Energy System
Human Digestive System
Structure and functions of digestive
organs: Digestive system of man consists of alimentary canal and digestive
glands. Alimentary canal is a long tube of varying diameter of about 8 to 10
metres in length. It is divided into following parts:
Mouth: It is a transverse slit bounded
by two soft movable lips which are covered by skin.
Vestibule: It is the narrow space
enclosed between the lips and cheeks externally and the gums and teeth
internally, and leads into oral cavity. Its lining contains mucous glands.
Oral or buccal cavity: It is a large
space bounded above by palate that separates oral cavity from nasal chamber,
below by throat and on the sides by the jaws. The throat supports tongue while
jaws bear teeth.
Palate: It is of hard palate and soft
palate. Hard palate is supported by bones and bears transverse ridges called rugae, which keep the food in place
during mastication. Soft palate is the posterior part of the palate and its
smooth surface makes swallowing easy. The posterior free end of it hangs down
as a small flap, the uvula or velum palate that closes internal
nares during swallowing of food bolus.
Tongue: It is a large, muscular, highly
mobile structure that lies at the floor of buccal cavity. Posteriorly it is
attached to floor of buccal cavity by a soft ligamentous fold, the frenulum. Structurally tongue is
skeletal muscle covered by mucous membrane that secretes mucus to keep the
tongue moist. Its dorsal surface is marked by a V-shaped furrow, the sulcus terminalis, and also bears lingual papillae which are of three
types, viz. filiform papillae are
thread-like and distributed over the anterior two-third of the tongue, fungiform papillae are mushroom-shaped
and are more numerous near the edges of tongue and circumvallate papillae form an inverted V at the posterior part of
tongue. The sides of fungiform and circumvallate papillae bears taste buds and the taste buds located
near tip of the tongue taste for
sweet, those present on sides taste
for sour and salt, and those on the posterior
part of tongue for bitter taste.
Function of tongue:
(i) Helps in
ingestion.
(ii) Taste the
food.
(iii) Mixes saliva with food and makes food soft and
slippery for chewing.
(iv) Moves food in buccal cavity during chewing.
(v) Turns
chewed food into a spherical mass called bolus.
(vi) Helps in speech.
Teeth: Teeth develop by ossification in
the mucous membrane of buccal cavity along the ridge of the jaw. The teeth in
human are of following types:
(i) Thecodont,
i.e., are fastened in the sockets of the jaw bones.
(ii) Diphyodont, i.e., develop twice. Twenty
deciduous teeth develop in the baby and are called milk teeth. These are replaced later between the ages of 6-13,
except for the last molar called wisdom
teeth that appears after the age of 18 years.
(iii) Heterodont, i.e., of various shapes,
size and structure adapted for different functions.
The arrangement
of permanent teeth can be represented by dental
formula:
Tonsils: These are two oval patches one
on either side in the posterior part of buccal cavity. These contain groups of
white blood corpuscles held in place by a thin tough covering. Lymphocytes are
formed in the tonsils and remove micro-organisms that enter buccal cavity with
food.
Pharynx: It is the cavity behind the
soft palate about 12-14 cm, conical in shape where food and air passages cross
each other. It has the following openings: two nasal openings or internal
nares, two openings of Eustachian tubes, opening of oesophagus the gullet, the opening of larynx or
trachea the glottis and the opening
of buccal cavity into pharynx. Pharynx is divided into three parts, viz., nasopharynx lies behind the nasal
chambers and has internal nares in its roof, oval-shaped openings of Eustachian
canals on the sides, and two bodies of lymphoid tissue called pharyngeal tonsils in children up to
the age of seven years; oropharynx
lies behind the buccal cavity and is the passage for food-bolus; and laryngopharynx is the lowermost part of
pharynx and has two apertures-anterior slit-like glottis and posterior gullet. Glottis
is closed during swallowing of food bolus by a leaf-like cartilage, the epiglottis.
Oesophagus: It is a muscular tube about
10 inches (25cm) long and passes food down by peristaltic movements of its
muscular wall. Its opening into the stomach is surrounded by a ring of muscle
called cardiac sphincter.
