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INTRODUCTION
DIGESTIVE SYSTEM
DIGESTION AND ABSORPTION16
Food is one of the basic requirements in living organisms. The major components of
our food are carbohydrates, proteins, fats, vitamins and minerals. Food provides energy
and organic materials for growth and repair of tissues. The food taken in has to be broken
down and converted into simple substances. This process of conversion of complex food
substances into simple absorbable forms is called digestion. It is carried out by our digestive
system by mechanical and biochemical methods.
• The digestive system is made up of the gastrointestinal tract (GIT) or alimentary canal
and associated glands. They help in the digestion and absorption of food.
• The alimentary canal begins anteriorly with the mouth and opens out posteriorly
through the anus (fig 16.1).
• It is a long coiled tube with muscular walls.
• The Alimentary canal has four divisions.
- Buccopharyngeal cavity: Vestibule, Buccal cavity, and Pharynx
- Oesophagus
- Stomach: Cardiac, Fundic, and Pyloric stomach
- Intestine: Small intestine, and Large intestineFig 16.1
2
Fig 16.2
Fig 16.3
• The oral cavity or mouth has several teeth and a muscular tongue.
• It is divided into two parts;
Vestibule: It is the space between gums and lips.
Buccal cavity: It is the space bounded;
- Dorsally by the palate.
- Ventrally by the throat.
- Laterally by the alveolar processes of jaws having teeth.
• The Palate is the roof of the buccal cavity or mouth. It separates the buccal cavity from
the nasal chamber.
UVULA: Hanging flap in the buccal cavity that closes the internal nares
during swallowing of the food bolus.
Tongue• The tongue is a freely movable muscular organ attached to the floor of the oral
cavity using the frenulum.
• The upper surface of the tongue has small
projections called the papillae (fig.16.2).
• Some papillae bear taste buds.
• There are mainly four types of papillae:
- Filiform: They are without taste buds,
located near the centre and the upper
surface of the tongue, most numerous.
- Fungiform: They contain few (8-10)
taste buds and are located over the
flat surface of the tongue.
- Foliate: Moderate number of taste buds and are located in folds along the lateral
surfaces of the tongue.
- Circumvallate: It contains approximately 200 papillae, located on the posterior
part of the tongue, and are many. They have a
V-shaped arrangement.
• Weber’s gland: Several tubule mucous glands
present over the tongue.
• Lingual tonsils: Tonsils present on the basal region
of the tongue. Tonsils are a set of lymphoid organs
and they contain lymphoid tissue.
• Waldeyer’s tonsillar rings: They mainly consist of
MOUTH
3
Fig 16.4
Fig 16.5
Adenoid tonsil/Pharyngeal, Tubal tonsil (2), Palatine tonsil (2), Lingual tonsil (fig.16.3).
• Tonsillitis: it is the inflammation of tonsils.
Teeth• Hard structures meant for tearing, cutting,
crushing, and holding food (fig. 16.4).
• Enamel: It is the hardest, white, shining outer
covering of the tooth which is secreted by
Ameloblasts.
• Dentine: 2nd layer of the tooth beneath enamel,
formed from Odontoblasts.
• Pulp: innermost soft tissue part.
• Human teeth are:
- Thecodont: In this type of arrangement each
tooth is embedded in a socket of the jaw
bone.
- Heterodont: Presence of different types of teeth in the life cycle.
- Diphyodont: Appearance of two sets of teeth in the life cycle.
• In human beings there occur 2 types of teeth in their life span;
• Milk, deciduous or temporary teeth: They begin to appear at the age of 6-11
months. They are completed by the age of 2 years. There are 20 milk teeth.
• Permanent teeth: They begin to appear between the age of 6-12 years when milk
teeth are lost. The last molar teeth come out late after 18 yrs of age, i.e wisdom
teeth.
• Permanent teeth of men are 32 in number and they are of four types (fig. 16.5):
- Incisors (8),
- Canines (4),
- Premolar (8),
- Molar (12)
• Dental formula: It is the number of teeth
in one half of the upper jaw divided by
teeth in one half of the lower jaw.
• The dental formula of permanent teeth =
4
PHARYNX
9. Bunodont 10. Lophodont 11. Solenodont 12. Secodont
• Teeths have low cusps.
• E.g. Human
• Have transverse ridges.
• E.g. Elephant
• Have crescent shaped cusps.
• E.g. Sheep
• Have pointed cusps.
• E.g. Carnivores
• The oral cavity leads into a short pharynx.
• The pharynx serves as a common passage for food and air.
• The oesophagus and the trachea (windpipe) open into the pharynx.
• A cartilaginous flap called epiglottis prevents the entry of food into the glottis
– opening of the windpipe – during swallowing.
• The pharynx can be divided into 3 regions (fig
16.6);
- Nasopharynx: It is the anteriormost part of
the pharynx that lies in between the nasal
chamber.
- Oropharynx: It lies behind the buccal cavity.
It acts as the passage for food bolus.
- Laryngopharynx: It is the lowermost part of the pharynx.
