UNIT 5
The reductionist approach to study of life forms resulted in increasing
use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell-
free systems. An explosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of molecules,
supra molecular assemblies, cells, tissues, organisms and indeed,
populations and communities, each create emergent properties. In the
chapters under this unit, major human physiological processes like
digestion, exchange of gases, blood circulation, locomotion and
movement are described in cellular and molecular terms. The last two
chapters point to the coordination and regulation of body events at the
organismic level.
HUMAN PHYSIOLOGY
Chapter 16
Digestion and Absorption
Chapter 17
Breathing and Exchange
of Gases
Chapter 18
Body Fluids and
Circulation
Chapter 19
Excretory Products and
their Elimination
Chapter 20
Locomotion and Movement
Chapter 21
Neural Control and
Coordination
Chapter 22
Chemical Coordination
and Integration
2020-21
ALFONSO CORTI, Italian anatomist, was born in 1822. Corti began
his scientific career studying the cardiovascular systems of
reptiles. Later, he turned his attention to the mammalian
auditory system. In 1851, he published a paper describing a
structure located on the basilar membrane of the cochlea
containing hair cells that convert sound vibrations into nerve
impulses, the organ of Corti. He died in the year 1888.
Alfonso Corti
(1822 – 1888)
2020-21
Food is one of the basic requirements of all living organisms. The major
components of our food are carbohydrates, proteins and fats. Vitamins
and minerals are also required in small quantities. Food provides energy
and organic materials for growth and repair of tissues. The water we take
in, plays an important role in metabolic processes and also prevents
dehydration of the body. Biomacromolecules in food cannot be utilised
by our body in their original form. They have to be broken down and
converted into simple substances in the digestive system. This process of
conversion of complex food substances to simple absorbable forms is
called digestion and is carried out by our digestive system by mechanical
and biochemical methods. General organisation of the human digestive
system is shown in Figure 16.1.
16.1 DIGESTIVE SYSTEM
The human digestive system consists of the alimentary canal and the
associated glands.
16.1.1 Alimentary Canal
The alimentary canal begins with an anterior opening – the mouth, and it
opens out posteriorly through the anus. The mouth leads to the buccal
cavity or oral cavity. The oral cavity has a number of teeth and a muscular
tongue. Each tooth is embedded in a socket of jaw bone (Figure16.2).
This type of attachment is called thecodont. Majority of mammals
including human being forms two sets of teeth during their life, a set of
temporary milk or deciduous teeth replaced by a set of permanent or
adult teeth. This type of dentition is called diphyodont. An adult human
D
IGESTION AND
A
BSORPTION
C
HAPTER
16
16.1 Digestive
System
16.2 Digestion of
Food
16.3 Absorption of
Digested
Products
16.4 Disorders of
Digestive
System
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258 BIOLOGY
Sigmoidî€ Colon
Figure 16.1 The human digestive system
has 32 permanent teeth which are of four different types (Heterodont
dentition), namely, incisors (I), canine (C), premolars (PM) and molars
(M). Arrangement of teeth in each half of the upper and lower jaw in the
order I, C, PM, M is represented by a dental formula which in human
is
2123
2123
. The hard chewing surface of the teeth, made up of enamel, helps
in the mastication of food. The tongue is a freely movable muscular organ
attached to the floor of the oral cavity by the frenulum. The upper surface
of the tongue has small projections called papillae, some of which bear
taste buds.
The oral cavity leads into a short pharynx which serves as a common
passage for food and air. The oesophagus and the trachea (wind pipe)
open into the pharynx. A cartilaginous flap called epiglottis prevents the
entry of food into the glottis – opening of the wind pipe – during swallowing.
The oesophagus is a thin, long tube which extends posteriorly passing
through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag
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259
like structure called stomach. A muscular
sphincter (gastro-oesophageal) regulates the
opening of oesophagus into the stomach.
The stomach, located in the upper left
portion of the abdominal cavity, has four
major parts – a cardiac portion into which
the oesophagus opens, a fundic region, body
(main central region) and a pyloric portion
which opens into the first part of small
intestine (Figure 16.3). Small intestine is
distinguishable into three regions, a ‘C’
shaped duodenum, a long coiled middle
portion jejunum and a highly coiled ileum.
