Alimentary Canal
Alimentary canal is the major part of digestive system. It is
long (8 to 10m) and coiled tube with uneven diameter starting from
mouth and ending on anus. It consists of mouth, vestibule, buccal
cavity, pharynx, oesophagus, stomach, small intestine, large
intestine and anus. 1. MOUTH: It is a transverse aperture present
at the face below the nose. Mouth is bordered by two fleshy and
movable lips. The lips are rolled outward exposing a small part of
buccal mucosa which bears a large number of touch receptors
(thigmoreceptors). The upper lip bears a shallow median depression
called philtron.Functions:
a. Opening and closing of mouth b. Holding the food c. Helps in
speech
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2. VESTIBULE: Vestibule is a narrow, vertical, slit-like space
between lips and cheeks and teeth and gum. Vestibule bears mucous
glands. 3. BUCCAL OR ORAL CAVITY: It is the cavity between upper
and lower jaw. The roof consists of palate, floor consists of
tongue and on lateral sides there are cheeks.
A. Palate: Palate forms the roof of buccal cavity, which
separates it from nasal chamber. Anterior part is hard palate and
posterior is soft palate. Hard palate is bony and made of maxilla
and palatine bones (2). It bears transverse ridges, which are
called rugae. It helps during mastication of food. Soft palate is
soft and fleshy. On the posterior free end, there is a uvula
hanging down as a muscular fold covered with mucous membrane. Uvula
closes internal nostrils while swallowing food. On either side of
soft palate, there is an irregular mass of lymphoid tissues called
palatine tonsil. B. Tongue: Tongue is a voluntary, muscular,
protrusible and flattened organ on the floor of buccal chamber.
Tongue is attached with hyoid bone by its base and it is attached
with floor by a fold of mucous membrane called frenulum.
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TASTE ZONES
Tip of tongue = Sweet Lateral sides = Sour Near the base =
Bitter Overlapping (maximum) with sweet taste zones =
SaltyFUNCTIONS
i.
Determination of Taste
DI GE ST
a. Circumvallate Papillae are the largest. They are about 8 to
12 in number. They are arranged in inverted V-shape close to the
base of the tongue. b. Fungiform Papillae are more numerous than
circumvallate papillae, small, bright red coloured and
mushroom-like. They are mostly concentrated at the tip and the
sides. c. Filiform Papillae are most numerous and the smallest.
They are white in colour and conical or thread-like. They are
mostly confined on anterior two third part of the tongue.
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The tongue is superficially lined by stratified squamous
epithelium. It bears raised buds (taste buds) or papillae that
enclose nerve endings of sensory nerves that find taste. Adult
human bears three types of papillae: circumvallate, fungiform and
filiform (foliate in young ones).
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The tongue is incompletely divided into two parts by an inverted
V-shaped depression called sulcus terminalis: anterior one is
called oral part and posterior part is called pharyngeal part. On
the pharyngeal part, there is a mass of lymphoid tissues called
lingual tonsil.
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ii. iii. iv. v. vi.
Helps in chewing of food Helps in mixing of food and saliva
Helps in swallowing of food materials Speech Acts as brush in
cleaning teeth
There are some gland cells in tongue. Ebners glands produce
lingual lipase. Webers glands produce mucus. C. Teeth: In both the
jaws, teeth are present. Human dentition is thecodont, heterodont
and diphyodont.
Thecodont teeth lodged in socket (cup-like) or jaws Heterodont
teeth are of different shape and function a. Incisors: These are
the teeth of anterior part. They have sharp and flat edges for
biting and cutting. Their number is 4 in each jaw.
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b. Canines: They are next to incisors and have pointed edges for
piercing and tearing. Their number is 2 in each jaw. c. Premolars:
They are broad and strong teeth. They have two cusps. Premolar of
lower jaw has single root while premolar of upper jaw bears two
roots. They are meant for crushing and grinding. Their number is 4
in each jaw. d. Molars: They are broad and strong teeth. They have
4 or 5 cusps. Roots are two in number in lower jaw and three in
upper jaw. Their number is 6 in each jaw. Diphyodont Only two sets
of teeth possible, so replacement is possible only once. a. Milk
teeth / Deciduous teeth / Primary dentition 20 in number Eruption
starts after 6 months of birth and ends by the age of 2 years b.
