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 Functions: : 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. a. Opening and closing of mouth b. Holding the food c. Helps in speech DIGESTIVE SYSTEM

Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

Embed Size (px)


Uploaded from Google Docs

Citation preview

Page 1: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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.



: 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.

a. Opening and closing of mouth b. Holding the food c. Helps in speech



Page 2: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)


: Vestibule is a narrow, vertical, slit-like space between lips and cheeks and teeth and gum. Vestibule bears mucous glands.


: 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.



Page 3: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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.

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).

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.

Tip of tongue = Sweet


Lateral sides = Sour

Near the base = Bitter

Overlapping (maximum) with sweet taste zones = Salty

i. Determination of Taste




Page 4: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

ii. Helps in chewing of food iii. Helps in mixing of food and saliva iv. Helps in swallowing of food materials v. Speech vi. Acts as brush in cleaning teeth

There are some gland cells in tongue. Ebner’s glands produce lingual lipase. Weber’s 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.



Page 5: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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 [Ca10(PO4)6(OH)2

Neck is embedded in the gum.

] crystals arranged perpendicular to tooth surface.

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.



Page 6: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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.


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).

: 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:

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.


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.

: 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.

Histology of Oesophagus

Histology of Oesophagus

Serosa Muscularis Sub mucosa Mucosa

Muscularis mucosa

Lamina propria

Mucosal epithelium



Page 7: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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.

It consists of few enzymes named pepsinogen, prorenin and few gastric lipases.


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.

: It is the widest organ of the alimentary canal. The stomach is a hollow, J-shaped 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:

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.

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. DIG


Page 8: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

Histology of Stomach

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 Castle’s intrinsic factor (for absorption of vitamin B12

Argentaffin or enteroendocrine cells secrete gastrin, serotonin and histamine. Gastrin Hormones regulate the secretion of gastric enzyme.

), mucous or goblet cells secreting mucous for protecting mucosa layer from action of acid.




Page 9: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

i. Storage of food temporarily ii. Churning movement of wall of stomach helps for breakdown of food and mixes it

with gastric juice iii. Partial digestion of food iv. Absorption of water, alcohol and some drops


: 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 Brunner’s glands and Crypts of Lieberkuhn.

B. Jejunum: Jejunum is narrower, longer part of intestine (2.5 metres long). 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.



Page 10: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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 Brunner’s 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:

I. Paneth/Zymogenic cells, which secrete digestive enzymes II. Argentaffin cells, which secrete secretin, cholecystokinin, etc.


: 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.



Page 11: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

A. Caecum: It is small, blind sac coming off the colon at the latter’s 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.

The caecum is very large in herbivores. It serves to digest cellulose by bacterial action.

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.

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 (S-shaped 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 (90º or >90º), 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



Page 12: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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.

Note: A painful condition, called haemorrhoids or piles, sometimes develops in the anal canal due to enlargement of rectal veins.


i. Absorption of water and electrolytes ii. Absorption of vitamins B and K

iii. Temporary storage and elimination of faeces

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.



Page 13: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

Digestive Glands

Salivary 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. PAROTID GLANDS (GR. PAR-CLOSE; OTID-EAR): They are the largest salivary glands present in

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 Stenson’s duct

into oral cavity at the cavity of second upper molar tooth.



Page 14: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)



: 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 Whartan’s Duct on wither side of lingual frenulum.

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 Bartholin’s

Ducts or Ducts of Rivinus. These glands bear more numerous cells, less serous cells.

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.

Secretion and Composition of Saliva

Saliva is slightly acidic (pH – 6.5 to 7), colourless fluid. It contains 97% to 99.5% water, salts of

Na and K in the form of Cl-, PO4--- and HCO3

---. It contains salivary amylase (ptylin), lysozyme,

immunoglobulin, etc.


Functions of saliva


It moistens and lubricates the dry food.


Ptylin acts on cooked starch (acts at pH 6.8) and converts into maltose. (polysaccharide to



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.

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




Page 15: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

Liver is mainly divided into two lobes: larger right lobe (5/6th

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.

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.



Page 16: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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


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 Glisson’s 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



Page 17: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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 star-

shaped 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.



SECRETION OF BILE: Hepatic cells secrete bile synthesizing its own constituents from mixed

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 NaHCO3


(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.


Carbohydrate Metabolism:


Glycogenesis: Excess of glucose is converted into glycogen in the presence of insulin for


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.



Page 18: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

Fat metabolism

Lipogenesis: Formation of fat from amino acid and glucose

Desaturation of fats

Protein Metabolism

: Conversion of stored fat into simple form such that it can be used by

tissues to release energy.

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

amino acid is used for Glucaneogenesis.


Heat Production: Liver is the major heat producing organ. It has high rate of metabolism.

: 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.


Liver stores fat soluble vitamins (A, D, E, K) and also B12

Synthesis of vitamin A from β-carotene

, minerals like Fe, Cu, etc.

Synthesis of albumin, fibrinogen, prothrombin, etc.

Production of heparin

Destroys worn out RBCs; in embryonic stage, liver produces RBCs.


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



Page 19: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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; α-cells, β-cells, δ-cells (γ-cells) and PP cells.



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++,

Mg++, Cl-, SO4--, HPO4


ii. Pancreatic juice neutralizes the acidic chime by the help of NaHCO

, etc.). Digestive enzymes include trypsinogen,

chymotrypsinogen and procarboxypeptidase. In addition, there are pancreatic amylase,

pancreatic lipase and nucleases.

3 and Na2HPO4

iii. Secretion of hormones

(Disodium biphosphate).



Page 20: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

Digestive Process


It refers to the intake of food into the buccal cavity.


Propulsion of food in alimentary canal takes place by peristalsis (due to alternate contraction and

relaxation of circular and longitudinal muscle fibres).


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.

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).



Page 21: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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


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

juice bears enzymes, mucous and HCl.

Churning of food into smaller particles and its mixing with gastric juice is carried out by

contraction of its muscular wall.

• Mucous is amphoteric. It protects the wall of stomach from action of acid and enzymes.

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.



Page 22: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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.



Page 23: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

The intestinal juice consists of many digestive enzymes – amino peptidase, dipeptidase,

enterokinase, maltase, sucrose, lactase, α-dextrinase, nucleotidase, nucleosidase, etc.

Enterokinase activates the proteolytic enzymes of pancreatic juice: trypsin and



It is the process by which digested nutrients cross the mucosal lining of intestine and enter into

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



Page 24: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)

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

vitamins by passive transport.

Minerals are absorbed with the help of binding protein.

Water is mainly absorbed in small intestine and colon.


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.



Page 25: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)



Page 26: Digestive System (Alimentary Canal + Digestive Glands + Digestive Process)