Digestion and absorption

DIGESTION AND ABSORPTION
SACHIN DAS
NEET SPECIAL

INTRODUCTION
 Process of conversion of complex food substances to simple absorbable
forms is called digestion.
 When the process of digestion occurs within the cell in the food vacuole it
is called intracellular digestion. E.g., protozoa, porifera, coelenterata and
free living platyhelminthes.
 When the process of digestion occurs outside the cell it is called
extracellular digestion. E.g., Coelenterates and phylum platyhelminthes to
phylum chordata.

DIGESTIVE SYSTEM
 Digestion in vertebrates occurs in the
digestive tract or alimentary canal.
 The various parts involved in digestion can
be broadly divided into two groups -
 Digestive tract or alimentary canal
 Accessory digestive glands

ALIMENTARY CANAL
 The alimentary canal is a long coiled tube having
muscular wall & glandular epithelium extending
from mouth to anus.
 The organs comprising the alimentary canal
includes mouth and pharynx (called
buccopharyngeal cavity), oesophagous, stomach
and intestine.

MOUTH
 Mouth consists of vestibule and buccal or oral cavity.
 Vestibule is a space which is bounded externally by lips, cheeks and internally by the
gums and teeth.
 Buccal or oral cavity is bounded by palate above (dorsally), throat with tongue below
and jaws with teeth on the sides.
 Palate forms the roof of buccal cavity and is differentiated into anterior hard palate and
posterior soft palate.
 Hard palate has uvula as a free hanging flap which closes the internal nares during
swallowing of food bolus.
 Soft palate bears transverse palatine rugae supported by bones for holding food during
mastication.
 The tongue is a freely movable muscular organ attached to the floor of the oral cavity by
the frenulum.
 Tongue not only helps in ingestion, mastication and deglutition of food but also help in
voice production and tasting of food.
 The upper surface of the tongue has small projections called papillae, some of which
bear taste buds.

 Teeth are hard structures meant for tearing cutting,
crushing and holding food.
 Each tooth is embedded in a socket of jaw bone. 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 has 32 permanent teeth which are of four different types
(heterodont dentition) - 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
 Dental formulaTemporary teeth in man =
 Teeth at the age of 20 year =
 Human beings (Adult) =

 Morphologically, teeth can be distinguished as homodont or heterodont.
 Homodont : When all the teeth are structurally and functionally similar. E.g., Vertebrates
except metatherian and eutherian mammals.
 Heterodont : When the teeth are different in structure and functions. They are
distinguished into four types - incisors, canines, premolars and molars. E.g., Metatherian
and eutherian mammals.
 Incisors (8) : These are the front, long, curved and possess sharp-edged teeth. They
are adapted for cutting or cropping and biting.
 Canines (4) : There is one pointed or (dagger shaped) canine in each maxillary of upper
jaw and each dentary of lower jaw next to the incisors. They are meant for piercing,
tearing and offence and defence. These are poorly developed in man.
 Premolars (8) : They are meant for crushing, grinding and chewing. They are also
called cheek teeth.
 Molars (12) : They are also called cheek teeth. They are specialized for crushing and
grinding the food.
 Enamel represents the hardest substance of the body. It is completely acellular and
avascular and non regenerable. It helps in mastication of food.
 The element that hardens the tooth enamel is fluorine.

PHARYNX
 The oral cavity leads into a short pharynx which 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.

OESOPHAGUS
 The oesophagus is a thin, long
tube (25 cm) which extends
posteriorly passing through the
neck, thorax and diaphragm and
leads to a ‘J’ shaped bag like
structure called stomach.
 A muscular sphincter (gastro-
oesophageal) regulates the
opening of the oesophagus into
the stomach.

STOMACH
 Stomach is the most distensible and widest organ of
the alimentary canal.
 Empty stomach possesses folds called gastric rugae,
which disappears when the stomach is distended with
food.
 Loss of gastric rugae are one of the earliest sign of
stomach cancer.
 The stomach, located in the upper left portion of the
abdominal cavity, has three major parts – a cardiac
portion into which the oesophagus opens, a fundic
region and a pyloric portion, which opens into the first
part of the small intestine.
 Cardiac part are so called because it is present near the
heart. It is broad at upper part with cardiac sphincter
which prevents regurgitation of food.
 Fonds is commonly filled with air and gases.

INTESTINE
 Intestine is responsible for most of the digestion and absorption of food and usually
formation of dry faeces.
 It is divided into two parts – small intestine and large intestine.
 Small intestine is distinguishable into three regions, a ‘U’ 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.
 Duodenum has ampulla of vater which receives both bile duct (from liver) and main
pancreatic duct (from pancreas) and whose opening was guarded by sphincter of
oddi.
 Ileum is thinner than jejunum and less vascular. It is the longest part (3.5m) of the
small intestine.
 Small intestine is the major site of digestion and absorption of nutrients.