5. Monophyodont 6. Diphyodont 7. Polyphyodont 8. Pleurodont
• One set of teeth• E.g. Platypus,
Toothed Whale
• Two sets of teeth.• Milk teeth temporary• Permenant teeth• E.g. Mammals
• Teeth can be replaced many times.
• E.g. Frog
• Types of teeth is fixed by sides to lateral surface of jaw ridge
• E.g. Reptiles
Types of teeth
Fig 16.6
OESOPHAGUS
• The oesophagus is a thin, long tube that extends posteriorly passing through the
neck, thorax and diaphragm and leads to the stomach.
• It extends between the pharynx and stomach.
1. Acrodont 2. Thecodont 3. Isodont or Homodont 4. Heterodont
• Teeth is a part of bone not embedded in sockets.
• Teeth is embedded in sockets.
• All teeth are similar.
• More than one type.
• E.g. Reptiles (except crocodiles), Amphibians.
• E.g. Mammals, Crocodiles
• E.g. Toothed whale
• E.g. 4 types in humans- Incisors, canine, premolar, molar
5
Glottis Gullets
• It leads to trachea
• It is guarded by a cartilaginous flap called epiglottis.
• Epiglottis is made up of elastic cartilage
• It leads to oesophagus or food pipe.
• It is normally closed.
STOMACH
• The stomach is a J-shaped bag-like structure.
• It is located in the upper-left portion of the abdominal
cavity.
• Has a capacity of about 1-1.5 L.
• The stomach helps in the mechanical chewing and
chemical digestion of food.
• The stomach consists of mainly four parts (fig 16.7);
- Cardiac portion: It is the part into which the oesophagus opens. The cardiac stomach
region is guarded by a cardiac sphincter (gastro-oesophageal sphincter) and this
cardiac sphincter helps to prevent significant reflux of stomach contents into the
oesophagus except under very abnormal conditions.
- Fundic portion: It is a small dome-shaped structure that produces gastric juices
including HCl. It is elevated above the level of oesophageal opening.
- Body: It is the large (75-85%) central region. It extends from the lowest region of
the fundus up to the pyloric region.
- Pyloric portion: This is the portion that opens into the first part of the small intestine
(duodenum). The pyloric sphincter regulates the passage of food into the intestine.
Fig 16.7
STOMACH OF RUMINANTS: It contains 4 parts: the rumen, the reticulum,
the omasum and the abomasum. Rumen, reticulum, and omasum are parts
of oesophagus and secrete digestive juice whereas abomasum is the glandular
part which functions as the true stomach.
• Length varies from 25 to 30 cm.
• Gastro-oesophageal sphincter: It is a muscular sphincter that regulates the opening of
the oesophagus into the stomach. It is also called the cardiac sphincter since it opens
into the cardiac region of the stomach.
Difference between Glottis and Gullets
6
INTESTINE
Intenstine
Small IntestineDuodenum JejunumIleum
Large Intestine
Small intestine • The small intestine is composed of 3 parts (fig 16.8).
- Duodenum: It is a ‘C’ shaped proximal part of the small intestine.
- Jejunum: It is the long coiled middle portion in between the duodenum and ileum.
- Ileum: It is the distal-most part of the small intestine. A highly coiled portion. The
ileum opens into the large intestine
• The small intestine is the longest part of GIT.
• They possess finger-like projections called villi which increase absorptive surface area.
• It is approximately 6-7 m long.
SHORT BOWEL SYNDROME: This condition is characterised by reduced length
of the small intestine (length reduced below 200 cm).
CaecumColon
Rectum
● Peyer’s patches
• Lymphoid aggregate present in the submucosa of the ileum. It is involved in the
production of B lymphocytes and also protects the intestine from infection.
● Brunner’s glands
• They are found in the duodenum. It is mucus-secreting glands.
● Sphincter of Oddi
• It guards the opening of the common bile and pancreatic duct into the duodenum.
● Ampulla of Vater
• It is a region of the duodenum where the common bile and pancreatic duct unite.
Ampulla of Vater is also known as hepatopancreatic ampulla or hepatopancreatic
duct.
● Chyme
• The semi-digested and acidified food that enters the duodenum is called chyme.
● Sacculus rotundus
• It is a small bulb-like structure present at the distal end of the ileum.
Large intestine• The large intestine is composed of 3 parts (fig. 16.8).
• Caecum: Caecum is a small blind sac that hosts some symbiotic microorganisms. The
caecum opens into the colon. There is an ileocaecal valve present at the junction
7
HISTOLOGY OF THE ALIMENTARY CANAL
between the ileum and caecum. The caecum is prominent in herbivorous animals.
- Vermiform appendix: It is a narrow finger-like tubular projection that arises from
the caecum. It is a vestigial organ.
• Colon: It is the largest part which consists of 4 segments- ascending, transverse,
descending and sigmoid. This region is concerned with the absorption of water and
electrolytes from the chyme to form solid faeces and the storage of faecal matter
until it can be expelled.
