GI physiology KKK.pptx

PHYSIOLOGY
OF
DIGESTIVE SYSTEM
2
K.R K.R

Outlines
Introduction
Functional Structures of Di
gestive System
Histology of the Alimentary
Canal
Regulation of GIT
GIT reflexes
Blood Supply to Digestive
System
3
Functional types of mov
ements in the GIT
Ingestion of food
Secretory Functions of
GIT
Digestion of food
Absorption of food
Defecation
Jaundice.
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THE GASTROINTESTINAL TRACT
Introduction:-
• Functions of the Gastro-intestinal tract (GIT):
1.Breaks down complex food particles into simpler ones
Digestion.
2. Transports products of digestion to the blood stream
Absorption.
3. Expels the remaining waste to outside of the body
Excretion.
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Also known as the digestive tract, alimentary tract, or gut.
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Functional Structures of Digestive System
 Organs involved in the process of digestion are:
Teeth aids mechanical breakdown of food
Tongue assists chewing and swallowing
Other accessory organs produce and send secretions that fa
cilitate chemical breakdown of food
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Accessory structures: – teeth, tongue, gallbladder, salivar
y glands, liver and pancreas
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Microbial defense by the GIT
GIT. is a hollow at both ends(mouth-anus)there fore can pos
sibly harbor micro organisms in its luminal surfaces
1. Mouth: Saliva contains lysozymes and Immunoglobulins (IgA
) that attacks microbes in the mouth
2. Stomach: HCl secreted in the stomach has bactericidal actions
3. Small intestine (SI) : Immuno-competent cells (Payer's patch
es) attack microbes in the SI wall
4. Macrophages: in SI, developed from Monocytes destroy micr
obes by Phagocytosis
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Different organs of the GIT and its accessories
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1. Mouth
2. Pharynx
3. Esophagus
4. Stomach
5. Small intestine
6. Large intestin
e
7. Rectum and
(Anus)
1. Salivary glan
ds
2. Teeth
3. Pancreas
4. Liver
5. Gallbladder
GI-Organs Accessories
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The Digestive Process
 Ingestion
Taking in food through the mouth
 Motility (movement of food)
Mastication, Swallowing
Peristalsis – propulsion by alternate contraction &relaxation
Segmentation, mass movement
 Mechanical digestion
Chewing
Churning in stomach
Mixing by segmentation 9
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Con….
 Chemical digestion
By secreted enzymes:
 Absorption
Transport of digested end products into blood and ly
mph in wall of canal
 Defecation
 Elimination of indigestible substances from body as f
eces
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Histological layers of the GIT
1. The Serosa
2. A longitudinal muscle layer,
3. A circular muscle layer,
4. The Submucosa, and
5. The mucosa.
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Layers of GIT from outer surface i
nward (from the lumen to outwa
rd)
 Each layer has a specific d
igestive function
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A. Mucosa
 Moist epithelial layer that lines the lumen of the aliment
ary canal
Its three major functions are:
 Secretion of mucus
 Absorption of the end products of digestion
 Protection against infectious diseases
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Con….
B. Sub-mucosal layer
Consists of :-
Loose connective tissues
Secretory glands
Lymph nodes and blood vessels
 Enteric nerve plexus
This plexus controls secretions by the GIT.
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Con…
C. Muscularis externa
 The muscularis of the
 Mouth,
 pharynx, and consists of skeletal muscles that
 Upper esophagus produce voluntary swallowing.
 The skeletal muscle also forms the external anal sphincte
r, which permits voluntary control of defecation.
 Through out the rest of the tract, the muscularis consists o
f smooth muscles that is generally found in two sheets:
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Con….
 Involuntary contraction of both smooth muscles hel
p breakdown of food :-
 Physically,
 Mix it with digestive secretions and
 Propel it along the tract.
 The muscular is also contains the major nerve supply t
o the GIT;
 The myenteric plexus (plexus of Auer Bach), which co
nsists of fibers from both autonomic divisions.
 This plexus mostly controls GIT motility.
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1. Inner sheet of circular fibers and
2. Outer sheet of longitudinal fibers
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Con……
• The serosa is the outer most protective layer of the GI
T
• It is a serous membrane composed of:-
 Connective tissue
 Epithelium.
• Below the diaphragm, this layer is also called the visce
ral peritoneum.
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D. Serosa
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Regulation of GIT
• Regulation of digestion involves:
1. Mechanical and chemical stimuli:– stretch receptors,
osmolarity, and presence of substrate in the lumen
2. Extrinsic control system by ANS
3. Intrinsic control system by enteric NS
4. The GIT hormonal system
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 Sympathetic NS = ↓GI function
 Parasympathetic NS = ↑GI function
 Submucosal plexus (plexus of Meissner)
 Myenteric plexus (plexus of auer bach)
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Nervous Control of the GI Tract
 Intrinsic control system:-
Involves the enteric nerve plexuses present with in the GIT.
Nerve plexuses near the GI tract initiate short reflexes
 Sub mucosal plexus (plexus of Meissner):
controls GI secretory activities
 Myenteric plexus (plexus of auer bach):
controls motility of the gut
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Con…..