Permanent teeth They replace milk teeth. 32 in number Eruption
starts at the age of 6 years and completes by 24 years of age
Dental Formula
Structure of a tooth A tooth consists of three parts: crown,
neck and root. Crown is the exposed part covered by enamel. Enamel
is made up of Ca++ salts bearing Hydroxyapatite [Ca 10 (PO 4 ) 6
(OH) 2 ] crystals arranged perpendicular to tooth surface. Neck is
embedded in the gum. Root is lodged in the socket of jaw bone. Root
is covered by a layer of calcified tissue called cement or cementum
that is enclosed by periodontal membrane. Dentine is the hard part
similar to the bones. It bears canaliculi. Dentine is produced by
odontoblast cells.
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Pulp cavity is lined by odontoblast cells. It bears connective
tissues, blood vessels and nerves. Blood vessels and nerves enter
and emerge out through apical foramen present at the tip of root.
4. PHARYNX: The buccal cavity leads into a short tube called
pharynx. This is a common passage of digestive and respiratory
tract. It is incompletely divided into three parts by soft pallet:
A. Nasopharynx: It is the pharynx above soft palate towards nasal
chamber. It has opening of internal nares and a pair of oval
opening of Eustachian tubes. It also bears mass of lymph nodules
called pharyngeal tonsil (also called adenoids). B. Oropharynx: It
is present below soft pallet towards buccal chamber. C.
Laryngopharynx: It is present below and behind uvula. It has the
openings of glottis and gullet. Opening of glottis is guarded by a
leaf like cartilage called epiglottis. It prevents entry of food
towards respiratory tract. Pharynx is lined by stratified squamous
epithelium. The wall of pharynx is with well developed constrictor
muscles which force the food-bolus into oesophagus during the
process of swallowing or deglutition. 5. OESOPHAGUS: Descending
down from the gullet through our neck behind the trachea and, then,
along the middle line in our thorax, is the oesophagus. It finally
pierces through the diaphragm at oesophageal hiatus reaching the
abdominal cavity, but immediately opening into the stomach by a
sphinctered cardiac orifice. It is about 25 cm ling and about 25 to
30 mm thick, distensible tube. Its cavity normally remains
obliterated by several longitudinal folds of mucosa and distends
only on arrival of food in it. Oesophagus is sphinctered at both
ends. The anterior part bears Crico-pharyngeal sphincter that
prevents entry of air into oesophagus during inspiration. The
posterior part bears Cardiac sphincter which is a physiological
sphincter and whose contraction causes narrowing of region only
when needed. Histology of Oesophagus
Serosa
DI GE STMuscularis
Histology of Oesophagus
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Sub mucosa
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Muscularis mucosa
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ST EMMucosa Lamina propria
Mucosal epithelium
Anterior part of oesophagus does not bear serosa. Instead, it
bears a layer of fibrous connective tissues called tunica
adventitia. Muscularis consists of outer longitudinal and inner
thicker layer of circular muscles. Sub mucosa consists of
connective tissues, blood and lymph vessels and nerves. Muscularis
mucosa consists of longitudinal muscles only. Proximal third is
lined with stratified squamous epithelium. Distal third is lined
with simple columnar epithelium. Middle third consists of a mixture
of two. Mucosa consists of 4 to 6 longitudinal major folds. These
folds flatten out when the food is in transit. Oesophageal mucosa
bears few mucus gland cells which lubricate food when in
transit.
The pyloric region is divided into the pyloric antrum and the
pyloric canal. The latter opens into the duodenum. The pyloric
sphincter guards the opening between the stomach and the duodenum
and periodically permits partially digested food to leave the
stomach and enter duodenum.
