Physiology of digestion

Physiology of
For 1st year Nursing students
Dr. M.Ashraf
M.Ashraf

Introduction
The digestive system consists of:
The gastro-intestinal tract (GIT): This
includes the mouth, pharynx, oesophagus,
stomach, small intestine (duodenum, jejunum
& ileum), large intestine, rectum & anal canal.
Accessory glands (salivary glands, pancreas,
liver & biliary system).
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Digestion Absorption Excretion
Of
undigested
or
unabsorbed
substances
Transport of
digested
substances
into blood or
lymphatics
breakdown of
food into smaller
components
that can be
absorbed

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Digestion
1
This achieved through group of secretions (juices)
salivary gastric pancreatic bile intestinal

Salivary Secretion
 Saliva is secreted by major and minor salivary gland:
 Major salivary glands: are parotid (produce 20%),
sublingual (produce 5%) & submandibular salivary glands
(produce 70%).
 Minor salivary glands: buccal glands & lingual =Ebners
glands (share in 5%).
 The parotid gland secretes serous secretion, the submandibular
and the buccal glands secrete mucous secretion mainly while the
sublingual secretes mixed secretion.
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Composition:
 99.5%  Water.
 0.5%  Solids: organic & inorganic.
Organic: as α-amylase (ptyalin), lysozyme,
antibodies & mucin.
Inorganic: as NaCl & KCl, & buffers as
bicarbonate & phosphate.
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Volume: about 1.5 L / day

Functions of saliva
1. Facilitation of chewing and swallowing: By
lubricating and moistening the food and buccal
cavity.
2. Facilitation of Articulation: By Moistening of the
buccal cavity structures essential for speech.
3. Start Digestion of Carbohydrate:
Cooked starch
α 𝑎𝑚𝑦𝑙𝑎𝑠𝑒
𝐶𝑙 𝑖𝑜𝑛𝑠
> maltose, maltriose & dextrins
Then it is inactivated by the low pH of the stomach.
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4. Antibacterial & cleaning: Saliva helps to fight
dental caries By
– The flow of saliva.
– the presence of lysozyme & Thiocyanate
– The presence of Antibodies (IgA).
5. Regulation of water balance:
 Decrease body water content  decreased salivary
secretion  dryness of the mouth and pharynx 
thirst sensation.
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6. Buffering action:
 By presence of bicarbonate, phosphates and mucin.
 This prevents loss of Calcium from teeth by acids and
help to neutralize gastric HCl relieving the heartburn
in regurgitation.
7. Excretion of Some drugs as mercury, iodides, and
lead. In chronic lead poisoning a blue line develops
at the margins of the teeth and gums.
 Some of Urea excreted in saliva and converted into NH3 by
bacterial action. This explains bad taste in patients with
renal failure.
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8. Solvent:
 Saliva dissolves some foods substances to stimulate
the taste buds of the tongue that initiate taste
sensation.
9. Regulation of body temperature:
 In animals with no sweat glands as dogs. The
evaporation of saliva during panting increases heat
loss in hot weather.
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Proteins
Carbohydrates
Uncooked
starch
Cooked
starch
After action
of saliva
Maltose
Cooked
starch
Proteins

Gastric Secretion
 Gastric secretion is secreted by
gastric glands.
 Each gland contain various cell
types that share in gastric
secretion:
 Parietal cells (oxyntic cells).
 Chief cells (peptic cells).
 Mucous secreting cells.
 Enterochromaffin like cells.
 G cells and D cells.
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Gastric juice
 Composition:
 99%  water.
 0.5%  HCl.
 0.5%  solids: organic & inorganic.
 Organic: as pepsin, rennin, lysozyme, lipase, intrinsic factor & mucin.
 Inorganic: as Na+, Cl- & K+, PO4
3-, Ca2+ & HCO3
-.
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Clear colourless fluid
pH is 0.9 – 1.5
(the most acidic fluid in the body)
Volume: about 2-3 Litres / day