 Extrinsic controls:-
Involve CNS and extrinsic autonomic nerves
system
,
,
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Para sympat
hetic NS
↑GI function
↑Motility
↑Secretions
Sympathetic
NS
↓GI function,
↓Motility
↓Secretion
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GIT reflexes
1. Reflexes that occur entirely within the ENS these in
clude reflexes that control
 GI-secretion,
 Peristalsis,
 Mixing contractions,
 Local inhibitory effects.
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Con….
2. Reflexes that arise from the gut go to the sympatheti
c ganglia and then back to the GI-tract.
Examples:
a.The gastro-colic reflex: signals send from the stoma
ch to cause evacuation of the colon.
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Con…..
b. The entero-gastric reflexes: signals from the colon and
small intestine To inhibit stomach motility and
secretion.
c. The colono-ileal reflex: reflexes from the colon T
o inhibit emptying of ileal contents into the colo
n.
3. Reflexes from the gut to the spinal cord or brain ste
m and then back to GIT.
Example: Defecation reflex
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Blood Supply to Digestive System
• The blood vessels of the GI-system are part of a more exte
nsive system called the splanchnic circulation.
• Includes blood flow through the:-
GIT
Spleen,
Pancreas and
Liver.
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Con….
• All of the blood that flows through the gut, spleen and pa
ncreas then passes into the liver by way of the portal vein
.
• In the liver, blood passes through millions of liver sinusoi
ds and finally leaves the liver by way of the hepatic veins
that empty into the inferior vena cava of the general circula
tion.
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The advantage of sinusoidal passage of blood is that:
The superior mesenteric and inferior mesenteric arteries:- su
pply blood to the wall of small and large intestine.
The celiac artery:- Supplies blood to the stomach.
Blood flow to the GIT is increased by 100% during meal tim
e.
What causes sleepiness and tiredness following a meal ??
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The reticulo-endothelial cells in the liver remove bact
eria and other particles → entering the blood from the
GIT & preventing pathogens.
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Functional types of movements in the GIT
Two basic types of movements occur in the GIT:
1. Propulsive movements: which cause food to mo
ve forward along the tract at an appropriate rate f
or digestion and absorption.
2. Mixing movements: which keep the GI contents
thoroughly mixed at all times.
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Con….
A. Propulsive Movements (Peristalsis):
 Peristalsis is the basic propulsive movemen
ts of the GIT that appears in the form of con
tractile rings around the gut and propels to t
he anal ward direction.
• It is an inherent property of the smooth mus
cles in the GIT that generate action potenti
al rhythmically (basic electrical rhythm, (B
ER).
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B. Mixing Movements
 Segmentation
Most areas of the small intestine.
These movements churn and fragment the digestive materi
als, mixing the contents with intestinal secretions.

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Con…
Haustration
 At the same time, the longitudinal muscle of the colo
n, which is aggregated into three longitudinal strips c
alled the teniae coli, contracts.
These combined contractions of the circular and long
itudinal strips of muscle cause the unstimulated porti
on of the large intestine to bulge outward into baglike
sacs called haustrations.
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THE MOUTH
• Also known as the buccal
cavity or the oral cavity.
Functions of the mouth:-
1- Orifice for food and water
intake.
2- Mastication "or chewing”.
3- Mixes food with saliva.
4- Initiates swallowing.
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Digestive processes in the mouth
• After food is ingested
 Mechanical digestion begins → chewing/mastication
 Propulsion is initiated → by swallowing
• Salivary amylase begins chemical breakdown of starch
• The pharynx and esophagus serve as conduits to pass fo
od from the mouth to the stomach
Mastication (Chewing)
• It is a process of mechanical breakdown of food.
• Salivary secretion containing amylase involves chemical d
igestion and lubrication of the food.
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Con…..
• Teeth, tongue, jaws and lips are involved in chewing.
• Teeth are well adapted for this function as: incisors for cu
tting, canine for tearing, molars and premolars for grindin
g.
• Mastication muscles are supplied mainly by the motor bra
nch of the trigeminal nerve (CN V) .
• Chewing center is located in the pons
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Pharynx
• From the mouth, the oropharynx and
laryngo pharynx allow passage of:
Food and fluids to the esophagus
Air to the trachea
• Lined with stratified squamous epithe
lium and mucus glands
• Has two skeletal muscle layers:
Inner longitudinal
Outer pharyngeal constrictors
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Esophageal Characteristics
• Muscular tube going from the laryngopharynx to the stomach
• Joins the stomach at the cardiac orifice
• Esophageal mucosa: non-keratinized stratified squamous
epithelium
• The empty esophagus is folded longitudinally and flatte
ns when food is present
• Glands secrete mucus as a bolus moves through the esop
hagus
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Con….
• Muscularis changes from skeletal (superior third) to smoot
h muscle (inferior third)
• The pharynx and esophagus serve as conduits to pass foo
d from the mouth to the stomach
Deglutition (Swallowing)
• It is the propulsion of food from mouth to the esophagus i.
e. controlled by the swallowing center in the medulla.
• Involves the coordinated activity of the tongue, soft palat
e, pharynx, esophagus and muscle groups.
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Con….
Has 3 stages
1.Voluntary stage of swallowing:
• Buccal/oral phase → bolus is forced into the orophar
ynx voluntarily.