• Rectum: This region leads into the anal canal
and opens out through the anus. It is guarded
by the anal sphincter muscle. When a mass
movement forces faeces into the rectum, the
desire for defecation occurs immediately,
including reflex contraction of the rectum
and relaxation of the anal sphincters.
PILES or HEMORRHOIDS: It is a painful
condition in which rectal veins get enlarged.
Fig 16.8
The wall of the alimentary canal from the oesophagus to rectum possesses four layers (fig 16.9);
• Serosa: It is the outermost layer and is made up of a thin mesothelium (epithelium
of visceral organs) with some connective tissues.
• Muscularis: It is formed by smooth muscles usually arranged into an inner circular
and an outer longitudinal layer. An oblique
muscle layer may be present in some
regions.
• Sub-mucosa: It is formed of loose connective
tissues containing nerves, blood and lymph
vessels.
- In the duodenum, glands are present in
the submucosa.
• Mucosa:The innermost layer lining the lumen of the alimentary canal is the mucosa
(fig 16.10). This layer forms irregular folds (rugae) in the stomach and small finger-
like foldings called villi in the small intestine.
- The cells lining the villi produce numerous projections called microvilli giving a
brush border appearance.
Fig 16.9
8
DIGESTIVE GLANDS
- Villi are supplied with a network of capillaries
and a large lymph vessel called the lacteal.
- The mucosal epithelium has goblet cells that
secrete mucus that help in lubrication.
- The mucosa can form glands in the stomach like
gastric glands.
- Mucosa forms crypts in between the bases of villi
in the intestine called Crypts of Lieberkuhn.
Fig 16.10
• Digestive glands secrete digestive juices for the digestion of food.
• Digestive juices contain digestive enzymes.
• The major digestive glands are :Salivary glands, Liver, and Pancreas.
- Gastric glands and intestinal glands also produce some digestive secretions.
Types of digestive juices from their glands and location
Salivary glands• Saliva is a complex mixture produced by salivary glands. It is secreted continuously
by the salivary gland under the control of ANS. Secretion of saliva is a reflex action.
• Secretion of salivary glands includes mucin, lysozymes, and a starch-splitting enzyme
called ptyalin (salivary amylase) (fig 16.11).
• Mucin: It is a glycoprotein and helps in lubricating the food (bolus) and swallowing. It
is mainly produced by sublingual and submaxillary
glands (submandibular glands).
• Ptyalin or salivary amylase: They are produced
by the parotid gland. They help in breaking down
carbon dioxide.
• Lysozyme: The enzyme lysozyme present in saliva
acts as an antibacterial agent that prevents
infections.
Digestive Juices Digestive Glands Secreted Into • Saliva • Gastric Juice • Bile • Pancreatic Juice • Intestinal Juice
• Salivary Gland • Gastric Gland • Liver • Pancreas • Intestinal Folds
• Mouth • Stomach • Duodenum • Duodenum • Small Intestine
Fig 16.11
9
Ducts and location of major salivary glands
MUMPS: Viral infection of the parotid gland caused by Myxovirus.
Gastric glands• They are numerous, simple, branched, or unbranched tubular glands.
• They are formed by the infolding of epithelium.
• There are mainly 3 types of glands;
• Fundic glands: They are also called oxyntic glands located in the body and the fundic
region of the stomach.
• Pyloric glands: They are present in the pyloric region of the stomach and secrete mucus.
• Cardiac glands: They are located in the cardiac region of the stomach.
• Secretions of gastric glands are called gastric juices, this secretion is under nervous and
hormonal control.
• Gastrin: Gastrin produced in the stomach stimulates gastric glands of the stomach to
secrete large amounts of gastric juices.
Glands Ducts Location • Parotid gland • Submaxillary gland • Sublingual gland
• Stensen duct • Wharton duct • Duct of Rivinus
• Open in vestibuli just outside the 2nd upper molar. • Opens near the lower central incisor. • Open under the tongue.
Secretory function of cells in gastric glands
Cells Secretory Products • Cheif cells • Parietal cells • Goblet cells • Gastrin cells • Enterochromaffin cells • Enterochromaffin like cells
• Pepsin • Hydrochloric acid • Mucin • Gastrin • Serotonin • Histamine
Digestive enzymes of gastric juices
Enzyme Activator Substrate End Product • Pepsin • Gastric lipase • Gastric amylase • Gelatinase • Urease
• HCl • Acid medium • Acid medium • Acid medium • Acid medium
• Proteins • Triglycerides • Starch • Gelatin and collagen of meat • Urea
• Proteases peptones polypeptides • Fatty acids and glycerols • Dextrin and Maltose • Peptides • Ammonia
Liver
• The liver is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult
human.
10
Fig 16.12• It is situated in the abdominal cavity, just below
the diaphragm and has two lobes (fig 16.12).
• The liver can be differentiated into 2 parts;
the small left lobe and the large right lobe.
• The hepatic lobules are the structural and
functional units of the liver containing hepatic
cells.
- Each lobule is covered by a thin connective
tissue sheath called the Glisson’s capsule.
