rectum
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This
opening is guarded by an ileocecal valve
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❖ About 2cm below the ileocecal orifice, a worm-like blind structure
arises from the caecum called a vermiform appendix (8 cm long)
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It serves as a safe storehouse of

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beneficial gut bacteria
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❖ It is thought to give early protection against deadly diseases
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Colon (140-155 cm long):
❖ Colon of human has an ascending (shortest part of colon, 18 cm
long), transverse (50 cm long), descending (25 cm long) and
sigmoid (S-shaped, 40-45 cm long) part
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❖ The bend between ascending colon and transverse colon is called
the right hepatic flexure whereas the bend between the transverse
colon and the descending colon is called the left splenic flexure
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As these bands are shorter
than the rest of the colon, between the taeniae, the wall of the colon
forms outpouchings (haustra)
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❖ There are no villi,
digestive glands and
plicae circulares on its
mucosa
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❖ Many symbiotic microorganisms reside in the colon and depend on
undigested food
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❖ The type of contraction that occurs only in the colon is the
progressive and continuous mass action contraction which pushes
the waste material forward into the rectum
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❖ Taeniae coli and haustra are absent here
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❖ Terminal end of large intestine is anal canal (3-4 cm long)
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Anal opening is controlled by anal
sphincters
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HISTOLOGY OF ALIMENTARY CANAL
The wall of alimentary canal is composed of four layers (outer to inner):
Serosa (visceral peritoneum)
• It is the outermost layer of the alimentary canal
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• Esophagus is lined with external adventitia which is made up of dense elastic
fibrous connective tissue
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The
external anal sphincter is also formed by skeletal muscles, thus permitting
voluntary control of defecation
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e
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• The thickest layer is found in the stomach (maximum peristalsis) and the thinnest
layer in rectum
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• Auerbach's plexus (myenteric plexus) is the network of nerve cells and
parasympathetic nerve fibers which is present in between longitudinal and
circular muscular fibers to control gastrointestinal movement (peristalsis)
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Meissner's plexus (submucosal plexus) is the network of nerve cells and
sympathetic nerve fibers which is present between muscularis and the mucosa
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In stomach, Meissner's
plexus lies between oblique muscle and submucosa
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It forms plicae circulares in the intestine
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Mucosa muscularis
❖ It is the outer layer (towards submucosa)
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❖ It plays an important role in exposing the surface area of intestinal wall for
absorption
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Lamina propria
❖ It is the middle layer and contains blood vessels and lymphatic tissue
called as MALT (Mucosa Associated Lymphoid Tissue) that provides
immunity e
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, Peyer's patches (in small intestine, mainly ileum)
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Epithelial mucosa
❖ It is the Innermost layer
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❖ Folds of mucosal epithelium in small intestine are conically shaped called
villi (maximum villi are found in jejunum)
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❖ Invaginations of epithelium called Crypts of Lieberkühn are present in the
small intestine
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• Ducts from glands outside intestine: e
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, Salivary glands, liver, pancreas
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g
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• Ducts from glands of mucosa: e
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, Gastric glands, intestinal glands
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Digestive glands present in the mucosa of alimentary canal are called gastric glands
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Digestive Glands

Mucosal Glands

Accessory Digestive Glands

1
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Intestinal glands

1
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Liver
3
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• Secretion of salivary glands is known as saliva
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)
❖ pH-6
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5% moisten the food
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❖ Antibacterial substances lactoferrin, thiocyanates Lysozymes,
❖ Few ions like Na, K, Cl, HCO₃- Buffering agents
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2-1
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• The largest gland of the body is liver
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Left lobe is
smaller in comparison to the right lobe
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• Right and left hepatic duct drains bile from right and left hepatic lobe respectively
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• Gall bladder is situated below right lobe of liver and drained via cystic duct
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Cystic duct of gall bladder
joins common hepatic duct to
form a common bile duct
(ductus choledochus)
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Sphincter of Oddi guards the
opening of hepatopancreatic
ampulla into the duodenum
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Hepatic Lobule
❖ Hepatic lobule is the functional and structural unit of liver
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❖ Each lobule comprises of radial rows of hepatic cells (hepatocytes) which
are called hepatic cords
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Hepatocytes pour bile into the canaliculi
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These branches are also present at the angular part
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Branch of hepatic portal vein,
hepatic artery and hepatic ducts are collectively called portal triad
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All the central veins unite to form a hepatic vein which emerges
out from liver and opens into inferior vena cava