Large intestine consists of caecum, colon and rectum.
 Caecum is a small blind sac which hosts some symbiotic microorganisms.
 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 three parts – an ascending, a transverse and a descending part.
The descending part opens into the rectum which opens out through the anus.
 Colon is concerned with absorption of water of undigested food, salts, vitamins etc. Hence,
concerned with faeces formation.
 Colon bacteria also synthesized vit. B12 and K.
 Rectum has strong sphincter muscle in its wall. The sphincter keeps the canal as well as
anus, closed when not used for defecation.
 Anal canal connects rectum with anus and it is about 3 cm. long.
 Anus is the terminal inferior opening of alimentary canal, which is guarded by an internal
involuntary sphincter and an external voluntary sphincter.
 The chief function of large intestine is the absorption of water and elimination of solid
waste.

 The wall of alimentary canal from oesophagus to rectum possesses four layers 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 submucosal 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.The cells lining the villi produce numerous microscopic 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. Section of small intestinal mucosa
showing villi.

DIGESTIVE GLANDS
The digestive glands associated with the alimentary canal
includes the
 salivary glands,
 gastric glands,
 intestinal glands,
 the liver
 the pancreas.

SALIVARY GLAND
 Saliva is mainly produced by three pairs of
salivary glands - the parotids (cheek), the
submaxillary/submandibular (lower jaw)
and the sublinguals (under the tongue).
These glands situated just outside the
buccal cavity secrete salivary juice into the
buccal cavity.
 The secretion of salivary glands is called
saliva or salivary juice.

SALIVARY JUICE
Amount : 1.0-1.5 litre/day
Chemical nature : Slightly acidic
pH : 6.3 - 6.8
Control of secretion : Autonomic reflex (parasympathetic nervous system increases
salivation while sympathetic nervous system inhibits secretion.)
Chemical composition : Water (99.5%), mucous (acts as lubricant), salts (NaCl, NaHCO3 etc.),
enzymes (ptyalin/salivary amylase, lysozyme) etc.
FUNCTION OF SALIVARY JUICE
It makes the medium slightly acidic for the action of its enzyme, help in taste detection,
deglutition, speaking etc.
Starch Maltose + Isomaltose + Limit dextrin
Bacteria (living) Bacteria killed.

GASTRIC GLANDS
There are approximately 35 million of gastric glands present in the human stomach .
The gastric gland (fundic gland) secretes acid and digestive enzymes.
Secretion of gastric gland is called gastric juice.
GASTRIC JUICE
Amount : 2-3 litres/day
Chemical nature : Highly acidic
pH : 1.0 - 3.5 (due to presence of HCl)
Control of secretion : By gastrin hormone
Chemical composition : Water (99%), mucous, inorganic salts, intrinsic factor, HCl
(0.5%, conc.) and enzymes - prorennin, pepsinogen and gastrin lipase

FUNCTION OF GASTRIC JUICE
 Inactivates the action of ptyalin.
 Makes the medium acidic for the action of gastric enzymes.
 HCl kills micro - organisms (prey etc.) if ingested.

INTESTINAL GLANDS
 Intestinal glands in mammals is a collective name for crypts of Lieberkuhn (secretes
alkaline enzymatic juice) and Brunner's glands (secretes mucus).
 Intestinal glands secrete intestinal juice or succus entericus.
INTESTINAL JUICE
 Amount : 1.5 - 2.0 l/day
 Chemical nature : Alkaline
 pH : 7.6-8.3
 Control of secretion : Nervous and hormonal (Enterocrinin, Duocrinin etc.)
 Chemical composition : Water (99%), mucous, inorganic salts, enzymes (like,
enterokinase, intestinal lipase, maltase, sucrase etc.)

FUNCTION OF INTESTINAL JUICE
 Inhibits the action of gastric enzymes.
 Makes the medium alkaline for the action of enzymes.

PANCREAS
 It is a single, endodermal, flat, leaf-like yellowish, heterocrine (mixed) gland, present
between the ascending and descending limb of duodenum and opens into the duodenum
through the pancreatic duct.
 Pancreas has two different kind of tissues–exocrine and endocrine.
 Exocrine is the major part (about 99%) of the pancreas. The exocrine tissue of the pancreas
consists of rounded lobules (acini) that secretes an alkaline pancreatic juice. The juice is
carried by the main pancreatic duct, into the duodenum through the hepatopancreatic
ampulla (ampulla of vater). An accessory pancreatic duct, may sometimes lead directly into
the duodenum.
 Endocrine tissue is the minor part (1% only) and also called as islets of Langerhans scattered
in the exocrine part. It consist of four various type of cells – α(A) cells, β(B) cells, δ(D) cells
and F or PP cells. α-cells secretes glucagon hormone, β-cells secretes insulin hormone and δ
cells secretes somatostatin. The PP or F-cells secrete pancreatic polypeptide hormone to
control somatostatin. The secretion passes directly into the blood.
 Pancreatic secretion is stimulated by cholecystokinin and secretin both.
 Complete digestive juice is pancreatic juice as it contains amylolytic, lipolytic and proteolytic
enzymes.