• The bile is secreted by the hepatic cells.
• The bile passes through the hepatic ducts (duct of the liver for bile passage) and is
stored and concentrated in a thin muscular sac called the gallbladder.
• Gall-bladder: It is a pea-shaped yellow-green sac-like structure that lies on the inferior
surface of the right liver lobe and it stores the bile.
• The duct of the gallbladder (cystic duct) along with the hepatic duct from the liver
forms the common bile duct.
• The common bile duct and the pancreatic duct open together into the duodenum as
the common hepato-pancreatic duct (Ampulla of Vater or hepatopancreatic ampulla).
• The liver stores food in the form of glycogen.
• Sphincter of Oddi: It guards the opening of the common bile duct and pancreatic duct
into the duodenum.
GALLSTONES: They are formed by bile salts and calcium also promoted by the high cholesterol level.
Pancreas • The pancreas consists of two types of glandular tissue;
• Exocrine part: Secretes the alkaline
pancreatic juice containing enzymes.
- The most important pancreatic
enzymes for digesting proteins
are trypsin, chymotrypsin, and
carboxypolypeptidase. By far the
most abundant of these is trypsin.
• Endocrine part: Secretes hormones
called insulin and glucagon.
- It is located between the limbs of the U shaped duodenum (fig 16.13).
HEPATOPANCREAS: The hepatopancreas is a digestive gland found in many
invertebratesv (e.g. Prawns) that perform the function of both the liver and
pancreas.
Fig 16.13
11 Intestinal glands
• They are formed by the surface epithelium of the small intestine.
• They occur at the Crypts of Lieberkuhn and Brunner's gland.
• Secretions of both these glands are called Succus entericus (intestinal juice).
• The composition of succus entericus includes 99.5% water, organic (enzymes, minerals,
etc.), and inorganic salts (Na, Ca, K, Chloride, etc).
• Brunner’s gland: They are located in the wall of the first few centimetres of the
duodenum, mainly between the pylorus of the stomach and the papilla of Vater where
pancreatic secretion and bile empty into the duodenum. These glands secrete large
amounts of alkaline mucus.
• Located over the entire surface of the small intestine are small pits called crypts of
Lieberkuhn. These crypts lie between the intestinal villi. The surfaces of both the crypts and the villi are covered by an epithelium composed of two types of cells;
- moderate number of goblet cells, which secrete mucus that lubricates and protects
the intestinal surfaces.
- A large number of enterocytes, which, in the crypts, secrete large quantities of
water and electrolytes and, over the surfaces of adjacent villi, reabsorb the water
and electrolytes along with end products of digestion.
Secretions of intestinal glands
Enzyme Substrate End Products • Peptidases • Sucrase • Maltase • Lactase • Dextrinase • Trehalase • Intestinal lipase
• Peptides • Sucrose • Maltose • Lactose • Dextrin, Maltose, • Trehalose • Triglycerides
• Aminoacids • Fructose and glucose • Glucose • Galactose and glucose • Glucose • Glucose • Fatty acids
DIGESTION OF FOOD
• The process of digestion is accomplished by mechanical and chemical processes.
Types of digestive enzymes
Carbohydrase Proteinase Lipase Nuclease • Salivary amylase • Gastric amylase • Pancreatic amylase • Sucrase • Maltase • Lactase • Dextrase • Trehalase
• Pepsin • Trypsin • Chymotrypsin • Carboxypeptidase • Dipeptidases • Tripepetidases • Aminopeptidase
• Lingual lipase • Gastric lipase • Pancreatic lipase • Phospholipase • Intestinal lipase
• DNAase • RNAase
12
Carbohydrase Proteinase Lipase Nuclease • Salivary amylase • Gastric amylase • Pancreatic amylase • Sucrase • Maltase • Lactase • Dextrase • Trehalase
• Pepsin • Trypsin • Chymotrypsin • Carboxypeptidase • Dipeptidases • Tripepetidases • Aminopeptidase
• Lingual lipase • Gastric lipase • Pancreatic lipase • Phospholipase • Intestinal lipase
• DNAase • RNAase
Digestion at various parts of the alimentary canal
Mouth• The buccal cavity performs two major functions;
- Mastication of food
- Facilitation of swallowing
• The teeth and the tongue with the help of saliva masticate and mix up the food
thoroughly.
- Mastication or chewing: It is the first mechanical process performed by GIT.
• The saliva secreted into the oral cavity contains electrolytes and enzymes like salivary
amylase, lysozyme, etc.
• Salivary amylase: The chemical process of digestion is initiated in the oral cavity by the
hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase.
• About 30% of starch is hydrolysed here by this enzyme (optimum pH 6.8) into a
disaccharide (maltose).
• Lysozyme:The enzyme lysozyme present in saliva acts as an antibacterial agent that
prevents infections.
• Mucous in saliva helps in lubricating and adhering the masticated food particles into a
bolus.
• Bolus: Tongue rolls up the masticated food into small balls called the bolus.