Functions of gastric HCl
A. It transforms the inactive pepsinogen into active
pepsin.
B. It provides the optimum pH for the action of
pepsin (PH 1.5-2.0).
C. Has an antibacterial function.
D. Help absorption of calcium (by preventing its
precipitation) and iron (by converting ferric iron
into ferrous).
E. Help milk clotting.
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Mechanism of secretion of HCl
Inhibited by carbonic
anhydrase inhibitors as
diamox
Inhibited by proton pump
inhibitors as omeperazole
These reactions can be
inhibited using 𝐻2 blockers
as cimitidine
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Digestive enzymes and other organic constituents
1- Pepsin
 Proteolytic enzyme (endopeptidase) secreted in an inactive
form (pepsinogen) then activated by HCl to pepsin.
 Its optimum PH is 1.5 - 2.
 It hydrolyzes the proteins into proteoses, peptones &
polypeptides.
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2- Rennin
 Milk clotting enzyme.
 Not present in human but found in gastric juice of young
animals.
 Its function in human is performed by pepsin and HCl.
 Its optimum PH is 6- 6.5.
Importance of milk clotting:
It prevents rapid evacuation of the milk from the stomach.
This allows sufficient time for gastric digestion & the
antibacterial action of HCl to take effect.
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3- Gastric lipase
 Weak lipolytic enzyme.
 Its optimum PH is 5.
 So, it is of less value in the stomach of adults.
 It hydrolyzes the fat (in infants as the gastric juice is less
acidic) into glycerol & fatty acids.
Explain: gastroenteritis is more common in children than
adults.
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4- Intrinsic factor
 glycoprotien secreted from parietal cells.
 Required for absorption of vitamin 𝑩 𝟏𝟐
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5- Lysozyme has antibacterial action
 Excellent lubricant.
 Chemical and mechanical barrier: it Protects the wall
of the stomach from HCl or mechanical irritation.
6- Mucin:

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A - The digestive enzymes are secreted
in an inactive form & activated only
in the lumen of the stomach.
B - The presence of thick layers of
mucous.
F - The presence of protective
prostaglandins which: Decrease HCl
secretion & Increase mucous secretion.
C - Secretion of bicarbonate by
mucosal cells.
D - The presence of tight junctions
between mucosal cells prevents the
penetration of HCl.
E - Rapid regeneration of mucosal
cells.
How the stomach is protected against self digestion?

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Vomiting
It is reflex expulsion of gastric content outwards through
the mouth and it often starts with sensation of nausea.
Mechanism of vomiting
Controlled by vomiting center in medulla oblongata.
The glottis remains closed, soft palate elevated to prevent vomitus
from entering airways. (the larynx is not elevated)
Excessive salivary secretion, sweating & rapid heart rate
accompanied by unpleasant sensation of nausea.1
Deep inspiration then Closure of the glottis with descend of the
diaphragm downwards, this elevates intrabdominal pressure.2
The abdominal muscles contract while the body of the stomach and
cardiac sphincter relax  marked elevation of intrabdominal
pressure  squeezing the relaxed stomach leading to ejection of its
contents into the eosophagus.
3
4

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Causes of Vomiting
Reflex vomiting
Mechanical irritation of the posterior
part of the tongue or throat
Central vomiting
Chemical irritation of the stomach or
mechanically by overdistention.
Intestinal obstruction
Intense pain as renal or biliary colic or
myocardial infarction
Rotation or acceleration of the head
such as occurs in motion sickness
Psychogenic vomiting
(conditioned reflex)
Acidosis
Head injuries
Increased intracranial tension
as in brain tumors
Anoxia of vomiting center as
in High altitudes
Drugs & chemical agents:
anaethetics, emetics, cancer
chemotherapy, tartar emetate,
apomorphine.

EFFECTS OF VOMITING
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1. Dehydration.
2. Alkalosis.
3. Loss of electrolytes especially 𝐾+
.

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Proteins
Carbohydrates
Uncooked
starch
Cooked
starch
After action
of saliva
Maltose
Cooked
starch
Proteins
After action
of stomach
Cooked
starch
Maltose
Peptons, proteases
& polypeptides

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Pancreas
Pancreas is a mixed gland
exocrine and endocrine
90% 2%
The remaining portion is
supporting tissue
concerned
with secretion
of pancreatic
juice
islets of Langerhans
Secrete pancreatic
hormones as insulin
& glucagon

Pancreatic secretion
Composition:
 98.5%  water.
 1.5%  solids: organic & inorganic.
0.5% Organic: as trypsin, chymotrypsin, carboxypeptidase, lipase
and amylase.
1% Inorganic: mainly HCO3
- responsible for its alkalinity.
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Clear colourless fluid
pH is 8.5
(the most alkaline fluid in the body)
Volume: about 1.5 Litres / day

Functions of pancreatic enzymes
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1. Proteolytic enzymes
Trypsinogen Chymotrypsinogen proCarboxypeptidase
secreted in an inactive form
(endopeptidases)
Activated by
enterokinase
Activated by trypsin
hydrolyze the proteins into proteoses,
peptones & polypeptides.
breaks polypeptides into
amino acids with a free
carboxyl group)
exopeptidase

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Functions of pancreatic enzymes
2. Pancreatic lipase
It is the most powerful lipolytic enzyme in GIT
l𝐢𝐩𝐢𝐝𝐬
𝑙𝑖𝑝𝑎𝑠𝑒
𝑏𝑖𝑙𝑒 𝑠𝑎𝑙𝑡𝑠
> glycerol & fatty acids
Absence of this enzyme or bile salts leads to passage of much
undigested fat in faeces (steatorrohea).
3. Pancreatic amylase
similar to salivary amylase but it is more powerful
hydrolyze both cooked & uncooked starch into
oligosaccharides and maltose