The opening of the stomach into the
duodenum is guarded by the pyloric
sphincter. Ileum opens into the large
intestine. It consists of caecum, colon and
rectum. Caecum is a small blind sac which
hosts some symbiotic micro-organisms. A
narrow finger-like tubular projection, the
vermiform appendix which is a vestigial
organ, arises from the caecum. The caecum
opens into the colon. The colon is divided
into four parts – an ascending, a transverse,
descending part and a sigmoid colon. The
descending part opens into the rectum
which opens out through the anus.
The wall of alimentary canal from
oesophagus to rectum possesses four layers
(Figure 16.4) namely serosa, muscularis,
sub-mucosa and mucosa. Serosa is the
outermost layer and is made up of a thin
mesothelium (epithelium of visceral organs)
with some connective tissues. Muscularis 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. The sub-
mucosal layer is formed of loose connective
tissues containing nerves, blood and lymph
vessels. In duodenum, glands are also
present in sub-mucosa. The innermost
layer lining the lumen of the alimentary
canal is the mucosa. This layer forms
irregular folds (rugae) in the stomach and
small finger-like foldings called villi in the
small intestine (Figure 16.5). The cells lining
the villi produce numerous microscopic
Figure 16.2 Arrangement of different types of
teeth in the jaws on one side and
the sockets on the other side
Figure 16.3 Anatomical regions of human
stomach
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260 BIOLOGY
Figure 16.4 Diagrammatic representation of transverse section of gut
projections called microvilli giving a brush border
appearance. These modifications increase the
surface area enormously. Villi are supplied with
a network of capillaries and a large lymph vessel
called the lacteal. Mucosal epithelium has goblet
cells which secrete mucus that help in lubrication.
Mucosa also forms glands in the stomach (gastric
glands) and crypts in between the bases of villi in
the intestine (crypts of Lieberkuhn). All the four
layers show modifications in different parts of the
alimentary canal.
16.1.2 Digestive Glands
The digestive glands associated with the
alimentary canal include the salivary glands, the
liver and the pancreas.
Saliva is mainly produced by three pairs of
salivary glands, the parotids (cheek), the sub-
maxillary/sub-mandibular (lower jaw) and the
sub- linguals (below the tongue). These glands
situated just outside the buccal cavity secrete
salivary juice into the buccal cavity.
Liver is the largest gland of the body weighing about 1.2 to 1.5 kg in
an adult human. It is situated in the abdominal cavity, just below the
diaphragm and has two lobes. The hepatic lobules are the structural and
functional units of liver containing hepatic cells arranged in the form of
cords. Each lobule is covered by a thin connective tissue sheath called
the Glisson’s capsule. The bile secreted by the hepatic cells passes through
the hepatic ducts and is stored and concentrated in a thin muscular sac
called the gall bladder. The duct of gall bladder (cystic duct) along with
Figure 16.5 A section of small intestinal
mucosa showing villi
Villi
Lacteal
Capillaries
Crypts
Artery
Vein
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261
the hepatic duct from the liver forms the common bile duct (Figure 16.6).
The bile duct and the pancreatic duct open together into the duodenum
as the common hepato-pancreatic duct which is guarded by a sphincter
called the sphincter of Oddi.
The pancreas is a compound (both exocrine and endocrine) elongated
organ situated between the limbs of the ‘C’ shaped duodenum. The
exocrine portion secretes an alkaline pancreatic juice containing enzymes
and the endocrine portion secretes hormones, insulin and glucagon.
Figure 16.6 The duct systems of liver, gall bladder and pancreas
16.2 DIGESTION OF FOOD
The process of digestion is accomplished by mechanical and chemical
processes.
The buccal cavity performs two major functions, mastication of food
and facilitation of swallowing. The teeth and the tongue with the help of
saliva masticate and mix up the food thoroughly. Mucus in saliva helps
in lubricating and adhering the masticated food particles into a bolus.
The bolus is then conveyed into the pharynx and then into the oesophagus
by swallowing or deglutition. The bolus further passes down through
the oesophagus by successive waves of muscular contractions called
peristalsis. The gastro-oesophageal sphincter controls the passage of food
into the stomach.The saliva secreted into the oral cavity contains
electrolytes and enzymes, salivary amylase and
lysozyme. The chemical process of digestion is initiated in the oral cavity
by the hydrolytic action of the carbohydrate splitting enzyme, the salivary
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262 BIOLOGY
amylase. About 30 per cent of starch is hydrolysed here by this enzyme
(optimum pH 6.8) into a disaccharide – maltose. Lysozyme present in
saliva acts as an antibacterial agent that prevents infections.