- The teeth and tongue with the help of saliva masticate and mix up the food thoroughly.
- Mucus in saliva helps in lubricating the food.
Pharynx and oesophagus• The bolus is then conveyed into the pharynx and then into the oesophagus.
• There is no digestion in the oesophagus. Oesophagus conducts the food from the mouth
into the stomach (fig. 16.14).
• Deglutition: It is the process by which bolus moves from mouth to stomach also called
swallowing.
• Peristalsis: It is the successive waves of muscular contractions that push the food through
the oesophagus into the stomach.
13 - It occurs in the oesophagus, stomach
and intestine
- The least peristalsis occurs in the
rectum.
• Peristalsis can be classified as two;
- Primary peristalsis: It is simply
a continuation of the peristaltic
wave that begins in the pharynx
and spreads into the oesophagus.
- Secondary peristalsis: If the
primary peristaltic wave fails to
move into the stomach all the
food that has entered the oesophagus, secondary peristaltic waves result from the
distention of the oesophagus itself by the retained food; these waves continue until
all the food has emptied into the stomach.
• The gastro-oesophageal sphincter would control the passage of food into the stomach.
• Vomiting: It is the reverse peristalsis for the ejection of harmful substances from the
gut.
Stomach • The mucosa of the stomach has gastric glands.
• Gastric glands have three major types of cells;
- Mucous neck cells: These cells secrete mucus.
- Peptic or chief cells: These cells secrete the proenzyme pepsinogen.
- Parietal or oxyntic cells: These cells secrete HCl and intrinsic factor (factor essential
for the absorption of vitamin B12).
• HCl: Parietal or oxyntic cells secrete HCl. The main functions of HCl include activation
of pepsinogen, killing microorganisms, and providing an acidic medium. HCl production
is stimulated by gastrin.
• The stomach stores the food for 4-5 hours.
• Chyme: The food is mixed thoroughly with the acidic gastric juice of the stomach by the
churning movements of its muscular wall and it is called the chyme.
• Pepsin: The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted
into the active enzyme pepsin (the proteolytic enzyme of the stomach).
- HCl secreted by parietal cells provides the acidic pH (pH 1.8 to 3.5) optimal for
pepsins.
Fig 16.14
14
• The vigorous mixing of the gastric juices transports the gastrin (secreted by G cells of
the pyloric gland) rapidly to the ECL cells (Enterochromaffin-like cells) in the body of
the stomach, causing the release of histamine directly into the deep oxyntic glands. The
histamine then acts quickly to stimulate gastric hydrochloric acid secretion.
• Pepsin (secreted by the chief cells of the stomach) converts proteins into proteoses and
peptones (peptides).
• The mucus and bicarbonates present in the gastric juice help in lubrication and protection
of the mucosal epithelium from the highly concentrated hydrochloric acid.
• Rennin: It is a proteolytic enzyme found in the gastric juice of infants which helps in
the digestion of milk proteins.
Small intestine • The movements provided by the muscular layer help in a thorough mixing up of the
food with various secretions in the intestine and thereby facilitate digestion.
• Secretions of the small intestine include:
- Pancreatic juice
- Bile
- Intestinal juice
• The bile and pancreatic juice are released through the hepato-pancreatic duct from the
liver and pancreas respectively.
● Pancreatic juice It contains inactive enzymes;
- Trypsinogen, Chymotrypsinogen, Procarboxypeptidase, Amylases, Lipases, and
Nucleases.
• Trypsinogen: It is an enzyme present in pancreatic juice. Trypsinogen is inactive and it
gets activated by the enzyme enterokinase.
• Enterokinase: It is secreted by the intestinal mucosa when chyme comes in contact with
the mucosa and converts inactive trypsinogen into active trypsin.
• Trypsin: It is the activated form of trypsinogen that activates other enzymes in the
pancreatic juice.
The action of pancreatic juice enzymes on chyme• Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching
the intestine are acted upon by the proteolytic enzymes of pancreatic juice and converts
15
to dipeptides.
• Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.
• Fats are broken down by lipases with the help of bile into di-and monoglycerides.
• Nucleases in the pancreatic juice act on nucleic acids to form nucleotides and nucleosides.
● Bile• The bile is produced in the liver and released into the duodenum. It contains bile pigments
(bilirubin and biliverdin), bile salts, cholesterol and phospholipids but no enzymes.
- Bile helps in the emulsification of fats, i.e., breaking down the fats into very small
micelles. They have detergent action on the fat particles in the food. This decreases
the surface tension of the particles and allows agitation in the intestinal tract to
break the fat globules into minute sizes. This is called the emulsifying or detergent
function of bile salts.
- Bile also activates lipases produced by gastric glands.
- Bile salts help in the absorption of fatty acids, monoglycerides, cholesterol, and other
lipids from the intestinal tract. They do this by forming very small physical complexes
with these lipids; the complexes are called micelles.
- Without the presence of bile salts in the intestinal tract, up to 40 per cent of the
ingested fats are lost into the faeces, and the person often develops a metabolic
deficit because of this nutrient loss.