2.Pharyngeal stage of swallowing:
• It is the involuntary process and contributes the pas
sage of food through the pharynx to the esophagus.
• Controlled by the medulla and lower pons.
3.The esophageal stage of swallowing:-
• Involuntary phase, promotes the passage of food to t
he stomach.
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Lower esophageal (Gastro-esophageal) sphincter
• It is a thickened circular smooth muscle at the junction b/
n the esophagus and the stomach.
• Function: prevents the reflux of gastric contents into the
esophagus.
• Gastro-esophageal reflux:
It is the entry of gastric contents into the lower part of the
esophagus due to incompetence of the LES → that leads t
o ulcer of the mucosa of lower esophagus.
• Achalasia:
Failure of LES to be relaxed, swallowing is inhibited.
Caused by increased in tone of LES due to high sensitivi
ty to gastrin, weak esophageal peristalsis
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Functional structure of the stomach
• Chemical breakdown of proteins begins and food is convert
ed to chyme.
Cardiac region: surrounds the cardiac orifice
Fundus: dome-shaped region beneath the diaphragm
Body: midportion of the stomach
Pyloric region: made up of the antrum and canal which t
erminates at the pylorus
The pylorus is continuous with the duodenum through
the pyloric sphincter
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Function of the stomach
Storage of large quantities of food until it can be pumped i
nto the duodenum.
Stomach can accommodate large amount of food up to
1.5 Liters.
Mixing of food with gastric secretion to form a semi-fluid
chyme.
Slow emptying the food from the stomach into the small i
ntestine at a rate suitable for proper digestion and absorpti
on by the small intestine.
Secretory function: HCl, mucous, pepsin, gastrin, IF
Sterilization, digestion, absorption
Facilitates defecation
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Movements in the stomach
1. Propulsive movement
2. Mixing movement
3. Receptive relaxation: relaxation of stomach muscles as
food moves through esophagus and enters stomach.
4. Hunger contraction: Strong contractions of the stomac
h associated with hunger pains.
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Types of Motility in stomach
1.Receptive Relaxation of theStomach.(Vasovagal Reflex )
 Ahead relaxation of the Stomach to receive food from t
he esophagus.
 The neurotransmitter is VIP(Vasoactive intestinal pol
ypeptide)
 Effects of vagotomy
 Sectioning the vagus nerve will prevent or diminish recepti
ve relaxation
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2.Peristalsis
Initiated near the fundal-corpus border and proceed c
audally, producing a peristaltic wave that propels the f
ood towards the pylorus.
 Its strength increases as it goes downward
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3.Retropulsion
Is the back and forth mo
vement of the chyme aga
inst closed pyloric sphin
cter.
The forward and backwa
rd movement of the chy
me breaks the chyme int
o smaller pieces and mix
es it with the gastric sec
retions.
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Chyme
After food in the stomach
has become thoroughly m
ixed with the stomach se
cretions, the resulting mi
xture that passes down th
e gut is called chyme.
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Hunger Contractions
 Occurs when the stomach has been empty for several hou
rs or more.
 They are rhythmical peristaltic contractions of the stomac
h.
 Sometimes cause mild pain, called hunger pangs.
Gastric emptying
 Occurs when the chyme is small enough to pass through t
he pyloric sphincter.
 Depends on the size of the particles.
 Liquids empty much faster than solids.
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Regulation of gastric emptying
A. Local reflexes
1. Excitatory reflexes
 Expansion of the antrum and
 Digestive products of food, increase gastric motilit
y.
2. Inhibitory reflexes.
 Enterogastric reflexes
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B. Hormones
 Gastrin
- promotes stomach emptying
 Cholecystokinin (CCK)
 Secretin Inhibit gastric emptying
 GIP
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Glands distribution
.
• Body of stomach secretes:
 Parietal cells (HCl, IF)
 Chief cells (pepsinogen)
• Antrum
 G-cells (gastrin)
 Chief cells (pepsinogen)
• Mucus producing cells: all parts
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Microscopic Anatomy of the Stomach
• Muscularis:
Allows the stomach to churn, mix, and pump food physi
cally
Breaks down food into smaller fragments
• Epithelial lining:
Goblet cells that produce a coat of alkaline mucus
• The mucous surface layer traps a bicarbonate-rich flu
id beneath it.
• Gastric pits:
Contain gastric glands that secrete gastric juice, mucus, a
nd gastrin
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Stomach Lining
• The stomach is exposed to the harshest conditions in the digestive tr
act so that why it is not digesting itself ?
• The stomach has a mucosal barrier with:
A thick coat of bicarbonate-rich mucus on the stomach
wall
Epithelial cells that are joined by tight junctions
Gastric glands that have cells impermeable to HCl
Damaged epithelial cells are quickly replaced
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Gross Anatomy of Small Intestine
• Runs from pyloric sphincter to the ileocecal valve
• 20 feet long &1 inch in diameter
• Large surface area for majority of absorption
Has three subdivisions:
Duodenum: the bile duct and main pancreatic duct join th
e duodenum
Jejunum: extends from the duodenum to the ileum
Ileum: joins the large intestine at the ileocecal valve
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I. Duodenum
25 cm long , proximal region
It is the first and shortest part of the small intestine.
 The duodenum is also where the bile and pancreatic juices enter th
e intestine.