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❖ All branches of hepatic duct of right and left lobe unite to form right and left
hepatic ducts which emerge out from the liver and form a common hepatic
duct
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❖ Bile juice does not have any digestive enzymes
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whereas bile from gall bladder has a pH of 7
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❖ Daily secretion is 500-1000ml
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❖ Bile pigments are the excretory materials of the liver
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❖ Bile contains two types of salts-inorganic salts (NaCl, Na2CO3, NaHCO3,
etc
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❖ Inorganic salt neutralizes the acidity of the food whereas organic salt help
in emulsification of fats
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Bile salts mix with fats, cholesterol, phospholipid (lecithin) and
fat-soluble vitamins to form compounds called micelles
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❖ Metabolism Carbohydrate: The site of carbohydrate metabolism is liver
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The main stored food in the liver is glycogen
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o Gluconeogenesis: Liver converts non carbohydrate compounds
(e
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; amino acids, fatty acids) into glucose
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❖ Storage of fats: Liver stores fats in a small amount
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❖ Liver converts ammonia (more toxic) into urea (less toxic) through
ornithine cycle
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❖ Synthesis of plasma proteins: All plasma proteins (except Gammaglobulins) synthesized in the liver
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These help in clotting of blood
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❖ Synthesis of heparin: Heparin is a natural anticoagulant
(mucopolysaccharide)
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❖ Synthesis of vitamin A: The liver changes B-carotene into vitamin A
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are
stored in liver
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❖ Hemopoiesis: The formation of blood cells is called hemopoiesis
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❖ Formation of Lymph
❖ Formation of Angiotensinogen: Angiotensinogen formed in the liver is
changed to angiotensin which helps in regulation of kidney
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❖ Emulsification: Sodium glycocholate and sodium taurocholate are bile
salts which break large fat droplets into the smaller ones thereby
increasing the surface area for lipase action
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❖ Excretion: Bile pigments (bilirubin and biliverdin) are excretory products
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Pancreas
• It is endodermal in origin
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It’s 99% part is exocrine while 1% part is
endocrine
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Acini is a
group of secretory cells that surrounds a cavity
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These acinar cells secrete the enzymes of pancreatic
juice
Each acini joins the pancreatic ductule
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The main pancreatic duct joins
with the common bile duct to form the hepatopancreatic ampulla which opens
into duodenum
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The endocrine part of pancreas consists of group of endocrine cells (a, b, d, and
PP cells) found in-between group of acini are called Islets of Langerhans
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Composition of pancreatic Juice:
❖ Daily secretion of pancreatic juice in human is about 500-800 ml (pH is
8
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❖ Pancreatic juice has water (98%), salt (sodium bicarbonate), inactive
enzymes like, trypsinogen, chymotrypsinogen, procarboxypeptidase, and