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6. STOMACH: It is the widest organ of the alimentary canal. The
stomach is a hollow, Jshaped organ lying between the oesophagus and
the small intestine. The lesser curvature short and lies on the
posterior surface of the stomach. The greater curvature is on the
anterior surface of the stomach. The fold of peritoneum which
attaches the stomach to the posterior abdominal wall extends beyond
the greater curvature. This is called the greater omentum. The
stomach has four parts: A. Cardiac Part: It is called cardiac part
because it is present near the heart. The cardiac sphincter lies in
the opening between oesophagus and stomach. It is not a true valve
but a functional sphincter. B. Fundus: It extends superiorly from
the cardiac part. The fundus is commonly filled with air or gases.
C. Body: It is the main part of the stomach. D. Pyloric Part
(Pylorus): It is the distal part of the stomach.
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It consists of few enzymes named pepsinogen, prorenin and few
gastric lipases.
Histologically, stomach consists of four fundamental layers: a.
Serosa: It is a single thin layer of squamous cells. b. Muscularis:
It bears three layers of muscles: i. Outer longitudinal muscle
fibres ii. Middle circular muscle fibres iii. Inner oblique muscle
fibres c. Sub mucosa: It consists of connective tissues, blood and
lymph vessels and nerves. d. Mucosa: It consists of simple columnar
epithelium. Many longitudinal folds called gastric rugae are
present. Mucosa consists of simple branched tubular glands. Mucosa
contains peptic cells (chief cells/zymogen cells) secreting pepsin,
oxyntic cells or parietal cells secreting HCl and Castles intrinsic
factor (for absorption of vitamin B 12 ), mucous or goblet cells
secreting mucous for protecting mucosa layer from action of acid.
Argentaffin or enteroendocrine cells secrete gastrin, serotonin and
histamine. Gastrin Hormones regulate the secretion of gastric
enzyme.FUNCTIONS:
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i. ii. iii. iv.
Storage of food temporarily Churning movement of wall of stomach
helps for breakdown of food and mixes it with gastric juice Partial
digestion of food Absorption of water, alcohol and some drops
7. SMALL INTESTINE: It is a long, coiled, narrow tube of length
6.35 metres. It extends from pyloric sphincter to iliocaecal valve
of caecum. It is the major site for digestion and absorption. It is
divisible into three parts: A. Duodenum: It is anterior, widest and
short part of small intestine (25 cm). It curves around the head of
pancreas (C-shaped). Duodenum receives bile juice from liver and
pancreatic juice from pancreas. Bile duct and pancreatic duct unite
in the wall of duodenum forming hepatopancreatic ampulla.
Hepatopancreatic ampulla is guarded by Sphincter of oddi. It
regulates release of bile juice and pancreatic juice into duodenum.
Its wall bears Brunners glands and Crypts of Lieberkuhn.Jejunum:
Jejunum is narrower, longer part of intestine (2.5 metres long). B.
C. Ileum: It is the longest and narrowest posterior part. It opens
into caecum of large intestine at iliocaecal valve.
Modifications for absorption A. Circular folds/Plicae
circularis: These are circular, permanent folds. It increases the
area of absorption. It also slows down the movement of food to
increase the efficiency of digestion. B. Villi: Villi are the
finger-like projections of mucosa which increase the surface area
of absorption.
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C. Microvilli: Microvilli are present in absorptive cells on
distal end which increase the surface area of absorption. 600
times, the surface area has increased due to presence of villi and
Microvilli. Histology i. Serosa ii. Muscularis: Circular muscle
layer is double in thickness than longitudinal muscle layer. iii.
Sub mucosa: In duodenum only, branched glands called Brunners
glands are present. They secrete alkaline mucous to neutralize
acidic chime. iv. Mucosa: Muscularis mucosa is poorly developed.
Lamina propria of ileum bears lymphoid nodules called Peyers
patches (yellowish). Mucosal epithelium is folded into finger-like
projections called villi. They are larger in duodenum (leaf like)
and are smaller in jejunum and ileum. Each villus encloses network
of blood capillaries, lacteal and smooth muscles. Base of villi
bears simple tubular branched glands called Crypts of Lieberkuhn.
They consist of two types of cells:
8. LARGE INTESTINE: The large intestine shorter than small
intestine. It is called large intestine as it is wider than small
intestine. It is arranged around the small intestine in the form of
an inverted C. It is about 1.5 metres long and diameter is about 6
cm. It lacks villi and Microvilli. It shows three regions viz.,
caecum, colon and rectum.
