1) Digestion is the breakdown of food into smaller molecules that can be absorbed and used by the body. It involves both mechanical and chemical digestion. 2) The digestive system includes the mouth, esophagus, stomach, small intestine, large intestine and associated glands. It functions to ingest, secrete digestive juices, mix and propel food, digest nutrients, absorb them, and excrete waste. 3) Digestive juices include saliva, gastric juice, pancreatic juice, bile and intestinal juice. Each contains different enzymes and components that play specific roles in digestion and are secreted in response to food in the digestive tract.

1. DIGESTION AND
ABSORPTION OF
NUTRITIONAL
CONSTITUENTS
Dr Ifat Ara Begum
Assistant Professor
Dept Of Biochemistry
Dhaka Medical College
Dhaka

2. DAY 1
Introduction to
digestion& absorption,
Local hormones of GIT,
Different digestive
juices

3. DIGESTION
 Digestion is the process of breakdown
of large/complex food molecules into
simple form, so that they can be
absorbed into the watery blood plasma
for subsequent utilization.
 It is often divided into two processes
based on how food is broken down:
mechanical and chemical digestion.

4. CONTD
 The term mechanical digestion refers to
the physical breakdown of large pieces
of food into smaller pieces which can
subsequently be accessed by digestive
enzymes.
 In chemical digestion, enzymes break
down food into the small molecules the
body can use.

6. CONTD
 The basic process involved in digestion :
hydrolysis.
 Enzymes involved: Hydrolases group of
enzyme

7. DIGESTIVE SYSTEM
 Digestive system is a 25 to 30 feet long
tube which includes organs that secret
digestive juices.
 It consists of alimentary canal/
gastrointestinal tract/ digestive tract
and the associated digestive
glands/organs.
 The digestive tract system comprise
of esophagus, mouth, stomach, small
intestine and large intestine or colon,
rectum and anus.

10. FUNCTION OF DIGESTIVE
SYSTEM
 Ingestion
 Secretion of water, acid, buffer and
enzyme in to gastrointestinal
lumen
 Mixing and propulsion
 Digestion
 Absorption
 Excretion (Defecation)

11. ABSORPTION
 It is the process of transfer of
nutrients/ end product of digestion (as
well as the vitamins , minerals and
water ) from digestive system in to the
lymphatic / circulatory system for
eventual distribution to the body cells.
 Glucose and amino acids are absorbed
in to the circulatory system
 Fatty acids are absorbed in to the
lymphatic system

12. CONTD
 The nutrients in the lymph and blood
mix in the heart and are then sent to
the body cells via the circulatory
system
 Most absorption takes place in small
intestine
 Water, sodium and end product of
bacterial digestion are absorbed in the
large intestine

14. DIGESTIVE JUICES
 The digestive juices are the
secretions of the digestive tract that
break down food.
 They include saliva, gastric juice,
pancreatic juice, bile and intestinal
juice.
 The digestive juices are secreted by
different organs, vary widely in
chemical composition, and play
different roles in the digestive process.

15. CONTD
 Each juice is constantly produced by
the body in small amounts, but the
presence of food as it passes through
the digestive tract causes increased
production and secretion.

16. IMPORTANCE OF DIGESTIVE
JUICES
 Digestive juices are the most
important part of the whole digestive
process as in their absence the
digestion process cannot take place.
These are very vital for breaking down
of food so that the
essential nutrients are absorbed and
properly utilized by our body.

17. COMMON DIGESTIVE JUICES
Digestive
juice
Source Daily
secretion
(L/day)
pH Important
components
Saliva Salivary
glands
0.75-1.5 6.0-7.0 Mucus,
Salivary
amylase,
Lingual lipase
Gastric
juice
Gastric
glands
1.5 1.0-3.5 Mucus, HCl,
Pepsin,
Intrinsic
factor, Gastric
lipase

18. CONTD
Digestive
juice
Source Daily
secretio
n (L/day)
pH Important
components
Pancreat
-ic juice
Pancreas 0.8-1.2 8.0-8.3 Amylolytic enzyme (P.
amylase), Lipolytic
enzyme (P. lipase,
cholesterol esterase,
Phospholipase A2),
Colipase, HCO3 ion,
Nucleases, Proteolytic
enzyme (Trypsin,
Chymotrypsin,
Carboxypeptidase,
Elastase, Collagenase)

19. CONTD
Digestive
juice
Source Daily
secretion
(L/day)
pH Important
components
Bile Liver 0.5-1.0 7.8 Bile salts, Bile
pigments (e.g.
bilirubin),
Cholesterol ester,
PL (lecithin)
Succus
entericus
Small
intestinal
mucosa
1.0-2.0 7.5 Amylolytic enzyme
(maltase, sucrase,
lactase, α-limit
dextrinase),
Proteolytic enzyme
(aminopeptidase, di
and tri peptidases),
Enterokinase
(enteropeptidase)

20. CONTROL OF DIGESTIVE JUICE
SECRETION
 Nervous factor
 Hormonal factor (Gut hormones):
Liberated from GIT mucosa in
response to presence of food in gut
lumen. They signal the brain
(hypothalamus) to achieve
efficient nutrient digestion &
absorption (Gut-brain interaction)