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active enzymes like pancreatic lipase, pancreatic amylase, nucleases
(DNase and RNase)
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• They are numerous, microscopic, simple, branched or unbranched tubular glands
formed by the invagination of stomach epithelium
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• Principal gastric glands are present in the fundus and body of the stomach
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❖ Oxyntic cells
(Parietal cells):
These secrete
hydrochloric acid
and Castle's Intrinsic
factor
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Microorganisms that enter the gut along with food are destroyed by HCl
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❖ Argentaffin cell: Secrete somatostatin (which suppresses the release of
gastrin hormone), a precursor of serotonin (5-hydroxy tryptamine), and
histamine
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❖ Stem cells: Undifferentiated cells which differentiate periodically to
replace damaged cells
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These are of two
types:
❖ Mucus secreting mucous neck cells or goblet cells
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Composition of Gastric juice
❖ Daily secretion - 2-3 L
❖ Water 99
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203%
❖ pH 1
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Histamine acts as a vasodilator and promotes gastric acid secretion
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• Crypts of Lieberkuhn
❖ Occur between the villi of small intestine
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❖ They contain Paneth cells and argentaffin cells
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They also release antibacterial
lysozymes
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• Brunner's glands
❖ They are present in the submucosal layer of duodenum and opens into the
crypts of Lieberkuhn
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Do You Know ?
The secretion of intestinal glands is called intestinal juice or succus entericus
with a pH 7
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Around 2-3 liters of intestinal juice is secreted per day
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DIGESTION OF FOOD
The process of digestion is completed by mechanical and chemical processes
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❖ Saliva contains salivary amylase (an amylase) or ptyalin
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8)
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e
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❖ The sweet taste of some foods after chewing is due to maltose
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Starch/Glycogen
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Ptyalin / pH 6
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❖ The activity of salivary amylase and the small amount of gastric amylase
present in the gastric juice is blocked by the hydrochloric acid secreted by
the oxyntic cells of the gastric mucosa
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This juice contains
amylase, also known as pancreatic amylase
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❖ The intestinal juice contains limit dextrinase, isomaltase and maltase
enzymes for hydrolyzing limit dextrins, isomaltose and maltose
(respectively) into glucose
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❖ There are two more carbohydrate digesting enzymes in the intestinal juice
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Sucrase hydrolyses sucrose into glucose
and fructose and lactase converts lactose into glucose and galactose
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Digestion of Proteins
The proteins consumed by human beings are chemically long chains of amino acids
bound by peptide bonds
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Digestion of Proteins in the Stomach
❖ Since saliva does not have any protein-digesting enzyme, the initial steps
for hydrolyzing proteins starts in the stomach
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❖ Once inactive pepsinogen (secreted by the chief cells of gastric mucosa)
gets converted into active proteolytic pepsin, it digests proteins into
proteoses, peptones and a few polypeptides by breaking the peptide
bonds between their amino acids
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Calcium paracaseinate is further broken down by pepsin to form peptones
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4) from the pancreas
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proteolytic enzymes namely
trypsinogen, chymotrypsinogen, procarboxypeptidase and proelastase
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❖ On activation, trypsin itself catalyses the activation of chymotrypsinogen to
active chymotrypsin, proelastase to active elastase and
procarboxypeptidase to active carboxypeptidase
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Digestion of Proteins in the Small Intestine by intestinal secretions
❖ The enterocytes produce two types of peptidases, aminopeptidases and
several dipeptidases
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Digestion of Fats
Most lipid digestion occurs in the small intestine by the action of pancreatic lipase
although some fat digestion also occurs in the stomach by lingual lipase and gastric
lipase
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of any
emulsifying agent
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To increase the
interface, fats must be
broken down to fine
droplets through a
process called
emulsification by bile
salts
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❖ Bile salts and lecithin are amphipathic compounds (having both waterloving, hydrophilic or polar part and water-repelling, hydrophobic or nonpolar part) and behave as biological detergents
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❖ On agitation in the lumen of duodenum, the big fat lobules with reduced
interfacial tension (because of the water-soluble polar projections) are
broken up into several minute particles very easily, and thus increase the
total surface area of the fats manyfold
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Action of Lipases:
❖ In the small intestine both pancreatic lipase (main fat-digesting enzyme)
and intestinal
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❖ Lipases hydrolyze triglycerides to fatty acids and diglycerides
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❖ Both cholesterol esters and phospholipids contain fatty acids
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Definition
Lipases: Fat digesting enzymes are known as lipases or steapsin
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Digestion of Nucleic Acids
The major amount of food consumed by human beings come from either plant or animal
source
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▪ Digestion of Nucleic acids in the Small Intestine
❖ Hydrolysis of DNA and RNA begins only in the small intestine under the
action of nucleases present in the pancreatic and intestinal juice
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❖ Further, the intestinal nucleotidases hydrolyses nucleotides into
nucleosides and inorganic phosphate
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GASTROINTESTINAL HORMONES IN DIGESTION
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Gastrin
❖ It is secreted by the G-cells of the antrum of the stomach (Pyloric
stomach) on stimulation by distension of stomach, products of proteins
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and gastrin-releasing peptide, which is released by vagus nerve
stimulation
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Motilin
❖ It is secreted by upper duodenum during fasting
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Cholecystokinin (CCK)
❖ It is secreted by 'I' cells present in the inner lining of duodenum and
jejunum as a result of response to digestive products of fatty acids,
monoglycerides and fats in the contents of intestinal lumen
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Secretin
❖ It is secreted by the 'S' cells of duodenal mucosa because of acidic gastric
juice entering in the duodenum
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pancreas to neutralize the acid of the chyme
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Vasoactive Intestinal Peptide
❖ Secreted by the small intestine and help in inhibition of gastric acid
secretion and dilation of peripheral blood vessels of the gut
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Its function
is to activate Brunner's glands
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It activates crypts of Lieberkuhn
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Gastric inhibitory peptide (GIP) or Enterogastrone
❖ It is secreted by the mucosa of Duodenum in response to majorly fatty
acid and amino acids and minorly carbohydrates
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On combustion of 1 g of food in a bomb
calorimeter, the amount of heat energy produced is its gross calorific value
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Human body is an open system
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Thus, Physiological value of a food component is lower
than its calorific value
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1
5
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45