 PANCREATIC JUICE
 Amount : 1-1.5 l/day
 pH : 7.1-8.2
 Control of secretion : Hormonal and normal mechanism. Secretin hormones stimulate the
production of more alkaline pancreatic juice but low in enzyme content. Pancreozymin or
Cholecystokinin stimulates the production of enzyme rich pancreatic juice.
 Chemical composition : Water (99%), enzymes and salts.

LIVER
 Liver is the largest digestive 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
(small left lobe and large right lobe).
 The hepatic lobules are the structural and functional units of the 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.
 Kupffer cells of liver sinusoids acts as phagocytes which eat up the dead cells &
bacteria by phagocytosis.
 The bile secreted by the hepatic cells passes through the hepatic ducts and is stored
and concentrated in a thin muscular sac called the gallbladder.
 The process of bile secretion is called choleresis.

Bile
 Amount : 800-1000 ml daily. On the average about 700 ml.
 Source : Secreted by hepatic cells
 Storage site : Gallbladder
 Colour : Greenish-blue
 pH : 7.6-8.6
 Chemical Composition : It contains 92% water, 6% bile salts (NaCl, sodium
bicarbonate and sodium glycolate & sodium taurocholate), 0.3% bile pigments
(bilirubin-yellow and biliverdin-green), 0.3 to 1.2% fatty acid, 0.3-0.9% cholesterol &
0.3% lecithin, but no enzymes.
 The duct of the gallbladder (cystic duct) along with the hepatic duct from the liver forms
the common bile duct.
 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.

DIGESTION OF FOOD
 Digestion is the process of breaking down of complex and insoluble inorganic substances
(carbohydrates, fats and proteins) into simpler and soluble substances like glucose, amino
acids and fatty acids so that they can easily be absorbed into the body.
 It can be done by mechanical or chemical means.
 Mechanical digestion comprises mastication or chewing, liquefaction of food by digestive
juices, swallowing and peristalsis.
 Chemical digestion includes the enzymatic action of food.
 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 which 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 (Na+, K+, Cl–, HCO3
–) 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, salivary amylase. Lysozyme present in saliva acts as an
antibacterial agent that prevents infections.
The mucosa of stomach has gastric glands. Gastric glands have three major types of cells
namely
 mucus neck cells which secrete mucus;
 peptic or chief cells which secrete the proenzyme pepsinogen; and
 parietal or oxyntic cells which secrete HCl and intrinsic factor (factor essential for
absorption of vitamin B12).

 The stomach stores food for 4-5 hours. The food mixes thoroughly with the acidic gastric
juice from 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 plays an important role in
lubrication and protection of the mucosal epithelium from 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 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,
procarboxy-peptidases, 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.
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 along with 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 along with the bicarbonates from the pancreas protects the intestinal
mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic
activities. Submucosal glands (Brunner's glands) also help in this.

No significant digestive activity occurs in the large intestine.
 The functions of 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 enters into the caecum of the
large intestine through ileo-caecal valve, which prevents the backflow of the faecal
matter. It is temporarily stored in the rectum until defecation.

ABSORPTION OF DIGESTED PRODUCTS
 Absorption is the process by which the nutrients are circulated throughout the body
by blood and lymph and supplied to all body cells according to their requirement.
 It is carried out by passive, active or facilitated transport mechanisms.
 Small amounts of monosaccharides like glucose, amino acids and some of 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
of the substances like fructose and some amino acids are absorbed with the help of
the carrier ions like Na+. This mechanism is called the facilitated transport.
 Transport of water depends upon the osmotic gradient. Various nutrients like amino
acids, monosaccharides like glucose, galactose, electrolytes (like Na+) are absorbed
into the blood by active transport which occurs against the concentration gradient and
hence requires energy.