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I. II.
Paneth/Zymogenic cells, which secrete digestive enzymes
Argentaffin cells, which secrete secretin, cholecystokinin,
etc.
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B. Colon: It is wider than small intestine and about 1.2 m long.
It gives sacculated appearance due to a series of constrictions.
The colon shows four regions, i.e. ascending colon that passes
upward on the right side of the abdominal cavity, transverse colon
that bends to the left and runs across the abdominal cavity,
descending colon that extends downward on the left side of the
abdominal cavity and sigmoid or pelvic colon (Sshaped part) that
turns to the right and joins the rectum. The bend between the
ascending and transverse colon is called right colic flexure or
hepatic flexure (90 or >90), and that between transverse and
descending colon is termed as left colic flexure or splenic flexure
(acute angle). The mucous membrane of colon is smooth. Its
longitudinal muscle fibres congregate in three thick longitudinal
bands called taenia coli (tape-like). The latter, by contraction,
draw the colon wall into small pouches termed as haustra. The
serous layer (visceral
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Note: Food may collect and decay in it or intestinal worms may
settle in it and cause inflammation. This condition is called
appendicitis. In such cases, it has to be removed immediately so
that it does not burst and spill infection in the abdominal cavity.
Its removal is called appendectomy.
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The caecum is very large in herbivores. It serves to digest
cellulose by bacterial action.
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A. Caecum: It is small, blind sac coming off the colon at the
latters junction with the ileum. It is just 6 cm wide. At the
iliocaecal junction is an ileocolic valve that regulates the
passage of material from the large to small intestine. The caecum
bears a short, slender, worm-like projection called the vermiform
appendix measuring about 8 cm long. Appendix bears lymph nodes or
lymphatic tissues.
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peritoneum) forms small, pendulous projections, the appendices
epiploicae, filled with adipose tissue. C. Rectum: It follows the
colon and lies in the pelvis region. It is about 15 to 20 cm long.
The rectum has longitudinal folds and large blood vessels. The
rectum leads through 2.5 cm long anal canal to the exterior at the
anus. The anal canal is guarded by internal anal sphincter
(involuntary) and external anal sphincter (voluntary). Anal canal
is lined by stratified squamous epithelium. Rectum has continuous
layer of longitudinal and circular muscles.
Functions: i. ii. iii. Absorption of water and electrolytes
Absorption of vitamins B and K Temporary storage and elimination of
faeces
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Histology: a. Serosa b. Muscularis: It is bilayered. In colon,
longitudinal muscles form three tape like bands called taenia coli.
Rectum and anal canal bear continuous layer of both circular and
longitudinal muscle fibres. c. Sub mucosa d. Mucosa: Mucosa is
thick. In colon and rectum, mucosa is lined by simple columnar
epithelium. It does not have circular folds, villi and glands
secreting digestive enzymes. It bears large number of goblet cells.
Mucosal epithelium form large number of folds but not villi.
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Note: A painful condition, called haemorrhoids or piles,
sometimes develops in the anal canal due to enlargement of rectal
veins.
Digestive GlandsSalivary Glands
A number of intrinsic salivary glands present in mucosa of oral
cavity secrete small amount of saliva. Extrinsic salivary glands
lie outside the oral cavity and pour their secretion into oral
cavity by ducts.
Salivary glands are compound tubule-alveolar glands. They are
enclosed in fibrous capsule. There are three pairs in number one
pair each of parotid glands, sub-mandibular glands and sub-lingual
glands. a. PAROTIDGLANDS
front and below the ear on each side of the face. These glands
bear serous cells only secreting digestive enzymes. These glands
pour their secretion through Stensons duct into oral cavity at the
cavity of second upper molar tooth.
DI GE ST(GR. PAR-CLOSE; OTID-EAR): They are the largest salivary
glands present in
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b.