Stomach: It is an elongated sac that
lies below the diaphragm in the upper part of abdominal cavity along with the
liver lobes. The greater part of stomach lies towards the left. Stomach is
divided into three parts, viz., (i) Cardiac
part is the left broad upper part into which oesophagus opens by cardiac aperture or cardia. The cardiac sphincter present around it checks
regurgitation of food, and in infant it is not well developed and hence
regurgitation is common. (ii) Fundus
is the upper projected part of cardiac stomach. (iii) Corpus or body is the
main, middle part of the stomach. (iii) Pyloric
part is the lower narrow part of stomach which opens into the duodenum by pyloric aperture which is guarded by a pyloric
sphincter that prevents predigested food to enter the duodenum.
The inner
surface or mucosa of stomach is raised into a large number of longitudinal
folds called gastric rugae that
dilate the stomach to store the food and also increase surface area for
digestion.
Functions of stomach:
(i) It stores
food until is partially digested.
(ii) It
secretes gastric juice to aid in digestion.
(iii) It churns food into a fine pulp and mixes
gastric juice.
(iv) Water and soluble foods such as sugar, alcohol and certain drugs are
absorbed by stomach wall.
(v) Its acid
kills bacteria that enter it along with food.
(vi) It produces hormone gastrin in pyloric stomach that stimulates
secretion of gastric juice by gastric glands.
(vii) It produces intrinsic factor that helps in the
absorption of vitamin B12.
Small intestine: It is 6 metres or 20
feet long tube of approximately 2.5 cm or 1 inch diameter. It lies coiled in
the abdominal cavity and is divided into three parts. (i) Duodenum is the first part of intestine, about 25 cm long and
U-shaped in appearance. Its descending limb receives of hepato-pancreatic duct
formed by the union of bile duct and
pancreatic duct. (ii) Jejunum is
about 2.5 metres long and narrower than duodenum. (iii) Ileum is about 3.5 metres, forms the largest part of small
intestine.
Functions of small intestine:
(i) Completes
digestion and absorbs the digested food.
(ii) Secrets
hormones like cholecystokinin, secretin, enterogastrone, duocrinin,
enterocrinine and villikinin that control the secretion of pancreatic juice,
bile and intestinal juice.
Large intestine (Colon): It is wider
than small intestine and about 1.5-1.8 metres long and 6 cm in diameter. It is
divided into three parts. (i) Caecum
is connected to ileum and opening of ileum into it is controlled by ileocaecal valve that permits food to
move into the large intestine but not back into the small intestine. From
caecum arises a worm-like blind tube called vermiform appendix and is a vestigial organ in man. Infection of
appendix leads toappendicitis. (ii) Colon is inverted U-shaped tube,
divided into four parts namely ascending
colon, transverse colon, descending colon and sigmoid colon. (iii) Rectum
is the 15 cm long terminal tube that opens to the exterior by anus. Anus is guarded by two (internal and
external) sphincter muscles.
Functions of large intestine:
(i) Absorption of water from food.
(ii) Secretion of mucous.
(iii) Egestion
of undigested waste matter
Structure and functions of digestive
glands: These associated glands secrete digestive juices for the digestion
of food and they are of following types:
(i) Salivary glands: It has three
types. (i) Parotid glands are the
largest salivary glands that lie on the sides of the face just below and in
front of the ears. Viral infection of parotid glands, causing swelling and
pain, is called mumps. (ii) Sublingual
glands lie under the front part of the tongue and sublingual ducts also
open under the tongue. (iii) Submaxillary
glands lie at the angles of the lower jaw and submaxillary ducts open under
the tongue.
Functions of salivary glands is to
secrete saliva which is watery, alkaline, fluid formed of 98-99% water, 0.2%
salts, mucus and a starch splitting enzyme ptyalin.
Daily 1-1.5 litres of saliva is secreted and its pH is 6.8.
a. Ptyaline hydrolyses starch into
maltose.
b. Mucus present
in saliva mixes with food and makes it soft and viscous to be easily masticated
by grinders.
c. Enzyme polysaccharidase present in saliva destroys bacteria. Lysozyme acts as an anti- bacterial
agent.