 Function Mainly secretary, mucus, hormones, enzymes
ii. Jejunum
~1.5m long ,middle region. It is different from the ileum due to fewe
r goblet cells and generally lacks Preyer's patches.
 Function Mainly absorptive
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Con….
iii. Ileum :-
~1.7m , distal small intestine ,joins the large intestine at t
he Ileo-cecal sphincter .
 The ileum is the longest part of the small intestine.
It is thicker, more vascular, and has more developed
mucosal folds than the jejunum.
 Function Mainly absorptive
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Movement in the small intestine
 Two types of movements occur in the SI:
A.Mixing movements (segmentation contractions)
 Mix intestinal contents at all times.
 Contractions cause "segmentation" of the small intestine
.
 Mix the chyme and expose it to pancreatic enzymes and
secretions of small intestine.
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• Mixing movement and
• Propulsive movement
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Con…..
B. Propulsive movements (Peristalsis)
Are peristaltic waves that propels the chyme forward t
hrough the intestinal tract.
Initiated by intestinal distension
Chyme is propelled in the SI until it reaches the termin
al ileocecal sphincter
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 Peristaltic rush = diarrhea
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Ileocecal sphincter
Function: prevents back flow of fecal matter from the
cecum to the ileum
Factors regulating the sphincter
Pressure and chemical irritation of ileum relax it
and initiates peristalsis
Pressure and chemical irritation of cecum inhibit
peristalsis of ileum and closes the sphincter
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Large Intestine (colon)
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 Overview
The large intestine is the portion of the digestive system
responsible for absorption of water from the
indigestible residue of food.
The ileocecal valve of the ileum (small intestine) passes
material into the large intestine at the cecum.
 Material passes through the ascending, transverse,
descending and sigmoid portions of the colon, and finally
into the rectum.
 From the rectum, the waste is expelled from the body.
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a) Caecum(6cm)-important for fermentation
 The caecum is a sac-like structure at the beginning of the
large intestine.
 Is attached to the appendix
b)Colon(1.3 m long )
 The colon is a long tube that functions as the main area
for water and electrolyte absorption
c)Rectum(20 cm)-for storage
The final region of the large intestine
The terminal 2-3cm of the rectum is called the anal canal
The opening of the anal canal to the exterior , is called th
e anus
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Movement in the large intestine
• Two types of movements
Mixing movements
Propulsive movements (mass movements)
• Mass movement is initiated by local distension gastro-co
lic reflex
• Poor motility of the transverse colon causes greater abso
rption and constipation
• Excess motility of the sigmoid colon causes less absorp
tion and diarrhea or loose stool
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Defecation
 Fecal material entering the rectum is evacuated by defe
cation.
 When a mass movement forces feces into the rectum, th
e desire for defecation occurs.
Defecation Reflexes
Involves two reflexes
• Intrinsic reflex
• Parasympathetic defecation reflex occurs. 76
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77
 Defecation signals enteri
ng the spinal cord initiat
e other effects, such as
 Taking a deep breath
Closure of the glottis
Contraction of the ab
dominal wall muscles
and
Relaxation of pelvic f
loor to relax downwar
d
 Afferent and efferent pathways o
f the parasympathetic mechanis
m
for enhancing the defecation refl
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SECRETORY FUNCTIONS OF GIT
• Primary secretory products of GIT are:
 Digestive enzymes
 GI-hormones
 Mucus
 Electrolytes, HCl, NaHCO3
• GIT secretory glands
1. Goblet cells: mucous producing glands
2. Brunner’s gland: mucous glands
3. Crypts of Lieberkuhn: water and electrolytes
4. Gastric glands: Oxyntic, pyloric and mucous glands
5. Complex glands: salivary, pancreatic glands and liver
6. Entero endocrine cells: produce hormones
• Secretory volume: 6~8 L/ day
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Fig. Volume of fluid entering & leaving the GI tract
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Factors stimulating GIT secretions
1. Local mechanical factors: distension, irritation, pH
2. Nervous stimulation: ANS, ENS
Sympathetic stimulation: inhibits GIT-secretions
Parasympathetic stimulation: increases GIT-secretions
Meissner’s plexus: increases GIT-secretion
3. Hormonal mechanisms:
Gastrin: increases HCl secretion
Secretin: increases NaHCO3 secretion from pancreas
Mucus
Function Composition
• Lubrication - Water
• Protection - Electrolytes
- Glycoproteins
- Polysaccharides 82
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• Produce and secrete saliva
Saliva:-
• is a fluid that is continuously secreted into the mouth for m
oistening, lubrication, dissolving and chemical breaking do
wn of food.
• Saliva contains two major types of protein secretion:
(1)A serous secretion : contains ptyalin (an α-amylase) (pH:
6-7) for starch digestion
(2)Mucus secretion: contains mucin for lubrication an
d surface protection
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Salivary Glands
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1. The parotid gland: The largest gland. Secretes 20-25% of saliva
2. The submandibular gland: The main gland. Secretes 70% of sali
va
3. The sublingual gland: The smallest gland. Secretes 5% of saliva
4. Minute glands in the mouth and pharynx: Secrete 5% of saliva
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Extrinsic Salivary Glands
Three pairs of glands
1. Parotid ..25%
• The largest glands
• Secrete mainly serous watery fluid rich in ptyalin.