SUB-MANDIBULAR GLANDS: They are present below the angle of two
jaws in either side of
face. They bear equal number of serous and mucous cells. They
pour their secretion into oral cavity through Whartans Duct on
wither side of lingual frenulum. c. SUB-LINGUAL GLANDS: These are
smallest sized salivary glands which lie below the tongue and open
at the floor of buccal cavity by a number of small sub-lingual or
Bartholins Ducts or Ducts of Rivinus. These glands bear more
numerous cells, less serous cells.Secretion and Composition of
Saliva
Secretion of saliva is controlled by autonomic nervous system.
Reflex secretion takes place when there if food in mouth; also
occurs from sight, smell and thought of food. About 1.5 litres of
secretion takes place per day.
Saliva is slightly acidic (pH 6.5 to 7), colourless fluid. It
contains 97% to 99.5% water, salts of
immunoglobulin, etc.Functions of saliva
i. ii.
It moistens and lubricates the dry food.
disaccharide) iii. iv. v.
It helps in cleaning of buccal cavity and its associated
structures. Lysozyme destroys bacteria, immunoglobulin acts as
antibody. It helps in taste determination by acting as
solvent.Liver
It is the largest gland of the body weighing about 1.5 kg. It is
reddish brown in colour. Liver is present in upper right side of
abdominal cavity just below diaphragm. It is enclosed in the folds
of peritoneum called Falsiform ligaments which attach the liver in
inferior surface of diaphragm and hold it in its position. Its
upper and anterior surfaces are smooth, while posterior surface is
irregular.
DI GE ST
Ptylin acts on cooked starch (acts at pH 6.8) and converts into
maltose. (polysaccharide to
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Na and K in the form of Cl-, PO 4 --- and HCO 3 ---. It contains
salivary amylase (ptylin), lysozyme,
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Liver is mainly divided into two lobes: larger right lobe (5/6th
of total) and smaller left lobe. The right lobe if further divided
incompletely into three lobes by shallow fissures: right central
lobe, small caudate lobe and rectangular quadrate lobe. Between
quadrate and right central lobe, there is a pear-shaped, dark gall
bladder. It stores bile, secretes mucus, concentrates the bile 10
times by absorbing water.
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From right and left lobes, right hepatic duct and left hepatic
duct arise. They unite to form hepatic duct, which joins with the
cystic duct from gall bladder and forms common bile duct. The
common bile duct joins with pancreatic duct at hepato-pancreatic
ampulla (Ampulla of Vater). Oxygenated blood is received from
hepatic arteries and blood with nutrients reach to liver from
hepatic portal vein. They open in posterior side at porta
hepatis.Histology of Liver
Each lobe of liver bears a large number of hexagonal hepatic
lobules. These lobules are separated by a thin layer of connective
tissue called septa or Glissons capsule. At the centre of each
hepatic lobule, there is a central vein. There are six triads on
the periphery. Each triad bears a bile canal, hepatic arteriole and
hepatic portal venule. Each lobule bears many cuboidal or
polyhedral hepatocytes. Each hepatic cell consists of granular
cytoplasm, large, rounded central nucleus, granules of glycogen and
lipid. The hepatic cells are arranged in
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radial rows of one or two layers called hepatic cords. In
between the hepatic cords, there are narrow, irregular spaces,
sinusoid and canaliculi. Each hepatic cord bears sinusoid in one
side and bile canaliculus on the other side. Sinusoids are narrow,
irregular spaces in which mixed blood from hepatic arteriole and
hepatic portal venule flows towards the central vein. Its wall
consists of endothelial cells, some of which are highly specialized
for phagocytosis. These cells are called Kupffer cells. These are
starshaped hepatic macrophages. Canaliculus is the irregular space
in which bile from hepatocytes flows towards the bile canal/duct of
portal triad. Canaliculi unite to form bile canal. Bile canals
unite to form right and left hepatic ducts. Right and left hepatic
ducts form common hepatic duct. Common hepatic duct + Cystic duct =
Common bile duct
Central veins of many lobules unite to form hepatic veins. They
open into inferior venecava.Functions
blood of sinusoid. Bile is dark, green in colour, alkaline in
nature. About 0.5 to 1 litre bile is secreted per day and stored in
gall bladder. Bile consists of water, mucous, bile salts and bile
pigments. Bile salts consist of NaHCO 3 (neutralizes the acidic
chime), Na or K Glycocholate and Taurocholate (formed from cholic
acid of cholesterol; responsible for emulsification of fats). Bile
pigments consist of bilirubin (yellow), biliverdin (greenish).