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Proteins
Carbohydrates
Uncooked
starch
Cooked
starch
After action
of saliva
Maltose
Cooked
starch
Proteins
After action
of stomach
Cooked
starch
Maltose
Peptons,
proteases &
polypeptides
After action
of pancreas
and bile
Maltose
Peptons,
proteases &
polypeptides
Amino acids

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Bile secretion
Bile is continuously formed by the liver
Normally, the sphincter of oddi surrounding the ampulla
of vatter is closed. So, bile is stored in gall bladder
which contract when the food reaches the duodenum 
increasing the pressure inside the biliary passages
leading to opening of the sphincter of oddi.
It passes through the intrahepatic biliary channels  to
hepatic ducts  unit with the cystic duct forming the
common bile duct which joins the pancreatic duct then
opens in the 2nd part of duodenum through the ampulla of
vatter.

Characters of bile
• Volume:
• 1.5 liters / day.
• pH:
• alkaline (8: 8.5) due to presence of NaHCO3 but this PH
falls to (6.8: 7.4).
• Specific gravity:
• liver bile = 1008
• gall bladder bile = 1050.
• Colour:
• Golden yellow: due to presence of bilirubin.
• Greenish: due to presence of biliverdin.
• White: if there is excess mucous secretion.
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• Composition of bile:
• 97%  water.
• Inorganic: mainly Na+, K+, Ca2+, Cl- &HCO3
–
responsible for its alkalinity.
• Organic: as bile salts, bile pigments, cholesterol,
mucous, Lecithin & Phospholipids.
• Bile contains NO digestive enzymes
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• Bile is both secretion & excretion:
• Secretion: of the bile salts which is important
for lipid digestion and absorption.
• Excretion: of bile pigments which are the end
products of Hb breakdown.

Functions of the gall bladder
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1- Storage of bile
Bile is continuously secreted by the liver cells and transported through
the biliary system to the gall bladder to be stored until needed.
2- Concentration of bile
The total secretion of bile is 1000 ml/ day.
The max. Volume of gall bladder is 70 ml.
During storage of bile, water, 𝐶𝑙−, 𝑁𝑎+, 𝐻𝐶𝑂3 are absorbed actively by
𝑁𝑎+- 𝐾+pump of the gall bladder mucosa while bile salts, bile pigments &
cholesterol are concentrated up to 10 times.
3- Acidification of liver bile:
The absorption of 𝑁𝑎𝐻𝐶𝑂3 by the gall bladder causes the pH of liver bile (8 -
8.6) to be less alkaline (7). This prevents precipitation of Calcium and calcium
stones formation.

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6- Evacuation of bile in duodenum:
When the food reach the duodenum by contraction of its wall and
relaxation of the sphincter of oddi.
4- Equalization of pressure
The gall bladder prevents the increase in biliary pressure by
concentration of bile keeping the pressure in the bile duct in normal
range. This allows the liver to secrete bile continuously as the liver can’t
secrete bile against high pressure.
In obstruction of biliary passages , marked increase in biliary pressure
leads to back pressure on the liver cells  atrophy of hepatic cells.
5- Secretion of mucous
The gall bladder secretes a large quantity of mucous helps to protect the
gall bladder mucosa & ducts from highly concentrated bile.

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Functions of bile salts
1- Digestion:
bile salts help fat digestion by:
• Activation of lipase.
• Emulsification of fat i.e.
break large fat globules into
fine particles exposing a
larger surface area for the
action of lipase.
• hydrotropic action: dissolve
the insoluble fatty acids in
water by micelle formation.
2- Absorption:
Bile salts are important for
absorption of:
•Long chain Fatty
acids: by their
hydrotropic action.
•Fat soluble vitamins
Iron and Calcium.

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3- Choleretic: Bile salts are the best stimulant of bile secretion
by the liver.
5- Stimulant of peristalsis:
Bile salts stimulate the movement of small and large intestine
and prevent constipation..
4- Solvent:
•The hydrotropic action of bile salts converts water insoluble
cholesterol to water soluble.
•This prevents precipitation in the gall bladder and formation of
gall stones.
•This is achieved when bile salts : cholesterol ratio is 13:1.

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6- anti-putrefactive:
•The effect is secondary to stimulation of fat
absorption.
• In the absence of bile salts, the unabsorbed fat forms a
layer around the protein particles that prevent their
digestion and absorption.
•The unabsorbed protein acts as a good medium for
growth of bacteria in the colon.