21. LOCAL HORMONES OF GIT
Biologically active PPs that are
secreted by nerve and gland cells in the
mucosa , act in a paracrine fashion ,
also enters the circulation
2 family:
 Gastrin family: Includes gastrin & CCK
 Secretin family: Includes secretin, GIP,
VIP etc
Others: Neurotensin, Substance B,
Bombesin, GRP (gastrin releasing PP)
etc

23. CONTD
Local
hormone
Origin Stimulus
for
secretion
Target
tissue
Action
Gastrin G cell of
stomach
Products of
protein
digestion
Stomach  Stimulates
secretion of
gastric acid
(HCl) by the
parietal cells of
the stomach
 Aids in
gastric motility.
CCK I cells in
mucosal
epi of
small
intestine
and
secreted in
duodenum
Product of
protein
digestion,
fat and FA
Pancreas,
GB
 Stimulates
enzyme rich
pancreatic
secretion
 Release of bile
from GB

24. CONTD
Local
hormone
Origin Stimulus for
secretion
Target
tissue
Action
Secretin S cell of
duodenum
Acid chyme,
FA
Stomach,
Pancreas
Inhibits
secretion of
gastric acid
(HCl) by the
parietal cells of
the stomach
 Stimulate
bicarbonate
rich pancreatic
secretion
GIP K cells in
the mucosa
of
duodenum
& jejunum
Nutrients Stomach Decrease
stomach acid
secretion
 Induce insulin
secretion

25. CONTD
(GIP: Glucose-dependent
insulinotropic peptide)
Somatostatin: Secreted from D cells of
GIT . It is secreted when fat is
ingested and decreases gastrin and
gastric acid secretion
VIP: Secreted by N. endings in GIT
and increases pancreatic secretion &
decreases gastric secretion

26. SALIVA (SECRETION OF
SALIVARY GLAND)

27. CONTD

28. COMPOSITION OF SALIVA
 Mucus
 Salivary α amylase: Initiates starch
digestion
 Lingual lipase: Initiates fat
digestion
 Electrolyte solution (Na+, Cl-, K+,
HCO3-): moistens food
 Others: Lysozyme, Salivary
Peroxidase etc

29. FUNCTION OF SALIVA
 Digestive function: by salivary
amylase, Lingual lipase
 Mastication of food, bolus formation
& swallowing of food
 Moistens oral mucosa, helps in
speech
 Moistens dry food, cools hot food,
dilutes irritant substances
 A medium for dissolving foods to
stimulate the taste bud
 Control bacterial flora of oral cavity

30. CONTD
 Buffers oral cavity contents due to
high concentration of bicarbonate
ions
 Mineralization of new teeth &
repair of precarious enamel lesion
(as it is rich in calcium and
phosphate)
 Protects teeth by a protein coat on
teeth which contains antibacterial
compound. (Prevent dental caries)

31. MUCUS
Thick secretion containing water,
electrolytes and glycoproteins.
Adherence (to food& other particles to
get spread as thin film over the
surfaces)
Coating of gut wall and prevention of
actual contact of food with mucosa
Low resistance for slippage
Resistant to digestion by digestive
enzymes

32. CONTD
 Glycoproteins: buffer acid/alkali
 HCO3 ion: neutralizes acid
 Function:
 Lubrication of ingested food to favor
their slippage along GIT
 Protection of GIT from
mechanical/chemical injury)

33. GASTRIC JUICE
 Mixture of secretions of all the glands
present in stomach.
 It is mixed also with swallowed saliva
& refluxed duodenal contents.

34. CONTD
 3 types of glands:
I. Cardiac glands: Present around cardiac
orifice of stomach, have mucous cells
(secret mucus)
II.Gastric/oxyntic glands: Have mucous
neck cells (secret mucus), peptic/chief
cells (secret pepsinogen) & parietal cells
(secret HCl & IF)
III.Pyloric glands: Present in antral part
of stomach , have mucous cells (secret
mucus) & G cells (secret gastrin)

35. CONSTITUENTS OF GASTRIC
JUICE

36. FUNCTION OF CHIEF
COMPONENTS OF GASTRIC JUICE
1. HCl:
 Denaturation of protein to facilitate its
digestion
 Converts pepsinogen to pepsin for chemical
digestion
 Provides optimal pH environment for pepsin
 Sterilizes upper GIT by destroying bacteria
 promotes the absorption of Fe2+
in small
intestine

37. CONTD
2. Pepsinogen (precursor of pepsin):
 Protein digestion (pepsin with HCl digest
protein up to peptone)
3. Mucus:
 Forms a protective barrier: mucus-bicarbonate
barrier
 Lubrication
4. Intrinsic factor of castle:
 Combines with vitamin B12 to make it
absorbable (ileum)

38. PRODUCTION
1. CO2 and Cl-
diffuse from
the blood into the
stomach cell.
2. Within cell, CO2 combines
with H2O in presence of
CA enzyme, to form
H2CO3.
3. H2CO3 dissociates into
HCO3-
and H+
.
4. H+
is secreted to gastric
lumen actively by proton
pump
5. HCO3-
is absorbed to
blood in exchange of Cl-
which subsequently
diffuse to gastric lumen
& combines with H+
to
form HCl.