(ii) Gastric glands: These are
numerous, simple or branched tubular glands present in the mucosa of the
stomach and have the following types of cells:chief or peptic (zymogen)
cells which secrete two proenzymes pepsinogen
and prorennin, and enzymes gastric lipase and gastric lipase.
Oxyntic (parietal) cells secrete
HCl and Castle’s intrinsic factor
that helps in the absorption of Vitamin B12 . Goblet cells secrete mucus; and Argentaffin cells secrete Serotonin, a vasoconstrictor.
Endocrine cells
or G cells secretes Gastrin, the hormone which stimulate
secretion of enzymes and HCl.
All these juices
collectively form gastric juices and
daily secretion is 2-3 litres per day. The pH of gastric juice is 1.2-1.8 and
HCl forms about 0.05 to 0.3 percent of it.
(iii) Intestinal glands: These are
numerous, microscopic glands present in the mucosa of small intestine between
the villi. These are of two types, viz., (i) Crypts of Lieberkuhn are multicellular, simple, straight, tubular
glands present throughout the mucosa of small intestine between villi. These
secrete digestive enzymes and mucus. (ii) Brunner’s
glands are compound tubular glands which are found only in the sub-mucosa
of duodenum. These secrete alkaline watery juice.
The secretions
of both the glands collectively are called intestinal juice or succus entericus
(pH 8.3) and secrete about 1 litre daily.It contains enzymes maltase, isomaltase, sucrose,lactase,α-dextrinase,limit
dextrinase,enterokinase, aminopeptidase, dipeptidase,nucleotidase, intestinal
lipase etc.
(iv) Liver: It is a reddish-brown multi-lobed gland and weighs about 1.5 kg.
It is formed of larger right lobe, smaller left lobe, small quadrate and
caudate lobes. Each liver lobe has numerous pentagonal or hexagonal hepatic lobules . It contains numerous interlobular blood vessels and interlobular bile ducts. Gall bladder is a thin
walled, pear-shaped sac present on the lower surface of the right liver lobe
and stores bile secreted by liver.
Functions of liver are
(i) Secretion of bile: It is yellowish or
reddish-brown in colour, alkaline (pH 7-8.6) and bitter liquid that contains bile salts (sodium taurocholate, sodium
glycocholate and sodium bicarbonate) and bile
pigments (bilirubin and biliverdin).Bile carries numerous functions like
stopping action of gastric juice, making the chime(partially digested
food that comes into duodenum) alkaline for the action of pancreatic juice, emulsification of fat
by bile salts, removal of excretory material (bile pigments from liver) absorption of vitamin K and inducing
peristaltic movement in wall of intestine.
(ii) Metabolism of glucose: Glycogenesis
is conversion of excess glucose in blood into glycogen to be stored in liver cells; glycogenolysis is hydrolysis
of glycogen into glucose; glyconeogenesis is synthesis of glucose from amino
acids or fatty acids and glycerol whenever needed by the body.
(iii) Synthesis and storage of
fat: Lipogenesis is conversion of excess glucose into fats to be stored in adipose
tissue. β–oxidation is denaturation of fatty acids and phosphorylation of fats
in liver cells.
(iv) Deamination of proteins:
Excess amino acids undergo deamination producing pyruvic acid and ammonia.
(v) Synthesis of urea: Ammonia produced in liver
cell is converted to urea (Kreb’s ornithine-arginine cycle).
(vi) Synthesis
of vitamin A from carotene and storage of vitamins A, B12 E and D.
(vii) Synthesis
of albumin from amino acids.
(viii) Formation of blood proteins like prothrombin
and fibrinogen.
(ix) Phagocytosis of dead RBCs by
Kupffer cells and breakdown of haemoglobin into bilirubin and biliverdin.
(x) Produce heparin,
an enzyme which prevents clotting of blood inside the blood vessels.
(xi) Formation
of RBCs during foetal development.
(xii) Detoxification:
Liver cells either inactivate the toxic substances like cresol, carbolic acid
etc.,
(produced by intestinal bacteria) or convert them into non-toxic substances. Also prussic
acid produced during metabolism is converted into non-toxic substance.
(xiii) Liver is the centre of heat production.