Bile pigments
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Metabolism of bile pigments:
Bilirubin: is a product of heme metabolism.
In Reticuloendothelial System:
•The body produces approximately 250–300 mg of heme daily, chiefly
from the breakdown of RBCs.
•heme is oxidized to biliverdin, then reduced to bilirubin (Indirect
bilirubin-unconjugated; water insoluble).
•Then it is transported to the liver in the bloodstream bound to
albumin.

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In Liver and Gallbladder:
•hepatocytes take up bilirubin by carriers. It is then conjugated
with glucuronic acid.
•Conjugated bilirubin (direct bilirubin), now in a water soluble
form, is excreted into bile.
In the Colon:
•gut bacteria hydrolyze conjugated bilirubin to form
urobilinogen (2/3 is excreted in the stool; 1/3 is passively
absorbed, and is recycled by the liver).
In the Kidneys:
Approximately 10% of the resorbed urobilinogen is filtered
and secreted by the kidney.

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Van den berg reaction
•Adding diazo reagent to blood sample gives violet colour.
•If the colour obtained immediately i.e. direct reaction  indicates the
presence of cholebilirubin.
•If the colour obtained after addition of methyl alcohol to the blood
sample then diazo reagent i.e. indirect reaction  indicates the presence
of haemobilirubin.
•If the violet colour obtained immediately after adding the reagent to the
blood sample then, a deep colour is developed after addition of methyl
alcohol  this is called biphasic reaction indicating the presence of both
haemobilirubin and cholebilirubin.
•Normally, the blood gives the indirect reaction only because the amount
of cholebilirubin isn’t sufficient to give the direct reaction.

Jaundice (icterus)
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Yellowish discoloration of the skin and mucous membranes
due to hyperbilirubinaemia.
Normal level
total bilirubin = 0.5 mg / dl.
Jaundice appears when bilirubin level reaches 1.5 mg / dl.
Usually seen in the sclera.
Types of jaundice:
•Haemolytic jaundice.
•Obstructive jaundice.
•Hepatocellullar jaundice.

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Physiological jaundice of newborn:
•Occurs in 50% of newly born infants.
•Appears in 2nd or 3rd day of life and disappears by the 7th or10th
day.
•It is due to transient deficiency of enzyme glucuronyl transferase
in the liver  failure of conjugation.

Small intestine
• Extends from the end of
pylorus to the ileo-caecal
valve.
• Length:
– 280 cm during life.
– 6 meters after death because
it loses its tone leading to its
relaxation and elongation.
• Divided into: duodenum (1st
25 cm or 12 inches), jejunum
& ileum.
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Intestinal secretion(succus entericus)
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Volume: 1.5 liters /day PH: 7.5Isotonic
Composition
Water Mucus
Inorganic
substances
digestive
enzymes
Functions of mucous:
 Protect the duodenal mucosa from the
acid chyme.
 Lubricant for intestinal epithelium.
Secreted by
Goblet cells &
Brunner’s glands
Erypsin
Disaccharidases
lipase

Digestive enzymes
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Erypsin
dipeptidase
Aminopolypeptidase
Carboxypeptidase
mixture of
enzymes that
hydrolyze
polypeptides
splits dipeptides into free amino acids
separate amino acids with free
amino group
separate amino acids with free
carboxyl group
Disaccharidases
Maltase
Sucrase
lactase
splits maltose into 2 of glucose
splits sucrose into glucose and
fructose
splits lactose into glucose and
galactose

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Digestive enzymes (cont.)
Intestinal lipase Its presence is doubtful
Enterokinase Activate trypsinogen into trypsin
Releasing factor
Help transfer of vit. B12 through
intestinal mucosa.
Other enzymes
Intestinal amylase, phosphlipase &
nucleases

Large intestine
The large intestine consists of: the caecum, vermiform
appendix, ascending colon, transverse colon,
descending colon, pelvic colon & rectum.
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Functionally it is divided into:
1. The proximal colon: from the caecum to middle of
transverse colon. It is for absorption.
2. The distal colon: from the middle of transverse colon
to the end of pelvic colon. It is for storage.
3. The rectum and anal canal: for defecation.

Functions of large intestine
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Absorption:
 Water and electrolytes and glucose when present can be
absorbed by the colon. The colon is the most efficient absorber
of water via actively transported sodium.
 This is important in rectal feeding as in unconsciousness or
severe vomiting.
Storage of faeces: Till the time of defecation.
Defecation.
Secretion of mucin Mainly for:
Protect the mucosa from mechanical injuries or
from acids produced by bacterial fermentation
Bind the fecal parts together.
Lubricant for defecation.

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Functions of large intestine (cont.)
Bacterial action
Synthesize vitamin K
Cellulose digestion
in animals by beta-amylase enzyme formed by bacteria
Synthesize Short-chain fatty acids (2–5-carbon)

Physiology of digestion

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