• Goblet cells: These cells are present in the intestinal mucosal epithelium.
- They secrete mucus.
- The mucus along with the bicarbonates from the pancreas protects the intestinal
mucosa from acid as well as provides an alkaline medium (pH 7.8) for enzymatic
activities.
● Intestinal juice• Also called Succus entericus.
- They consist of various enzymes like disaccharidases (e.g., maltase), dipeptidases,
lipases, nucleosidases, etc.
16
REGULATION OF ACTIVITIES IN THE GASTROINTESTINAL TRACT
• The activities of the gastrointestinal tract are under neural and hormonal control for
proper coordination of different parts.
- The sight, smell and the presence of food in the oral cavity can stimulate the secretion
of saliva.
- Gastric and intestinal secretions are also, similarly, stimulated by neural signals.
- The muscular activities of different parts of the alimentary canal can also be moderated
by neural mechanisms, both local and through CNS.
- Hormonal control of the secretion of digestive juices is carried out by local hormones
produced by the gastric and intestinal mucosa.
The action of intestinal juice enzymes on chyme• The enzymes in the succus entericus or intestinal juice act on the end products of the
above reactions to form simple absorbable forms.
• The breakdown of biomacromolecules occurs in the duodenum region of the small
intestine.
• The simple substances thus formed are absorbed in the jejunum and ileum regions of
the small intestine.
• The undigested and unabsorbed substances are passed on to the large intestine
Large intestine
• No significant digestive activity occurs in the large intestine.
• The functions of the large intestine are;
- Absorption of some water, minerals and certain drugs
- Secretion of mucus, which helps in adhering the waste (undigested) particles
together and lubricating it for an easy passage.
• The undigested, unabsorbed substances called faeces enter into the caecum of the large
intestine through the ileocaecal valve, which prevents the backflow of the faecal matter.
• It is temporarily stored in the rectum till defecation.
17SUMMARY OF THE DIGESTION EVENTS
Fig 16.15
• Absorption is the process by which the end products of digestion pass through
the intestinal mucosa into the blood or lymph.
• Absorption is carried out by:
- Passive or simple diffusion
- Active diffusion
- Facilitated diffusion
• Maximum absorption takes place in the ileum.
• Amino acids and glucose are carried by hepatic portal veins and water is absorbed by
osmosis.
• Assimilation: The process in which the absorbed substances finally reach the tissues
which utilise them for their activities.
• The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex
causing an urge or desire for its removal.
ABSORPTION OF DIGESTED PRODUCTS
18• Defecation (ejection): Elimination of undigested food through the anus.
- With the accumulation of faecal matter, the colon develops a strong peristaltic
movement, which produces the urge for defecation.
Transport mechanisms
Fat absorption• Fatty acids and glycerol are insoluble in water
and are not absorbed directly by the intestine.
• Digested fats are first incorporated into small,
spherical droplets called micelles.
• This moves into the intestinal mucosa.
• Fats are re-synthesized into very small fat
molecules called chylomicrons.
• They are further transported into lymph vessels
(lacteals) in the villi.
• These lymph vessels ultimately release the
absorbed substances into the bloodstream. (fig 16.16)
Fig 16.16
Simple Diffusion Facilitated Transport Active Transport
Small amounts of monosaccharides like glucose, amino acids and electrolytes like chloride ions are transported freely, without energy requirement.
Take place with help of carrier proteins like Na+. Substances absorbed include fructose and amino acids.
It occurs against the concentration gradient and requires energy. Substances absorbed include amino acids, monosaccharide like glucose, electrolytes like Na+ absorbed into blood.
Mouth Stomach Small Intestine Large Intestine
Certain drugs coming in contact with the mucosa of mouth and lower side of the tongue are absorbed into the blood capillaries lining them.
Absorption of water, simple sugars, and alcohol, etc. take place.
Principal organ for absorption of nutrients. The digestion is completed here and the final products of digestion such as glucose, fructose, fatty acids, glycerol and amino acids are absorbed through the mucosa into the blood stream and lymph.
Absorption of water, some minerals and drugs takes place.
SUMMARY OF ABSORPTION IN VARIOUS ORGANS
19
PEM (PROTEIN-ENERGY MALNUTRITION)
DISORDERS OF THE DIGESTIVE SYSTEM
• The inflammation of the intestinal tract is the most common ailment due to bacterial
or viral infections.
• The infections are also caused by the parasites of the intestine like tapeworm, roundworm,
threadworm, hookworm, pinworm, etc.
1. Jaundice • The liver becomes affected.
• Skin and eyes turn yellow due to the deposit of bile pigments.
• Causes: Jaundice has many causes, including hepatitis, gallstones and tumours.
2. Vomiting • It is the ejection of stomach contents through the mouth.
• This reflex action is controlled by the vomiting centre in the medulla.
• A feeling of nausea precedes vomiting.
• Causes: Foodborne diseases, indigestion and many other reasons result in vomiting.
3. Diarrhoea• The abnormal frequency of bowel movement and increased liquidity of the faecal
discharge.