(xiv) Storage of inorganic substances like iron and
copper.
(xv)
Manufactures lymph.
(v) Pancrease: It is diffused leaf-shaped
compound gland formed of exocrine part
and endocrine part. (i) The exocrine
part is formed of lobules or acini held
together in connective tissue. Each acinus is formed of single layer of large
glandular acinous cells that
manufacture pancreatic juice
containing enzymes trypsinogen
(inactive trypsin), chymotrypsinogen
(inactive chymotrypsin) and procarboxypeptidase.
(ii) The
endocrine part is represented by patches of cells in exocrine part called islets of Langerhans formed of two
types of cells, viz., α-cells that
produce glucagon that induce conversion of glycogen into glucose in liver
cells, causes release of glucose in blood and elevation of blood sugar level; β-cells that produce insulin that
promotes formation of glycogen from glucose for storage in liver and muscle
cells, and reduce blood sugar level.Deficiency of insulin causes Diabetes
Mellitus.
Physiology of digestion
Animals exhibit holozoic nutrition. The process
involves the following steps:
Ingestion: Intake of food through mouth.
Deglutition (swallowing): It is the movement of bolus from oral
cavity to stomach passing through pharynx and oesophagus.
Digestion: The process by which the complex food materials are changed to
simple molecules by the action of digestive juices.
Absorption by intestinal wall and
transported to blood stream.
Assimilation: it is the process of
utilization of absorbed nutrients by the body cells for energy and synthesis of
new protoplasm for growth and repair.
Egestion (defecation): The process of
rid of undigested food matter through anus.
Digestion
During digestion,
carbohydrates are hydrolysed into glucose, proteins into amino acids and fat
molecules into fatty acids and glycerol.
Mechanical digestion: It includes
movement of food along the digestive track, breaking solid food into minute
particles and churning of food for proper mixing of enzymes.
Chemical digestion includes hydrolysis
of food molecules with the help of enzymes.
Digestion in buccal cavity
A
small part (about 30%) of starch is
hydrolysed into Maltose, Isomaltose and
α-dextrines by the action of salivary amylase (ptyaline) present in the saliva.
Digestion in stomach
In
the stomach, food mixes with gastric juice secreted by the gastric gland. Gastric
juice contains Mucus, HCl, pepsinogen,
prorennin and a small amount of lipase.
HCl softens and disinfects food, stops
the action of salivary amylase and activates pepsinogen and prorennin.
No carbohydrate digestion in stomach as
carbohydrate enzymes are absent in stomach.
Digestion of proteins in stomach:
In
acidic medium, Pepsinogen is activated into pepsin. It catalyse hydrolysis of protein into proteoses and peptones.
Pepsin also helps in coagulation of milk viz. hydrolyses milk protein,
casein into paracasein and whey protein. Paracasein is precipitated as calcium paracaseinate to form solid
curd.
In
infants enzyme Prorennin catalyses
coagulation of milk.
Digestion of fats in stomach: a small
amount of gastric lipase is produced
in the stomach which hydrolyses fat into
fatty acid and glycerol. Lipase is soon destroyed by HCl present in gastric
juice.
The
thick acidic mixture of gastric juice and semi digested food formed in the
stomach is called chyme.
Digestion in small intestine
Bile
and pancreatic juice are poured in to the chyme when it passes from stomach
into duodenum.
Bile neutralize HCl of chyme (by its Sodium carbonate salt) and makes it alkaline.
Digestion
of fat: Bile salts break down fat molecules into very small micelles (emulsification of fat) and activate
pancreatic lipase and intestinal lipase. In duodenum, pancreatic lipase hydrolyses emulsified fat into triglycerides,
diglycerides and mono glycerides and then to fatty acids and glycerol which are the end product of fat digestion.
Digestion of carbohydrate:
1. In duodenum, pancreatic amylase
catalyses the remaining starch into disaccharides.
2.
In ileum, disaccharidases ( maltase,
isomaltase,sucrose, lactase, dextrinase) present in intestinal juice carry out final digestionof disaccharides
into monosaccharides.