39. CONTD

40. CONTROL OF GASTRIC ACID
SECRETION
By three mechanisms:
 Neurocrine (denoting an endocrine
influence on or by the nerves).
 Endocrine (gastrin)
 Paracrine (histamine) in contrast to
true endocrines these hormones are
not released into the bloodstream but
into the surrounding tissues and act
in the immediate vicinity, e.g.
intestinal mucosal hormones.

41. MUCUS- BICARBONATE BARRIER
 A protective barrier composed of unstirred
layer of mucus plus bicarbonate ion over the
mucosal surface of stomach & duodenum.
 Protects gastro-duodenal mucosa from acid-
peptic digestion & prevents ulceration
 Disruption of the barrier &/or excess HCl
secretion causes dev of PUD

42. CONTROL OF GASTRIC
HYDROCHLORIC ACID
SECRETION
Parietal cell is stimulated by
histamine, gastrin, Ach
Parietal cell is inhibited by PGE2,
somatostatin, VIP, GIP, CCK,
secretin
Proton pump is inhibited by
proton pump inhibitors (like
omeprazole)

43. PHASES OF GASTRIC
SECRETION
Phase Action
Cephalic phase
(accounts for 20% of
gastric secretion)
The sight, taste, smell or
thought of foods triggers
parasympathetic reflex (via
vagus nerve), gastric juice is
secreted in response
Gastric phase
(accounts for 70% of
gastric secretion)
Food in stomach chemically and
mechanically stimulates release
of gastrin, which in turn,
stimulates the secretion of
gastric juice. Reflex responses
(vagovagal reflex, local enteric
reflex) also stimulate gastric
juice secretion

44. CONTD
Phase Action
Intestinal phase Starts once acid chyme from
stomach goes to upper SI (esp.
duodenum). Intestinal cell release
intestinal gastrin, which
promotes gastric juice secretion
from stomach.
On the other hand, duodenal
activity, intestinal distension,
presence of breakdown products
of fat & protein initiate release of
VIP, GIP, somatostatin, & CCK
from SI, all of which inhibit
gastric secretion

46. ALKALI TIDE
 A condition, normally encountered
after eating a meal, where during the
production of HCl by parietal cells in
the stomach, the parietal cells secrete
bicarbonate ions across their
basolateral membranes and into the
blood, causing a temporary increase
in pH
 This is to maintain the plasma's
electrical balance, as the chloride
anions have been extracted

48. COMPOSITION
1. Enzymes
2. Trypsin inhibitor

49. ZYMOGENS/PROENZYME
An inactive enzyme precursor
Requires a biochemical change (such
as a hydrolysis reaction revealing the
active site, or changing the
configuration to reveal the active site)
for it to become an active enzyme.
The biochemical change usually
occurs in a lysosome where a specific
part of the precursor enzyme is
cleaved in order to activate it.

50. CONTD
The pancreas secretes zymogens
partly to prevent the enzymes from
digesting proteins in the cells in
which they are synthesized.
Enzymes like pepsin are created in
the form of pepsinogen, an inactive
zymogen. Pepsinogen is activated
when chief cells release it into HCl
which partially activates it.

51. CONTD
Another partially activated
pepsinogen completes the activation
by removing the peptide turning the
pepsinogen into pepsin.

54. CONTD
Accidental activation of zymogens
can happen when the secretion duct
in the pancreas is blocked by a
gallstone resulting in
acute pancreatitis.

55. TYPE OF PANCREATIC
SECRETION
Hydroelactic secretion:
 Watery secretion
 Rich in HCO3-
 Poor in enzyme
Ecbolic secretion:
 Thick & viscid
 Rich in enzyme
 Poor in HCO3-

56. FUNCTION OF PANCREATIC
JUICE
1. Neutralization of acid chyme
emptied in to the duodenum from
stomach by the HCO3- content of
pancreatic juice to provide
optimum pH for activity of
pancreatic enzymes
2. Digestion of CHO, fat, NA &
cholesterol ester

57. BILE

58. COMPOSITION
Bile Salts:
 Na taurocholate
 K taurocholate
 Na glycocholate
 K glycocholate
Bile acid:
 Cholic acid
 Chenodeoxycholic acid
Bile pigment:
 Bilirubin
 Biliverdin

59. JUSTIFY BILE AS
DIGESTIVE JUICE
It activates pancreatic lipase
It helps in emulsification of fat
It forms micelle and helps in
absorption of fat

60. FUNCTION OF BILE
Bile salts of bile help in fat digestion
by emulsification of fat
Bile salts of bile help in the absorption
of the end products of digestion of fat
& other lipids
Bile serves as a mean for excretion of
some important waste products from
blood ( bile pigment/bilirubin,
cholesterol, drugs)