Absorption of amino acids and protein :
 Under normal circumstances, dietary proteins are almost completely digested to their
constituent amino acids and that these end products of protein digestion are then
actively transported from the intestine into the portal blood. Surplus amino acids are
also withdrawn from portal blood by liver cells and deaminated into ammonia and keto
acids. The ammonia is converted to urea and released into blood for excretion by the
kidneys, while the keto acids are converted to glucose or pyruvic acid and utilized for
energy-production or for storage as glycogen and fat.
 The dietary fat is digested, by the action of the pancreatic lipase present in the intestine,
partially into glycerol and fatty acids and partially to split products such as
monoacylglycerols.
 Fatty acids and glycerol being insoluble, cannot be absorbed into the 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
chylomicrons which are transported into the lymph vessels (lacteals) in the villi.

 By the lacteals, the fat is carried to the cisterna chyli (meaning 'the receiver of the
chyle') and then by the thoracic (lymph) duct to the left brachiocephalic vein, where it
enters the blood. The lymph reaching the thoracic duct from the intestines contains an
excess of fat giving it a milky appearance. It is called chyle. In this way, fatty acids and
glycerol are eventually brought into the bloodstream and so, by a circuitous route, to
the liver. In the liver, they are reorganized and recombined to form human fat.
 Water-soluble vitamins like members of B complex (except B12) and vitamin C readily
diffuse across the walls of the intestine into the blood. The fat-soluble vitamins A, D, E
and K are dissolved in micelles, which enter the mucosal cells of the intestine, by simple
diffusion. The absorption of these fat-soluble vitamins is markedly decreased in the
absence of bile.
 Absorption of substances takes place in different parts of the alimentary canal, like
stomach, small intestine and large intestine.

 Almost no absorption takes place in the mouth and esophagus.
 Maximum absorption occurs in the small intestine.
 There are two general pathways for the transport of materials absorbed
by the intestine - the veins of the hepatic portal system (which lead
directly to the liver); and the lymphatic vessels of the intestinal area
(which eventually lead to the blood by way of the lymphatic system and
the thoracic duct).

 The absorbed substances finally reach the tissues which utilize them for their activities.
This process is called assimilation.
 After assimilation, the various nutrients and divided into 4 categories according to their
requirements -
 Energy producers : Carbohydrate, fats
 Body builders : Proteins
 Metabolic regulators : Vitamins and water minerals
 Hereditary substances : Nucleic acid.
 Calorific value of carbohydrate, fat and protein are 4.1 Kcal, 9.45 Kcal and 5.65 Kcal
respectively.
 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 (defecation) is a voluntary process and is carried out by a mass
peristaltic movement.

 ROUGHAGE
Roughage are the dietary fibre derived from plants which is indigestible compound that
the human body cannot absorb. Roughage has two main components - soluble fibre and
insoluble fibre. Soluble fibre dissolves in water and it gets fermented in the colon into
gases and physiologically active by products. Insoluble fibre does not dissolve in water and
is metabolically inert and provides bulking.
 FUNCTION
 It helps in preventing constipation by increasing the volume of stool in our body.
 It helps in retaining water in the body.
 It helps in getting rid of undigested food.
 Nutrients can be classified into macronutrients or micronutrients, both of which are
essential. Macronutrients are needed in large quantities and provide bulk energy for
the metabolic system to function.

VITAMINS
Vitamins are organic compounds essential in trace amounts to the health of animals.
Vitamins can be water soluble or fat soluble.
FAT SOLUBLE VITAMINS
These vitamins are stored in the liver in the form of fat droplets.

WATER SOLUBLE VITAMINS
Water soluble vitamins travel freely through the body, and excess amounts are usually
excreted by the kidneys. The body needs water soluble vitamins in frequent small doses.
Important water soluble vitamins are summarized below in the table.

NUTRITIONAL AND DIGESTIVE DISORDERS
 Jaundice : The liver is affected, skin and eyes turn yellow due to the deposit of bile
pigments.
 Vomiting is the ejection of stomach contents through the mouth. This reflex action is
controlled by the vomit centre located in the medulla. A feeling of nausea precedes
vomiting.
 The abnormal frequency of bowel movement and increased liquidity of the faecal
discharge is known as diarrhoea. It reduces the absorption of food.
 Constipation is a condition in which the faeces are retained within the rectum as bowel
movements occur irregularly.
 Indigestion is a condition in which 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.

 PEM is protein energy malnutrition. It is primarily due to inadequate intake of food,
particularly protein. It generally affects infants and children. Two very commonly
occurring disease due to protein malnutrition are Kwashiorkor and Marasmus.
 Kwashiorkor develops in children whose diets are deficient of protein. It is common in
infants between 1 to 3 years of age. Symptoms of Kwashiorkar are retarded growth of
body and brain, protruiding belly, oedema, bulging eyes and diarrhoea etc.
 Marasmus is due to deficiency of proteins and calories. It occurs in infants under 1 year
of age. Symptoms of marasmus are mental retardation, lean and weak body, dry, thin
and wrinkled skin etc.

Digestion and absorption

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