α-Dextrins α-Dextrinase Glucose
Digestion of proteins:
1. By pancreatic juice in duodenum
a. Trypsinogen
is activated to trypsin by enterokinase present in intestinal juice. It
catalyses hydrolysis of proteins into peptides.
b.
Chymotrypsinogen is activated to chymotrypsin by tripsin. It also catalyses
hydrolysis of proteins into peptides.
c.
Procarboxypeptidase is activated by trypsin
to carboxypeptidases. These
enzymes act on the carboxylic end of
poly peptide and removes single aminoacids.
2. by intestinal juice in ileum
I.
Dipeptidase
hydrolyses dipeptides into amino acids.
II.
Aminopeptidase
hydrolyses terminal peptide bond from the amino end and remove amino acids
one by one.
Endopeptidases and exopeptidases:
Endopeptidase
or endoproteinase are proteolytic peptidases
that break peptide bonds of nonterminal amino acids
(i.e. within the molecule).eg. pepsin,
trypsin and chymotrypsin.
An
exopeptidase is any peptidase that catalyzes the
cleavage of the terminal (or the penultimate) peptide
bond; the process releases a single amino acid
or dipeptide
from the peptide chain. exopeptidase
is further classified as an aminopeptidase
or a carboxypeptidase. An aminopeptidase will cleave a single
amino acid from the amino terminal, whereas carboxypeptidase will cleave a
single amino acid from the carboxylic end of the peptide.
What happens to
a starchy part of meal from the time it is eaten till it is made available to
body tissue. [3,00]
Ans. The changes that take place to a
starchy part of a meal from the time it is eaten till it is made available to
body tissues are:
(i)
In buccal cavity salivary amylase or ptyalin
hydrolyses about 30% of starch into maltose, isomaltose, and small dextrins in
almost neutral or slightly alkaline medium.
(ii)
In stomach gastric juice does not contain any
carbohydrate hydrolyzing enzymes and also HCl in it prevents ptyalin activity,
hence no digestion of starch or its products occurs.
(iii)
In duodenum part of small intestine the
pancreatic amylase or amylopectin present in pancreatic juice hydrolyzes
remainder of starch into maltose, isomaltose and dextrin.
(iv)
In jejunum and ileum part of small intestine the
intestinal juice, saccus entericus,
containing different carbohydrate hydrolyzing enzymes hydrolyzes maltose to
glucose, isomaltase to glucose and dextrin, dextrin to glucose by maltase,
isomaltase and α–dextrinase respectively. (Sucrose to glucose and fructose, and
lactose to glucose and galactose by sucrase and lactase respectively also occur
here.)
The
monosaccharide so formed are absorbed by intestinal walls and then circulated
through blood and then made available to the tissues.
9.Differentiate
between renin and rennin
Ans. Renin is produced in kidney and
catalyzes synthesis of angiotensin.
While Rennin is
produced in gastric glands in ruminant mammals meant for coagulation of milk.
Hormonal regulation of digestive juices:
The secretion of
digestive juices is regulated by nervous, mechanical and chemical (hormonal)
stimuli. Regulation of different digestive juices is given below:
(i) Secretion of saliva is controlled
by reflex mechanism (nervous stimuli) where chemical, mechanical, olfactory or
visual stimuli are carried to the salivatory nuclei in the posterior part of
medulla.
(ii) Secretion of gastric juice has
three phases controlled by nervous and chemical excitations.
(a) Cephalic phase is under nervous
control influenced by both unconditioned and conditioned reflexes. The taste of
food (unconditioned reflex), the sight, smell or even thought of food
(conditioned reflex) stimulate secretion of gastric juices even before the food
enters the stomach and hence called appetite
juice.
(b) Gastric phase where entry of
products of protein digestion into stomach stimulates mucosa of pyloric stomach
to release a hormone gastrin into
the blood capillaries of stomach. Release of gastrin can be also made by
distension of stomach due to presence of food. Gastrin accelerates the
secretion of gastric juices by gastric glands.
(c) Intestinal phase is where entry of
chyme containing fat components stimulates intestinal mucosa to secrete enterogastrone hormone which on entry
into blood and reaching stomach inhibits
the secretory activity of gastric glands and also influences the gastric
motility.