These pigments are formed from decomposition of hemoglobin. b.
METABOLIC FUNCTIONS:Carbohydrate Metabolism: Glycogenesis: Excess
of glucose is converted into glycogen in the presence of insulin
for
storage.Glycogenolysis: Glycogen is converted into glucose when
needed in the presence of glucagon. Glucaneogenesis: Formation of
glucose from non-carbohydrate sources like amino acid or
fatty acid in the presence of glucocorticoids.
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a. SECRETION OF BILE: Hepatic cells secrete bile synthesizing
its own constituents from mixed
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Fat metabolism Lipogenesis: Formation of fat from amino acid and
glucose Desaturation of fats: Conversion of stored fat into simple
form such that it can be used by
tissues to release energy.Protein Metabolism Deamination:
Removal of nitrogenous group or amino group from amino acid.
That
nitrogenous portion is used to synthesize urea which is less
toxic. The remaining part of
Trans-amination: Removal of amino group from amino acid that is
attached with carbohydrate
molecule to form new amino acid
Detoxification: Liver detoxifies drugs, alcoholic products and
toxins from bacteria.
Synthesis of albumin, fibrinogen, prothrombin, etc. Production
of heparin
Destroys worn out RBCs; in embryonic stage, liver produces
RBCs.Pancreas
Pancreas is soft, lobulated, elongated, yellowish compound
tubular gland present behind stomach. It is leaf-like or tadpole
like that bears head, body and tail. Head is present in C-like
curve of duodenum, body behind stomach and narrow tail in front of
left kidney. The length is about 15 cm and weight is about 60
grams. Pancreas is a heterocrine gland. Histologically, exocrine
part bears many branched tubules called pancreatic acini (lobules).
Each lobule consists of many conical secretory cells. These cells
bear large number of rough ER, zymogen granules. These cells
secrete pancreatic juice into the
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Synthesis of vitamin A from -carotene
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Liver stores fat soluble vitamins (A, D, E, K) and also B 12 ,
minerals like Fe, Cu, etc.
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Heat Production: Liver is the major heat producing organ. It has
high rate of metabolism.
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amino acid is used for Glucaneogenesis.
lumen of lobule. The pancreatic ductules unite to form major
pancreatic duct, which joins with common bile duct at
hepatopancreatic ampulla and opens into duodenum. Endocrine part
consists of islets of langerhans dispersed among pancreatic
lobules. They bear four types of cells; -cells, -cells, -cells
(-cells) and PP cells.Functions
i.
SECRETION OF PANCREATIC JUICE: It is alkaline (pH 7.5 to 8) in
nature and produced 1,200 to
1,500 ml. per day. It consists of water, digestive enzymes and
electrolytes (Na+, K+, Ca++,
chymotrypsinogen and procarboxypeptidase. In addition, there are
pancreatic amylase, pancreatic lipase and nucleases. ii.
Pancreatic juice neutralizes the acidic chime by the help of
NaHCO 3 and Na 2 HPO 4 Secretion of hormones
iii.
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(Disodium biphosphate).
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Mg++,
Cl-,
SO 4 --,
HPO 4 --,
etc.).
Digestive
enzymes
include
trypsinogen,
Digestive ProcessIngestion
It refers to the intake of food into the buccal
cavity.Propulsion
Propulsion of food in alimentary canal takes place by
peristalsis (due to alternate contraction and relaxation of
circular and longitudinal muscle fibres).Digestion
It is mechanical and chemical breakdown of complex organic food
matter into simpler diffusible form such that it can be absorbed by
the walls of the alimentary canal. Minerals, vitamins and water can
be absorbed directly.PHYSIOLOGY OF DIGESTION
Digestion of food in oral cavity
Both mechanical and chemical breakdown of food occurs in buccal
chamber. Mastication of food into fine particles is carried out by
teeth with the support of tongue (mechanical breakdown). At the
same time, food is mixed with saliva (salivary amylase and lingual
lipase).