• Causes:The inflammation of the stomach or intestine as a result of microbial infection
can be a cause of diarrhoea.
• It reduces the absorption of food.
4. Constipation • The faeces are retained within the colon as bowel movements occur irregularly.
• Causes: Eating foods low in fibre, not drinking enough water (dehydration), not getting
enough exercise, changes in routine all these can lead to constipation.
5. Indigestion• In this condition, the food is not properly digested leading to a feeling of fullness.
• Causes: Causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning,
overeating, and spicy food.
• Dietary deficiencies of proteins and total food calories are widespread in many
underdeveloped countries of South and Southeast Asia, South America, and West and
Central Africa.
• Protein-energy malnutrition (PEM) may affect large sections of the population during
drought, famine and political turmoil (e.g. Bangladesh-liberation war and Ethiopia-
severe drought in mid-eighties).
20
• PEM affects infants and children to produce Marasmus and Kwashiorkor.
Marasmus• It is produced by a simultaneous deficiency of
proteins and calories. (fig 16.17)
• Cause: It is found in infants less than a year
in age if mother’s milk is replaced too early by
other foods which are poor in both proteins and
caloric value.
• This often happens if the mother has a second
pregnancy or childbirth when the older infant is still too young.
• Symptoms: In this condition, protein deficiency impairs growth and replacement of
tissue proteins; extreme emaciation (being thin and weak) of the body and thinning of
limbs, the skin becomes dry, thin and wrinkled, growth rate and bodyweight decline
considerably.
• Even growth and development of the brain and mental faculties are impaired.
Kwashiorkor• It is produced by protein deficiency unaccompanied by calorie
deficiency. (fig 16.18)
• Cause: It results from the replacement of mother’s milk by a high-
calorie low protein diet in a child more than one year in age.
• Symptoms: Like marasmus, kwashiorkor shows wasting of muscles,
thinning of limbs, failure of growth and brain development, but
unlike marasmus, some fat is still left under the skin; moreover,
extensive oedema and swelling of body parts are seen.
Fig 16.17
Fig 16.18
21
QUESTION DISCUSSION
Qn - 1 Qn - 5
Qn - 6
Qn - 2
Qn - 7
Qn - 8
Qn - 3
Qn - 4
1 1
1
1
1
1
1
1
2
3
4
2
2
3
3
4
4
2
3
42
3
4
2
3
4
2
3
4
2
3
4
Identify the correctly matched
structure and its secretion :
Brunner’s gland – salivary amy-
lase
Intestinal mucosa - Insulin
Gall bladder - Bile
Salivary gland - Lysozyme
In vertebrates, lacteals are found
in
Oesophagus Ear
Ileum Ischium
Oxyntic cells secrete
HCl NaOH
Pepsin TrypsinDentine is secreted by :
Odontoblast
Elaioblast
Osteoblast
Chondroblast
Succusentericus is secreted by
Auerbach’s plexus
Brunner’s glands
Peyers patches
Crypts of Leiberkuhn
The types of teeth present in
humans are
Monophyodont and homodont
Diphyodont and heterodont
Diphyodont and homodont
Monophyodont and thecodont
Parotid glands are located below :
The floor of the mouth
The external acoustic meatus
The tongue
The eye
Brunner’s glands are found in
Liver
Oesophageal wall
Pancreas
Surface epithelium of intestine
22
Qn - 9
Qn - 12
Qn - 11
Qn - 10
1 1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
Match the column I with column II
and choose the correct option.
Column I Column II
A. Goblet cells 1. Antibacterial agent
B. Lysozyme 2. Mucus
C. Saliva 3. HCl
D. Oxyntic cells 4. Sublingual gland
A-3, B-1, C-4, D-2
A-1, B-3, C-4, D-2
A-2, B-3, C-1, D-4
A-2, B-1, C-4, D-3
Identify A, B, C and D in the given
figure and choose the incorrect
statement about them:
‘A’ is serosa and it contains thin
mesothelium with connective tissue
‘B’ is muscularis and it is formed by
smooth muscles
‘C’ is submucosa and it contains
loose connective tissues containing
nerves, blood and lymph vessels
‘A’, ‘B’, ‘C’ and ‘D’ all layer re-
mains as such without modification
throughout the GIT
Which of the following tooth is
premolar?
I II
III IV
Read the following statements and
select the correct option.
Statement A: Mucosal epithelium of gut
has goblet cells which secrete mucus.
Statement B: The mucus in the gastric
and pancreatic juice protects the mucosa
from excoriation by acidic secretion.
Statement B is true but Statement
A is false
Both Statements A and B are true
Statement A is true but Statement
B is false
Both Statements A and B are false Qn - 13
1
2
3
4
Pepsinogen is activated by :
Trypsin
Chymotrypsin
Hydrochloric acid
Pepsin
23
Qn - 14
1
2
3
4
What is cholecystokinin?