(iii) Secretion of pancreatic juice is
both under nervous and hormonal control. Nervous
control includes the reflexes caused by seeing or tasting food where
secretory cells of pancreas are excited by acetylcholine
released at the nerve endings of vagus nerve
fibres. While, hormonal control
involves stimulation of duodenal mucosa to liberate hormone secretin and pancreozymin (cholecystokinin) by the acid and fat components of
chyme entering it from stomach. Two hormones reach pancreas via hepatic portal
system. Secretin stimulates intralobular cells of pancreas to secrete watery
portion of pancreatic juice and bicarbonates, while cholecystokinin stimulates
acinous cells to secrete pancreatic juice.
(iv) Secretion of Bile Juice is
exclusively under chemical control where cholecystokinin released by duodenal
mucosa when stimulated by entry of amino acids into it brings about contraction
of gall bladder and relaxation of sphincter of Oddi thereby allowing the
passage of bile juice into the duodenum.
(v) Secretion of Intestinal juice by
intestinal mucosa is more or less continuous but it is considerably increased
during digestion. It is under both mechanical and chemical control. The food
material stimulates the local sense organs Meissner’s
plexus that leads to release of intestinal juice. While hormone enterocrinin secreted by intestinal
mucosa induces increased production of digestive enzymes. Secretin also plays
some role in stimulation of intestinal mucosa.
Absorption of digested food:
Absorption is
the process by which nutrient molecules are taken into the cells of a living
organism. It occurs mainly in small intestine by villi on its inner surface
along with microvilli on the free surface of the intestinal epithelium, which
provide enormous surface area for absorption.
(i) Absorption of carbohydrates. Essentially
all carbohydrates are absorbed in the form of monosaccharides mainly as glucose directly into the blood stream
and then carried to liver where they are stored as glycogen.
(ii) Absorption of proteins. Proteins
are absorbed as amino acids directly
into the blood stream and carried to different tissues where they become part
of the metabolic pool.
(iii) Absorption of fats. Fats are
mostly absorbed in the duodenum and jejunum in human beings. Fatty acids with
chain length less upto 10 carbon atoms are primarily absorbed through the blood
capillaries, while larger ones are absorbed through lymphatic route. Since
monoglycerides, diglycerides, and fatty acids are insoluble in water, they are
first incorporated into small, spherical, water-soluble droplets called micelles with the help of bile salts
and absorbed. Absorbed micelles are accumulated in the spaces of the
endoplasmic reticulum of mucosal cells and are subsequently discharged into
intercellular spaces of villi. There
they are synthesized into fats and are released in the form of droplets called chilomicrons into the lymph contained
in the lacteals (lacteals are network of lymph capillaries in the centre of each villi
and are surrounded by blood capillaries).
Absorption can
occur by simple diffusion (fatty
acids and glycerol, electrolytes vitamins etc),facilitated diffusion( fructose),cotransport( glucose and galactose, amionoacids with Na+ ions), and
active transport (aminoacids,glucose,
galactose)
Q. Give one function of lacteals .
Ans. It absorbs chilomicrons from
intestinal villi.
11. Give one
function of microvilli.
Ans. They increase surface area and
enhance absorption capacity of intestine.
Assimilation of digested food:
Assimilation is anabolic process by
which simple nutrients absorbed are utilized to resynthesize the complex
bio-molecules like proteins, carbohydrates, lipids and nucleic acids inside
cells. Assimilation of principal nutrients are given below:
(i) Amino acids. Most amino acids are
used in protein synthesis in the cellular ribosomes and these proteins act as
structural proteins that help in growth and repair of body tissue or act as
enzymes to control metabolism. Some amino acids are changed into glucose by gluconeogenesis for use as respiratory
fuel. Excess amino acids are changed into urea by liver using ornithine cycle.
(ii) Monosaccharides. Most of glucose
acts as the respiratory fuel and provides energy for metabolism while some are
changed into amino acids and fats. Excess of glucose is converted into glycogen
and stored in liver cells and muscle cells by glycogenesis.
(iii) Fats. Most of fats are used in
for the formation of bio-membranes while some are used as respiratory fuels.
Excess fat is stored in liver, bones and adipose tissue.
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