Starch
Salivary
.
Amylase
pH
Maltose
6 8
 →
The mucosa of stomach has gastric glands. Gastric glands have three
major types of cells namely -
(i) mucus neck cells which secrete mucus;
(ii) peptic or chief cells which secrete the proenzyme pepsinogen; and
(iii) parietal or oxyntic cells which secrete HCl and intrinsic factor
(factor essential for absorption of vitamin B
12
).
The stomach stores the food for 4-5 hours. The food mixes thoroughly
with the acidic gastric juice of the stomach by the churning movements
of its muscular wall and is called the chyme. The proenzyme pepsinogen,
on exposure to hydrochloric acid gets converted into the active enzyme
pepsin, the proteolytic enzyme of the stomach. Pepsin converts proteins
into proteoses and peptones (peptides). The mucus and bicarbonates
present in the gastric juice play an important role in lubrication and
protection of the mucosal epithelium from excoriation by the highly
concentrated hydrochloric acid. HCl provides the acidic pH (pH 1.8)
optimal for pepsins. Rennin is a proteolytic enzyme found in gastric juice
of infants which helps in the digestion of milk proteins. Small amounts of
lipases are also secreted by gastric glands.
Various types of movements are generated by the muscularis layer of
the small intestine. These movements help in a thorough mixing up of
the food with various secretions in the intestine and thereby facilitate
digestion. The bile, pancreatic juice and the intestinal juice are the
secretions released into the small intestine. Pancreatic juice and bile are
released through the hepato-pancreatic duct. The pancreatic juice
contains inactive enzymes – trypsinogen, chymotrypsinogen,
procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is
activated by an enzyme, enterokinase, secreted by the intestinal mucosa
into active trypsin, which in turn activates the other enzymes in the
pancreatic juice. The bile released into the duodenum contains bile
pigments (bilirubin and bili-verdin), bile salts, cholesterol and
phospholipids but no enzymes. Bile helps in emulsification of fats, i.e.,
breaking down of the fats into very small micelles. Bile also activates lipases.
The intestinal mucosal epithelium has goblet cells which secrete
mucus. The secretions of the brush border cells of the mucosa alongwith
the secretions of the goblet cells constitute the intestinal juice or
succus entericus. This juice contains a variety of enzymes like
disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc.
The mucus alongwith the bicarbonates from the pancreas protects the
intestinal mucosa from acid as well as provide an alkaline medium (pH
7.8) for enzymatic activities. Sub-mucosal glands (Brunner’s glands) also
help in this.
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Proteins, proteoses and peptones (partially hydrolysed proteins) in
the chyme reaching the intestine are acted upon by the proteolytic
enzymes of pancreatic juice as given below:
Proteins
Peptones
Proteoses
Trypsin
/
Chymotrypsin
Carboxyp





eeptidase
Dipeptides →
Carbohydrates in the chyme are hydrolysed by pancreatic amylase
into disaccharides.
Polysaccharides starch Disaccharides
Amylase
( )  →
Fats are broken down by lipases with the help of bile into di-and
monoglycerides.
Fats Diglycerides Monoglycerides
Lipases
 →  →
Nucleases in the pancreatic juice acts on nucleic acids to form
nucleotides and nucleosides
Nucleic acids Nucleotides Nucleosides
Nucleases
 →  →
The enzymes in the succus entericus act on the end products of the
above reactions to form the respective simple absorbable forms. These
final steps in digestion occur very close to the mucosal epithelial cells of
the intestine.
Dipeptides
Dipeptidases
 → Amino acids
Maltose
Maltase
 → Glucose + Glucose
Lactose Glucose + alactose
Lactase
 → G
Sucrose Fructose
Sucrase
 → +Glucose
Nucleotides Nucleosides
Nucleotidases Nucleosidases
 →  →→ +Sugars Bases
Di and Monoglycerides Fatty acids Glycerol
Lipases
 → +
The breakdown of biomacromolecules mentioned above 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.