2. Submandibular …70%
• Produce both serous and mucous fluid.
3. Sublingual…. 5%
• Secrete mainly thick mucous with little serous fluid
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Functions of Saliva
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• Daily secretion of saliva 1000 - 1500 ml/day
• Secretion is controlled by:
Nervous parasympathetic stimulation  salivary out
put
Chemical stimulation of taste buds
Mechanical stimulation
Psychic stimulation → smell, sight, hearing about food
Phases of Salivary secretions
•Three phases of salivary secretions include
1.Cephalic (brain) phase: triggered by thought, smell, or sight of food
2. Oral phase: triggered by food that stimulate touch & test receptors i
n the mouth
3. Gastric phase: triggered by substances which stimulate the gastric
mucosa (acids or sour tastes) in the stomach.
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Constituents of saliva
A. H2O (99.5%):
B. Electrolytes ( 0.5%): Na+, Cl-, K+, HCO3-, Mg, Iodine, etc.
Initially…isotonic Finally…hypotonic b/c Na+ &Cl- ar
e reabsorbed.
C. Other organic substances include:
 Enzymes: Amylase, lingual lipases
 Lysozymes , IgA
 Glycoprotein (albumin, globulin)
 Mucus
 Total secretion = about 1-1.5 L/day
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Regulation of saliva production
 Controlled by parasympathetic and sympathetic nervous
systems.
 Saliva production is unique in that it is increased by both
parasympathetic(cause more cupous;serous secretion) an
d sympathetic activity (more mucus sec.).
 Parasympathetic activity is more important, however.
Saliva production
 Is increased by food in the mouth, smells, conditioned ref
lexes, and nausea.
 Is decreased by sleep, dehydration, fear, and anticholiner
gic drugs
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• The stomach mucosa has two important types of tubular glands:
i. Oxyntic glands/gastric glands/acid-forming glands:
 secrete HCl, pepsinogen, IF and mucus.
 located on the inside surfaces of the body and fundus of the st
omach
 Constituting the proximal 80% of the stomach.
ii. Pyloric glands:
 secrete mucus for protection of the pyloric mucosa from the s
tomach acid.
 also secrete the hormone gastrin.
 located in the antral portion of the stomach, the distal 20% of
the stomach
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Gastric Secretion
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Areas of the Stomach, Cell types in each, and their Secretions
STOMACH REGIONS CELL TYPE SECRETIONS
Oxyntic gland area Parietal (oxyntic) cells Acid, instrinsic factor
Chief (peptic) cells Pepsinogen, gastric lipase
Pyloric gland area G cells Gastrin
(antrum) Mucus cells Mucus, Pepsinogen
D cells Somatostatin
This table lists the stomach areas, the cell types present in each area, and the secreti
ons of the cell types.
92
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Components of gastric juice
• Secretory volume: 1~2.5 L/d
• Character:
Achromic
Acidity（pH 0.9-1.5）
• Component:
Water
Inorganic salt: HCl, HCO3
-, Na+, K+, etc.)
Organics: pepsinogen, muco-protein & intrinsic factor
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• Pepsinogen:
• Secreted by the peptic and mucous cells of the gastric glands
• It comes in contact with hydrochloric acid, it is activated to form
active pepsin.
• Pepsin:
• An active proteolytic enzyme in a highly acid medium (optimu
m pH 1.8 to 3.5)
• but above a pH of about 5 it has almost no proteolytic activity
• Intrinsic factor:
• Essential for absorption of vitamin B12 in the ileum,
• Pernicious anemia developed because of failure of maturation of
the RBCs in the absence of vitamin B12 stimulation of the bone
marrow.
• Gastrin: plays a key role in controlling gastric secretion
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Pepsinogen Pepsin
protein
peptone
pH 2-3.5
HCl
Function of pepsinogen
95
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Regulation of Gastric Secretion
• Neural, hormonal and mechanical mechanisms regulate the release
of gastric juice
Stimulatory and inhibitory events occur in three phases
1. Cephalic (reflex) phase: prior to food entry
2. Gastric phase: once food enters the stomach
3. Intestinal phase: as partially digested food enters the duodenum
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Function of hydrochloric acid
① Activating pepsinogen
②Supplying acid environment for pepsin decomposing prot
ein
③ Food protein denaturation and easy decomposition
④ Kill bacteria in food into the stomach；
⑤Promoting pancreatic, small intestinal and bile secretion
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Mechanism of HCl Secretion
HCL is synthesized from hydrogen ions (H+) and chloride ions (Cl-).
- Sources of hydrogen ions (H+):
1- From dissociation of H2O into H+ & OH-: (H2 O = H+ + OH-)
2- From the reaction of CO2 with water at the presence of carbonic an
hydrase enzyme: (CO2 + H2O = H2CO3 = H+ + HCO3-).
- H+ is secreted into the canaliculi against a very high concentration
gradient by the H+-K+ ATPase pump (The proton pump).
- HCO3- enters the blood in exchange to Cl-, which enters the parietal
cell.
- Cl- enters the canaliculi with K+.
- K+ returns back to inside the cell by the H+-K+ ATPase pump (i.e.
K+ recycles between the cytoplasm and the canaliculi).