4
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0
9
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Passive mechanisms include simple diffusion of amino acids, some
monosaccharides like glucose and some electrolytes like chloride ions based on
their concentration gradients
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This mechanism is called the facilitated transport
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Molecules like glucose and galactose are absorbed into the blood by this
mechanism with the help of sodium pumps present in the cell membrane
(glucose and galactose are absorbed along with Na ions
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❖ Only some chemicals and alcohol are absorbed in the buccal cavity
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Absorption in stomach
❖ Due to lack of villi containing absorptive surface and tight junctions
between epithelial cells, stomach is poor in absorption
...
(glucose), some salts and few drugs like aspirin are
absorbed in the stomach
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❖ From the chyme, water is absorbed by osmosis
...

❖ In the duodenum and jejunum, chloride ions are readily absorbed by
diffusion and bicarbonates by active absorption
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❖ Maximum absorption takes place in jejunum
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❖ Vitamin B12, is absorbed with Castle's intrinsic factor by complex
formation
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❖ Proximal one-half of the colon is mainly involved in most of the absorption
...
The distal colon principally stores
faeces
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An osmotic gradient is created across the large
intestinal mucosal by the absorption of sodium and chloride ions which in
turn causes absorption of water
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villi and microvilli increase the absorptive area
of the mucosa of small intestine by about a thousand folds (more than 250
square meters i
...
; about the surface area of a tennis court)
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Therefore, they should be removed quickly from the duodenum
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Each bile salt
molecule has a highly fat-soluble sterol nucleus carrying the fat-digestate
(monoglycerides and free fatty acids) and a highly water-soluble polar
groups projecting outwards
...
These triglycerides
combine with proteins to form chylomicrons which move into the lacteals
of the villi
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This process is assimilation
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Excess amino acids are
deaminated in the liver
...

Ammonia is then converted into urea and excreted by the kidneys
...
Excess
glucose, fructose and galactose are stored in muscle cells and liver as glycogen
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EGESTION (DEFECATION)
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The discharge of faeces from the alimentary canal is known as egestion or
defecation
...

The solid matter in the faeces is composed of about 30 percent undigested
roughage, 30 percent dead bacteria, 10-20 percent inorganic matter, 10-20
percent fat and 2-3 percent protein, bile pigments and discarded epithelial cells
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❖ Bilirubin from the blood reaches the liver where it gets mixed with the bile
and leaves the body along with faeces
...

Do You Know ?
• Flatus: Gases in the gastrointestinal tract are collectively called flatus
...
Much of the swallowed air is released by
belching or eructation (burping)
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The brown color of the faeces is due to the presence of stercobilin and urobilin,
derivatives of bilirubin
...

This reflex action is regulated by two centers of vomiting in the medulla
oblongata
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Diarrhea
❖ The abnormally rapid frequency of bowel movement and increased
liquidity of the faecal content is known as diarrhea
...
It is caused due to increased motility of the intestinal
wall due to:
➢ Inflammation of the intestine (infectious diarrhea)
...

Constipation
❖ In constipation, the faeces are retained within the colon and rectum as the
bowel movements takes place irregularly
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Indigestion
❖ The food is not properly digested leading to a feeling of fullness during
such condition
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Protein Energy Malnutrition (PEM)
❖ Malnutrition is an inadequate amount of nutrients received by a body
...
Protein Energy
Malnutrition can be classified into two types: Kwashiorkor and Marasmus
...

• Retching: Strong involuntary contractions of diaphragm, abdominal muscles
and thoracic wall without any expulsion of gastrointestinal contents
...
It is characterized by watery
stool
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• Dysentery: Amoebic infection of the large intestine
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The stool has too much foul smell

VITAMINS
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Vitamins are organic molecules and essential micronutrients that an organism
needs in small quantities for the proper functioning of its metabolism and growth
...

The term 'Vitamin' was given by Funk
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