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264 BIOLOGY
No significant digestive activity occurs in the large intestine. The
functions of large intestine are:
(i) absorption of some water, minerals and certain drugs;
(ii) 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 enters into the caecum
of the large intestine through ileo-caecal valve, which prevents the back flow of
the faecal matter. It is temporarily stored in the rectum till defaecation.
The activities of the gastro-intestinal tract are under neural and
hormonal control for proper coordination of different parts. The sight, smell
and/or 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.
CALORIFIC VALUE OF PROTEIN, CARBOHYDRATE AND FAT
(Boxed item – Not for evaluation)
The energy requirements of animals, and the energy content of food, are
expressed in terms of measure of heat energy because heat is the ultimate form
of all energies. This is often measured to as calorie (cal) or joule (J), which is the
amount of heat energy required to raise the temperature of 1 g of water by 1 °C.
Since this value is tiny amount of energy, physiologists commonly use kilocalorie
(kcal) or kilo joule (kJ). One kilo calorie is the amount of energy required to raise
the temperature of 1 kg of water by 1 °C. Nutritionists, traditionally refer to kcal
as the Calorie or Joule (always capitalised). The amount of heat liberated from
complete combustion of 1 g food in a bomb calorimeter (a closed metal chamber
filled with O
2
) is its gross calorific or gross energy value. The actual amount of
energy combustion of 1 g of food is the physiologic value of food. Gross calorific
values of carbohydrates, proteins and fats are 4.1 kcal/g, 5.65 kcal/g and 9.45
kcal/g, respectively, whereas their physiologic values are 4.0 kcal/g, 4.0 kcal/g
and 9.0 kcal/g, respectively.
16.3 ABSORPTION OF DIGESTED PRODUCTS
Absorption is the process by which the end products of digestion pass
through the intestinal mucosa into the blood or lymph. It is carried out by
passive, active or facilitated transport mechanisms. Small amounts of
monosaccharides like glucose, amino acids and some electrolytes like
chloride ions are generally absorbed by simple diffusion. The passage of
these substances into the blood depends upon the concentration gradients.
However, some substances like glucose and amino acids are absorbed with
the help of carrier proteins. This mechanism is called the facilitated transport.
Transport of water depends upon the osmotic gradient. Active
transport occurs against the concentration gradient and hence requires
energy. Various nutrients like amino acids, monosaccharides like glucose,
electrolytes like Na
+
are absorbed into the blood by this mechanism.
Fatty acids and glycerol being insoluble, cannot be absorbed into the
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265
blood. They are first incorporated into small droplets called micelles which
move into the intestinal mucosa. They are re-formed into very small protein
coated fat globules called the chylomicrons which are transported into
the lymph vessels (lacteals) in the villi. These lymph vessels ultimately
release the absorbed substances into the blood stream.
Absorption of substances takes place in different parts of the alimentary
canal, like mouth, stomach, small intestine and large intestine. However,
maximum absorption occurs in the small intestine. A summary of absorption
(sites of absorption and substances absorbed) is given in Table 16.1.
The absorbed substances finally reach the tissues which utilise them
for their activities. This process is called assimilation.
The digestive wastes, solidified into coherent faeces in the rectum
initiate a neural reflex causing an urge or desire for its removal. The
egestion of faeces to the outside through the anal opening (defaecation) is
a voluntary process and is carried out by a mass peristaltic movement.
16.4 DISORDERS OF 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, pin worm, etc.
Jaundice: The liver is affected, skin and eyes turn yellow due to the
deposit of bile pigments.
Vomiting: It is the ejection of stomach contents through the mouth. This
reflex action is controlled by the vomit centre in the medulla. A feeling of
nausea precedes vomiting.
Diarrhoea: The abnormal frequency of bowel movement and increased
liquidity of the faecal discharge is known as diarrhoea. It reduces the
absorption of food.
Constipation: In constipation, the faeces are retained within the colon
as the bowel movements occur irregularly.
Indigestion: In this condition, the food is not properly digested leading to
a feeling of fullness. The causes of indigestion are inadequate enzyme
secretion, anxiety, food poisoning, over eating,
and spicy food.
Stomach
Absorption of
water, simple
sugars, and
alcohol etc.
takes place.
Small Intestine
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.
Mouth
Certain drugs
coming in contact
with the mucosa
of mouth and
lower side of the
tongue are
absorbed into the
blood capillaries
lining them.