- In the canaliculi, HCL is formed as follows: (H+ + Cl- = HCL).
- HCL escapes from the canaliculi to the lumen (through the gastric pi
ts).
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Regulation of mucus secretion
• Soluble and insoluble mucus are secreted by cells of the
stomach.
• Soluble mucus mixes with the contents of the stomach a
nd helps to lubricate chyme.
• Insoluble mucus forms a protective barrier against the hi
gh acidity of the stomach content.
• Secretion of both forms of mucus increase in association
with a meal.
• Vagal nerve stimulation increases the secretion of solubl
e mucus while chemicals and physical irritation stimulat
es the secretion of insoluble mucus.
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Mucosa of the SI secretes:
• Digestive enzymes
• Mucous: protective and lubricant
• Electrolytes Intestinal secretory out put = 2-3 L/d, pH=7.0
• Hormones
Intestinal secretory glands:
1. Brunner’s gland: mucous glands, duodenal in distribution
2. Crypts of Lieberkun: mucous and electrolytes. Distributed in the
SI below the duodenum and in the LI.
3. Goblet cells: mucous glands
4. Enterocytes: digestive enzymes
5. Enteroendocrine cells: produce hormones
6. Enterochromaffin cells: serotonin producing cells 101
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Secretion of the Small Intestine
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Digestive enzymes secreted in the SI
1. Peptidase: splits peptides into amino acids
2. Four enzymes hydrolysing disaccharides into monosaccharides
: sucrase, maltase, isomaltase and lactase
3. Intestinal lipase: splits neutral fats into glycerol and fatty acids.
Regulation of SI secretion
1. Local factors: tactile, distension, irritation, pH.
2. Hormonal: secretin, CCK, VIP, glucagon, GIP
3. Nervous:
 Vagal stimulation increases intestinal secretion
 Sympathetic stimulation decreases intestinal secretion
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Enteroendocrine cells
• G-cells = secrete gastrin
• S-cells = secrete secretin
• I-cells = produce CCK
• EG-cells = entero glucagon and Glucagon-like Pept
ide
• Gland-cells = GIP and VIP
• D-cells = Somatostatin
• Other cells = motilin, substance-P
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Secretion of the large intestine
Glands
• Crypts of Lieberkuhn Secrete H2O, electrolytes
• Goblet cells and mucous
• Regulated by local (tactile) factors
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105
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Duodenum and Related Organs
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THE PANCREAS
• Pancreas Location:
 Lies deep to the greater curvature of the stomach
 Divided into: Head, body and tail
 The head is encircled by the duodenum and the tail abuts the sp
leen
 Connected to the duodenum via the pancreatic duct (duct of Wi
rsung) and accessory duct (duct of Santorini).
• Pancreas contains two types of secretory glands:
1.Endocrine cells (islets of Langerhans): secrete hormones and
2.Exocrine cells (acinar and duct cells): secrete a mixture of fluid rich
in NaHCO3 and digestive enzymes called pancreatic juice.
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•Acinar cells = Secrete digestive enzymes
Digestive enzymes of pancreas:
Trypsin( digest protein into peptides
Pancreatic amylase(starch digestion) and
pancreatic lipase(for lipid digestion)
•Duct cells
Secrete sodium bicarbonate => basic solution that neutr
alizes pH of duodenum
Two types of exocrine pancreatic cells:
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Regulation of Pancreatic Secretions
 Secretin
 Acidity in intestine
causes increased so
dium bicarbonate re
lease
 GIP
 Fatty acids & sugar
causes increased ins
ulin release
 CCK
 fats and proteins ca
use increased dige
stive enzyme rele
ase
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LIVER and GALL BLADDER
 The liver is the heaviest gland in the body and the second
largest organ in the body after the skin.
 The liver is divisible into left and right lobes, separated b
y the falciform ligament.
 Associated with the right lobe are the caudate and quadrat
e lobes.
 The gallbladder is a sac located in a depression on the pos
terior surface of the liver.
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• Weighs 3 lbs.
• Located below diaphrag
m
• Right lobe is larger
• Gall bladder on right lob
e
• Causes the right kidney t
o be lower than the left
• Gallbladder has
fundus, body, neck
111
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Liver
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Functions of the Liver
1. On Carbohydrate Metabolism:
• It is the site of glycogenesis, gluconeogenesis and gly
cogenolysis
• Turns proteins into glucose
• Turns triglycerides into glucose
• Turns excess glucose into glycogen & stores
• Turns glycogen back into glucose as needed
2. On Lipid Metabolism
• It is the site of β-oxidation, formation of phospholipi
ds, lipoproteins, synthesis of cholesterol, and convers
ion of CHO into fat
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3. Protein Metabolism
• Deamination of amino acids = removes NH2 (amine grou
p) from amino acids so can use what is left as energy sou
rce
• Converts resulting toxic ammonia (NH3) into urea for ex
cretion by the kidney
• Synthesizes plasma proteins utilized in the clotting mech
anism and immune system
• Converts one amino acid to another
114
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Other Functions of the liver
4. Inactivation of drugs & hormones (Sulfonamide, penic
illin, thyroid, steroids)
5. Removes the waste product; bilirubin
6. Releases bile salts
7. Stores fat soluble vitamins: A, B12, D, E, K
8. Stores iron and copper
9. Filtration of blood: phagocytizes worn out blood cells
& bacteria. Removes blood clots and toxins from port
al circulation
10. Activates vitamin D
11. Storage of blood: a major blood reservoir
12. Synthesis of blood clotting factors (F-I, II, VII, IX, X)
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Secretion of Bile
Bile is secreted by hepatocytes in the liver for two purposes
1. It facilitates fat digestion and absorption of fat
2. Serves as a means of excretion of waste products (bilir
ubin and cholesterol)
Bile secretion has two stages:
1. Primary secretion: contains bile salt, cholesterol, lecithi
n, electrolytes (Na, Ca, K ions)
2. Secondary secretions: contains primary secretions plus
water, NaHCO3, Cl-.