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The food is changed into a pulpy substance called bolus. The
bolus goes towards pharynx then to oesophagus through gullet by
tongue and pharyngeal muscles. It is called swallowing or
deglutition.
Digestion of food in stomach
When the food reaches stomach, distension of its wall takes
place. Enteroendocrine cells secrete gastrin, which stimulates the
gland cells of gastric glands which secrete gastric juice.
Gastric
Churning of food into smaller particles and its mixing with
gastric juice is carried out by contraction of its muscular
wall.
HCl destroys bacteria and germs, makes food acidic and stops the
action of salivary amylase. HCl also activates the inactive
enzymes.
Digestive enzymes digest the food.
Gastric lipase digests fats into fatty acid and glycerol.
The food in stomach is semi-digested and semi-solid called
chime. The chime passes to duodenum intermittently through pyloric
sphincter.
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Mucous is amphoteric. It protects the wall of stomach from
action of acid and enzymes.
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juice bears enzymes, mucous and HCl.
Digestion of food in duodenum / small intestine
When chime enters the duodenum, the enteroendocrine and Paneth
cells are stimulated. The enteroendocrine cells secrete hormones
like secretin, cholecystokinin, enterocrinin, etc. Secretin
stimulates pancreas to produce pancreatic juice. Cholecystokinin
stimulates liver to produce bile juice and causes constriction of
gall bladder. Enterocrinin stimulates gland cells of intestine to
produce succus entericus or intestinal juice. Bile Juice Does not
have digestive enzymes.
Bile salts like Sodium Bicarbonate neutralizes the acidic chime.
Sodium Glycocholate and Taurocholate emulsify fats. Pancreatic
Juice
Neutralizes chime and makes it slightly alkaline.
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The intestinal juice consists of many digestive enzymes amino
peptidase, dipeptidase, enterokinase, maltase, sucrose, lactase,
-dextrinase, nucleotidase, nucleosidase, etc. Enterokinase
activates the proteolytic enzymes of pancreatic juice: trypsin and
carboxypeptidae.
Absorption
blood or lymph vessels. Ileum is the principal site for
absorption. Small intestine bears permanent circular fold, villi,
Microvilli on luminal surface of enterocytes. All these
modifications increase the absorptive surface area.Types of
Absorption
1. Passive transPort: In passive transport, absorption takes
place along the concentration gradient i.e. from higher
concentration of nutrients in intestinal lumen to lower
concentration in mucosal cells. It is a slow process and does not
require energy. Complete absorption of nutrients is not possible by
this method. Passive transport takes place by simple diffusion or
facilitated diffusion. 2. active transPort: In active transport,
absorption takes place against the concentration gradient i.e. from
low concentration in intestinal lumen to high
DI GE ST
It is the process by which digested nutrients cross the mucosal
lining of intestine and enter into
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concentration in mucosal cells and blood. It is a rapid process
and requires energy. Complete absorption can occur in active
transport. Glycerol is water soluble and therefore it is directly
absorbed by enterocytes. Fatty acids are insoluble in water and
have long chains. They get mixed with bile salts and form micelles.
Micelles are water soluble and they diffuse into enterocytes. Then,
fatty acids reesterify into phospholipids.
Chyle passes into lacteals. Through the lymph vessel, through
thoracic duct, chyle enters into duodenum. Water soluble vitamins
(B and C) are absorbed by active transport wile fat soluble
Minerals are absorbed with the help of binding protein.
Egestion
It is the process by which undigested food or faecal matter is
discharged out through anus. Faecal matter gets collected in
rectum. When rectum is filled, the reflex contraction of rectal
muscles develops the feeling of defecation. The abdominal muscle
contracts and anal sphincters relax and faecal matter is passed out
through anus.
DI GE ST
Water is mainly absorbed in small intestine and colon.
IV
vitamins by passive transport.
E
SY
ST EM
DI GE ST IV E SY ST EM
DI GE ST IV E SY ST EM