Large Intestine
Absorption of
water, some
minerals and
drugs takes
place.
TABLE 16.1 The Summary of Absorption in Different Parts of Digestive System
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266 BIOLOGY
PEM
Dietary deficiencies of proteins and total food calories are widespread in
many underdeveloped countries of South and South-east 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. This happened in Bangladesh during the liberation
war and in Ethiopia during the severe drought in mid-eighties. PEM affects
infants and children to produce Marasmus and Kwashiorkar.
Marasmus is produced by a simultaneous deficiency of proteins and
calories. 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 second pregnancy or
childbirth when the older infant is still too young. In Marasmus, protein
deficiency impairs growth and replacement of tissue proteins; extreme
emaciation of the body and thinning of limbs results, the skin becomes
dry, thin and wrinkled. Growth rate and body weight decline considerably.
Even growth and development of brain and mental faculties are impaired.
Kwashiorkar is produced by protein deficiency unaccompanied by calorie
deficiency. 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. 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.
SUMMARY
The digestive system of humans consists of an alimentary canal and
associated digestive glands. The alimentary canal consists of the mouth,
buccal cavity, pharynx, oesophagus, stomach, small intestine, large
intestine, rectum and the anus. The accessory digestive glands include the
salivary glands, the liver (with gall bladder) and the pancreas. Inside the
mouth the teeth masticates the food, the tongue tastes the food and
manipulates it for proper mastication by mixing with the saliva. Saliva
contains a starch digestive enzyme, salivary amylase that digests the starch
and converts it into maltose (disaccharide). The food then passes into the
pharynx and enters the oesophagus in the form of bolus, which is further
carried down through the oesophagus by peristalsis into the stomach. In
stomach mainly protein digestion takes place. Absorption of simple sugars,
alcohol and medicines also takes place in the stomach.
The chyme (food) enters into the duodenum portion of the small
intestine and is acted on by the pancreatic juice, bile and finally by the
enzymes in the succus entericus, so that the digestion of carbohydrates,
proteins and fats is completed. The food then enters into the jejunum and
ileum portions of the small intestine. Carbohydrates are digested and
converted into monosaccharides like glucose. Proteins are finally broken
down into amino acids. The fats are converted to fatty acids and glycerol.
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The digested end products are absorbed into the body through the epithelial lining
of the intestinal villi. The undigested food (faeces) enters into the caecum of the large
intestine through ileo-caecal valve, which prevents the back flow of the faecal matter.
Most of the water is absorbed in the large intestine. The undigested food becomes
semi-solid in nature and then enters into the rectum, anal canal and is finally egested
out through the anus.
EXERCISES
1. Choose the correct answer among the following :
(a) Gastric juice contains
(i) pepsin, lipase and rennin
(ii) trypsin, lipase and rennin
(iii) trypsin, pepsin and lipase
(iv) trypsin, pepsin and renin
(b) Succus entericus is the name given to
(i) a junction between ileum and large intestine
(ii) intestinal juice
(iii) swelling in the gut
(iv) appendix
2. Match column I with column II
Column I Column II
(a) Bilirubin and biliverdin (i) Parotid
(b) Hydrolysis of starch (ii) Bile
(c) Digestion of fat (iii) Lipases
(d) Salivary gland (iv) Amylases
3. Answer briefly:
(a) Why are villi present in the intestine and not in the stomach?
(b) How does pepsinogen change into its active form?
(c) What are the basic layers of the wall of alimentary canal?
(d) How does bile help in the digestion of fats?
4. State the role of pancreatic juice in digestion of proteins.
5. Describe the process of digestion of protein in stomach.
6. Give the dental formula of human beings.
7. Bile juice contains no digestive enzymes, yet it is important for digestion. Why?
8. Describe the digestive role of chymotrypsin. Which two other digestive enzymes
of the same category are secreted by its source gland?
9. How are polysaccharides and disaccharides digested?
10. What would happen if HCl were not secreted in the stomach?
11. How does butter in your food get digested and absorbed in the body?
12. Discuss the main steps in the digestion of proteins as the food passes through
different parts of the alimentary canal.
13. Explain the term thecodont and diphyodont.
14. Name different types of teeth and their number in an adult human.
15. What are the functions of liver?
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