Enzyme
Bile-pigment
Gastrointestinal hormone
Lipid
Qn - 15
Qn - 16
Qn -19
Qn - 20
Qn -21
Qn -22
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
3
4
4
4
4
4
4
In human, pH of saliva is
6.5 8
7 9.5
Food bolus after passing through
alkaline medium is
Chyle Chyme
Chylomicron Fat body
Enzyme present in saliva is
Maltase
Ptyalin
Sucrase
Invertase
Bile salts help in ______________ of
fats.
Dehydration
Deglutition
Emulsification
Peristalsis
Proteins after complete hydrolysis
yield
Glucose
Amino acids
Triglycerides
None of these
Gastric secretion is stopped by
hormone
Enterogastrone
Gastrin
Pancreozymin
Cholecystokinin
Qn - 17
Qn - 18
1
1
2
3
4
2
3 4
Carbohydrate digestion occurs
first in
Mouth
Intestine
Stomach
None of these
If pancreas is removed, the
compound which will remain
undigested is
Proteins Carbohydrates
Fats All of these
24
Qn - 23
Qn - 28
Qn - 29
Qn - 24
1
11
2
3
4
2
3
4
1
2
2
3
4
What is common among amylase,
rennin and trypsin?
These are produced in stomach
These act a pH lower than 7
These are all proteins
These are all proteolytic enzymes Assertion : Thick layers of muscl-
es are present in the wall of alim-
entary canal.
Reason : These muscles help in mixing of
food materials with the enzymes coming
from different glands in the alimentary
canal.
Both Assertion and Reason are true
and Reason is the correct explana-
tion of Assertion
Both Assertion and Reason are true
and Reason is not the correct ex-
planation of Assertion
Assertion is true but Reason is false
Both Assertion and Reason are false
Assertion : Saliva secretion is a
reflex action.
Reason : Saliva secretion is controlled by
only brain and not the hormones.
Both Assertion and Reason are true
and Reason is the correct explana-
tion of Assertion
Both Assertion and Reason are true
and Reason is not the correct ex-
planation of Assertion
Assertion is true but Reason is false
In the process of digestion in
human body, the carbohydrates
are broken down by
Amylolyticenzyes
Lipolytic enzymes
Proteolytic enzymes
Gastric lipase
Qn - 25
Qn - 27
Qn - 26
1
1
1
2
2
2
3
3
3
4
4
4
Digestion of which component of
food will be affected if the pH of
stomach is made 7?
Fat Protein
Sucrose Vitamins
Glucose is stored as glycogen in
Pancreas Liver
Stomach Kidney
Maximum digestion of food occurs
in
Duodenum
Jejunum
Ileum
Caecum
25Both Assertion and Reason are false3
4
Qn - 30
Qn - 34
Qn - 35
Qn - 36
Qn - 31
Qn - 32
1
1
1
1
1
1
2
3
4
2
2
2
3
3
3
4
4
4
2
2
3
3
4
4
Which of the following are absorbed
in the alimentary canal as such?
Albumin of egg
Polysaccaharides
Fat soluble vitamins
ProteinsSelect the option which is incorr-
ectly matched regarding the abso-
rption of nutrients.
Stomach – alcohol, water
Large intestine – NaCl, some drugs
Duodenum – vitamins produced by
bacteria, amino acids
Ileum – Vitamin B12 , bile salts
Which of the following is not a
function of liver?
Secretion of heparin
Synthesis of Vitamin A
Destruction of RBCs
Secretion of pepsinogen
Carrier ions like Na+ facilitate the
absorption of substances like
Amino acid and glucose
Glucose and fatty acids
Fatty acids and glycerol
Fructose and some amino acids
Calcium deficiency in the body
occurs in the absence of
Vitamin C
Vitamin D
Vitamin B
Vitamin E
Marasmus in children is caused by
deficiency of
Carbohydrates
Protein
Fats
Vitamins
Qn - 33
1
Which of the following statements
is true about kwashiorkor
It usually occurs in children in the
age group of 1 to 5 years
It is caused due to deficiency of
proteins and calories
Its symptoms are prominent ribs,
wrinkled and thin skin due to less
fat in body
It leads to non-recoverable growth
retardation in preschoolers
2
3
4
26
Qn - 37
Qn - 38
Qn - 39
1
1
1
2
2
2
3
3
3
4
4
4
Two friends are eating together
on a dining table. One of them
suddenly starts coughing while swallowing
some food. This coughing would have been
due to improper movement of
Epiglottis Diaphragm
Neck Tongue
The purplish red pigment rhodop-
sin contained in the rods type of
photoreceptor cells of the human eye, is a
derivative of
vitamin B1 vitamin C
vitamin D vitamin A
Which one of the following enzymes
carries out the initial step in the
digestion of milk in human adults?
Pepsin Rennin
Lipase Trypsin
Qn - 40
1
2
3
4
Which one of the following correc-
tly represents the normal adult
human dental formula?
3,1,3,1 / 3,1,2,1
2,1,3,3 / 2,1,2,3
2,1,2,3 / 2,1,2,3
3,1,3,3 / 3,1,3,3