• Average biliary out put: 600 -1200 ml/day
• yellow-green in colour b/c of bilirubin
pH = 7.6 - 8.6
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Function of Bile Salts in Fat Digestion and Absorption
• The liver cells synthesize about 6 grams of bile salts daily.
• The precursor of the bile salts is cholesterol, which is eithe
r present in the diet or synthesized in the liver cells during
the course of fat metabolism
• The cholesterol is first converted to cholic acid.
• These acids in turn combine principally with glycine and t
o a lesser extent with taurine to form glyco- and tauro-conj
ugated bile acids.
• The salts of these acids, mainly sodium salts, are then secr
eted in the bile.
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• The bile salts have two important actions in the intestinal
tract:
1. They have a detergent action on the fat particles in the foo
d.
This decreases the surface tension of the particles and
allows agitation in the intestinal tract to break the fat gl
obules into minute sizes.
This is called the emulsifying or detergent function of
bile salts.
2. Bile salts help in the absorption of: fatty acids, monoglyce
rides, cholesterol and other lipids from the intestinal tract.
119
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Enterohepatic Circulation of Bile Salts
• About 94 % of the bile salts are reabsorbed into the blood fro
m the small intestine
• about one half of this by diffusion through the mucosa in t
he early portions of the small intestine and
• the remainder by an active transport process through the i
ntestinal mucosa in the distal ileum.
• The small quantities of bile salts lost into the feces are replac
ed by new amounts formed continually by the liver cells.
• This recirculation of the bile salts is called the enterohepatic c
irculation of bile salts.
• The daily rate of liver bile salt secretion is actively controlled
by the availability/lack of availability of bile salts in the enter
o-hepatic circulation.
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Regulation of Bile Secretion
121
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Gallbladder
1. Functions
 Stores (20-50 ml of bile ) and concentrates bile
2. Control
a.CCK
 Major stimulus.
b. Vagal stimulation
3. Effects of Cholecystectomy
 Bile empties slowly but continuously into the intestine, all
owing digestion of fats.
 Only high-fat meals need to be avoided.
122
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123
Fig. Formation of gallstones.
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K.R 125
 In the mouth: Salivary α amylase (acts on polysaccharides "st
arch")
 In the stomach: No specific enzyme. Salivary α amylase is inh
ibited (by the low pH).
 In the intestine: Pancreatic α amylase (also acts on starch).
Brush border enzymes (maltase, sucrase & lactase)
Act on disaccharide and produce monosaccharides--fructose,
glucose & galactose
 Maltase: Acts on maltose to give glucose + glucose
 Sucrase: Acts on sucrose to give glucose + fructose
 Lactase: Acts on lactose to give glucose + galactose
Digestion of Carbohydrates
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Digestion of Carbohydrates in Stomach
• However, starch digestion sometimes continues in the bo
dy and fundus of the stomach for as long as 1 hour before
the food becomes mixed with the stomach secretions.
• Activity of the salivary amylase is blocked by acid of the
gastric secretions because the amylase is essentially non-a
ctive as an enzyme once the pH of the medium falls belo
w about 4.0.
• Nevertheless, on the average, before food and its accomp
anying saliva do become completely mixed with the gastr
ic secretions, as much as 30 to 40 percent of the starches
will have been hydrolyzed mainly to form maltose.
126
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Digestion of Carbohydrates
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Digestion of Proteins
Stomach
• HCl denatures or unfolds protei
• HCl activates pepsinogen in to pepsin
• Pepsin turns proteins into partially digested proteins (peptides)
Pancreas
• Digestive enzymes: split peptide bonds between different
amino acids
Small Intestine
• Brush border enzymes:
• aminopeptidase or dipeptidase: enzymes break peptide bonds that attach ter
minal amino acids to carboxyl ends of peptides → carboxypeptidases: enzy
mes break peptide bonds that attach terminal amino acids to amino ends of p
eptides → aminopeptidases: enzymes split dipeptides to amino acids (dipepti
dase)
129
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Digestion of Proteins
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Digestion of Lipids
• Mouth: lingual lipase
• Most lipid digestion, in an adult, occurs in the small intestine.
132
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Definition
• Absorption: is the passage of the end products of digestion from th
e GI tract into blood or lymph
• It occurs by
Diffusion,
Facilitated diffusion,
Osmosis, and Active transport.
• Essentially all carbohydrates are absorbed as monosaccharides.
They are absorbed into blood capillaries.
• Absorption of Amino Acids, Dipeptides, and Tripeptides
Most proteins are absorbed as amino acids by active transport p
rocesses.
They are absorbed into the blood capillaries in the villus.
135
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Absorption of Monosaccharides
• Absorption into epithelial cell
• Glucose & galactose by active transport
• Fructose by facilitated diffusion
• Movement out of epithelial cell into bloodstream by facilitated d
iffusion
• Glucose and Galactose are transported by a sodium co-transport
mechanism SGLUT-1 and fructose by GLUT-5.
• Glucose, galactose and fructose are transported out of the entero
cyte through another hexose transporter (called GLUT-2) in the
basolateral membrane.
136
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Absorption of Amino Acids & Dipeptides
• Absorption into epithelial cell through active transport wi
th Na+ or H+ ions
• Movement out of epithelial cell into blood via diffusion
137
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Absorption of Lipids
• Dietary lipids are all absorbed by simple diffusion.
• Long-chain fatty acids and monoglycerides are absorbed
as part of micelles, resynthesized to triglycerides and for
med into protein-coated spherical masses called chylomi
crons.
• Chylomicrons are taken up by the lacteal of a villus.
• From the lacteal they enter the lymphatic system and the
n pass into the cardiovascular system, finally reaching th
e liver or adipose tissue.
• The plasma lipids - fatty acids, triglycerides, cholesterol -
are insoluble in water and body fluids.
138
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• In order to be transported in blood and utilized by body c
ells, the lipids must be combined with protein transporters
called lipoproteins to make them soluble.
• The combination of lipid and protein is referred to as a lip
oprotein.
• Small fatty acids enter cells & then blood by simple diffu
sion
• Larger lipids exist only within micelles (bile salts coating
)
• Lipids enter cells by simple diffusion leaving bile salts be
hind in gut
• Bile salts reabsorbed into blood & reformed into bile in th
e liver
• Fat-soluble vitamins enter cells within micelles
139
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Absorption in the Large Intestine:
Formation of Feces
• About 1500 ml of chyme normally pass through the ileocecal valve
into the large intestine each day.
• Most of the water and electrolytes in this chyme are absorbed in the
colon, usually leaving less than 100 ml of fluid to be excreted in the
feces.
• Also, essentially all the ions are absorbed, leaving only 1 to 5 mEq
each of Na+ and Cl- to be lost in the feces
• Most of the absorption in the large intestine occurs in the proximal
one half of the colon, giving this portion the name absorbing colon
Functions: principally for feces storage until a propitious time f
or feces excretion and is therefore called the storage colon.
140
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Absorption and Secretion of Electrolytes and Water
•The mucosa of the large intestine, like that of the small in
testine, has a high capability for active absorption of Na, a
nd the electrical potential gradient created by absorption o
f the Na causes Cl absorption as well.
•In addition, as occurs in the distal portion of the small int
estine, the mucosa of the large intestine secretes HCO3
-wh
ile it simultaneously absorbs an equal number of Cl- in an
exchange transport.
•Absorption of Na+ and Cl- creates an osmotic gradient acr
oss the large intestinal mucosa, which in turn causes absor
ption of water.
141
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Jaundice
•Jaundice Excess Bilirubin in the Extracellular
Fluid.
• Refers to a yellowish staining of the body tissues, in
cluding a yellowness of the skin as well as the deep
tissues.
The usual cause of jaundice is
Large quantities of bilirubin in the ECFs, either free bi
lirubin or conjugated bilirubin.
The normal plasma concentration of bilirubin is 0.5 m
g/dl of plasma.
In certain abnormal conditions, this can rise to as high
as 40 mg/dl, and much of
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Con….
• The common causes of jaundice are
1. Increased destruction of RBCs, with rapid rele
ase of bilirubin into the blood, and
2. Obstruction of the bile ducts or damage to the
liver
• These two types of jaundice are called, respectivel
y, hemolytic jaundice and obstructive jaundice.
• They differ from each other in the following ways.
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Con….
• Hemolytic Jaundice Is Caused by Hemolysis of R
ed Blood Cells.
• In hemolytic jaundice, the excretory function of the liver is not
impaired, but red blood cells are hemolyzed so rapidly that the
hepatic cells simply cannot excrete the bilirubin as quickly as it
is formed. Therefore, the plasma concentration of free bilirubin
(unconjugated)or(Albumin bound) rises to above normal levels
.
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Con….
• Obstructive Jaundice Is Caused by Obstruction of Bile Du
cts or Liver Disease.
• When a gallstone or cancer blocks the common bile du
ct) or
• By damage to the hepatic cells (which occurs in hepati
tis),
• The rate of bilirubin formation is normal, but the biliru
bin formed cannot pass from the blood into the
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Diagnostic Differences Between Hemoly
tic and Obstructive Jaundice.
 In hemolytic jaundice,
Almost all the bilirubin is in the “free” form;
 In obstructive jaundice,
It is mainly in the “conjugated” form.
 negative urine test for urobilinogen
the stools become clay colored
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149
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Gall Bladder Disease
 Crohn’s Disease
Peritonitis
Peptic ulcer disease (PUD)
Vomiting
Read on the following Clinical correlatio
ns
 Liver Disorders
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End of the chapter
Thank you!!!
150
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