Digestion is the process of reducing
food to smaller molecules that can be absorbed in the body. Three organs play
an important role in the digestion process. These are the gastrointestinal
tract, the liver and the pancreas.
The organs that compose the GIT are
the:
1. Mouth
2. Pharynx
3. Esophagus
4. Stomach
5. Small
intestine
6. Large
intestine
THE STOMACH
Structure
1. The
size of the stomach varies according to factors such as gender and amount of
distention.
2. When
no food is in the stomach, it is about the size of a large sausage.
a. In
adults, capacity ranges from 1.0 to 1.5 L.
3. Stomach
location: upper part of the abdominal cavity under the liver and diaphragm.
4. Divisions
of the stomach
a. Fundus
– enlarged portion to the left and above the opening of the esophagus into the
stomach.
b. Body
– central portion of the stomach.
c. Pylorus
– lower part of the stomach.
5. Curves
of the stomach
a. Lesser
curvature – upper right curve of the stomach.
b. Greater
curvature – lower left curve of the stomach.
6. Sphincter
muscles – circular fibers arranged so that there is an opening in the center
when relaxed and no opening when contracted
The Stomach Wall
1.
Gastric mucosa
a. Epithelial
lining has rugae marked by gastric pits
b. Gastric
glands – found below level of the pits which secrete most of the gastric juice.
c. Chief
cells – secretory cells found in the gastric glands and secrete the enzymes of
the gastric juice.
d. Parietal
cells – secretory cells found in the gastric gland which secrete hydrochloric
acid and thought to produce intrinsic factors needed for Vitamin B12
absorption.
2.
Gastric
muscularis
– thick layer of muscle with three distinct sublayers of smooth muscle tissue
arranged in a crisscrossing patter. This pattern allows the stomach to contrast
strongly at many angles.
Functions of the stomach
1. Reservoir
for food until it is partially digested and moved further along the GI tract
2. Secretes
gastric juice to aid in digestion of food.
3. Breaks
food into small particles and mixes them with gastric juice.
4. Secretes
intrinsic factor
5. Limited
absorption
6. Produces
gastrin
7. Helps
protect body from pathogenic bacteria swallowed with food.
Major constituents secreted by
the stomach
1.
Hydrochloric
acid
2.
Mucus
a. Major
function is to protect the mucosal epithelium from mechanical damage during the
passage of food.
b. Provides
a lubricant for the passage of food and its strongly adhesive properties ensure
that much of the mucus remains stuck to the mucosa.
c. It
does not provide and impenetrable barrier to acid since it is permeable to H+.
3.
Electrolytes
a. In
the basal states, the major electrolytes in gastric juice are Na+
and Cl– with small amounts of H+ and K+.
b. As
the rate of secretion increases, there is a marked increase in H+
concentration and small increases in Cl– and K+
concentrations. The sodium concentration decreases.
c. At
all rates of secretion, H+, K+ and Cl–
concentration are greater than those in serum, whereas, Na+ is lower
than that found in serum.
4.
Enzymes
a.
Pepsinogen – secreted by
the gastric mucosa as a zymogen and converted to proteolytically active pepsis
by the acid pH of the stomach.
(1) Pepsin
aids in the digestion of protein by splitting peptide bonds, especially those
involving aromatic amino acids such as phenylalanine, tyrosine and leucine.
(2) The
activity of pepsin is terminated when the acid gastric contents are mixed with
alkaline pancreatic secretions in the small intestine.
b.
Gastric lipase – has a broad
pH optimum of 4.0 to 7.0 and is stable at pH 2.0. It is important in the
digestion of short–chain triglycerides in milk.
5.
Hormones
a. Gastrin
– produced and stored within the G cells located in the antral mucosa, however,
is release is inhibited by an acid pH (usually 2.0).
b. Biologically
active forms of gastrin:
(1)
Big gastrin or
G34
– twice as abundant as “little gastrin” in blood sample, eventhough it only
accounts for 10% of the gastrin contained within the G cells. The reason for
this difference is that big gastrin has a half–life of 42 minutes while little
gastrin only has 7 minutes.
(2)
Little gastrin
or G17
– results from the cleavage of G34
(3)
Minigastrin or
G14
Physiologic
effects of gastrin
a.
Gastrin
stimulation
(1) Acid
secretion by parietal cell
(2) Pepsinogen
secretion
(3) Gastric
blood flow
(4) Contraction
of the circular muscle of the stomach
(5) Growth
of gastric and small intestinal mucosa and pancreas
b.
Gastrin
inhibition
(1) Contraction
of the pyloric sphincter, ileocecal sphincter and sphincter of the
hepatopancreatic ampulla.
(2) Gastric
emptying
(3) Absorption
of glucose and electrolytes in the small intestine
c.
Gastrin
release
(1) Insulin
(2) Glucagon
(3) Calcitonin
Stimulants
for gastrin release
(1)
Major
stimulants
Phenylalanine,
cysteine, tryptophan, hydroxyproline, tyrosine
(2)
Minor
stimulants
(a) Calcium,
beer, wine, coffee
(b) Vagal
stimulation by electric current
(c) Insulin
hypoglycemia
(d) Injection
of 2–deoxyglucose
Concentration
Fasting = 21 – 105 pg/ml
Postprandial = increase to 42 – 84
pg/ml (20 – 40 pmol/L)
THE SMALL
INTESTINE
Structure
1. The
tube is approximately 2.5 cm in diameter and 6 m in length; coiled loops fill
most of the abdominal cavity.
2. It
has the following divisions:
a. Duodenum
– uppermost division; approximately 25 cm long, shaped roughly like the letter
C.
b. Jejenum
– approximately 2.5 m long.
c. Ileum
– approximately 3. 5 m long.
3. Wall
of the small intestine:
a. Intestinal
lining has plicae with villi.
b. Villi
– important modification of the mucosal layer.
(1) Each
villus contains an arteriole, venule and lacteal
(2) Covered
by a brush border made up of 1,700 ultrafine microvilli per cell.
(3) Villi
and microvilli increase the surface area of the small intestine hundreds of
time.
Function
1. Completion
of digestion of food.
2. Selective
absorption of nutrients and water.
3. Passage
of unabsorbed foodstuff along the digestive tract.
The process of absorption
1. Absorption
is the passage of substance through the intestinal mucosa into the blood or
lymph.
2. Mechanism
of absorption
a. Secondary
active transport (e.g. Na+)
b. Sodium
cotransport (coupled transport) – e.g. glucose
c. Fatty
acids, monoglycerides, cholesterol are transported with the aid of bile salts
from lumen to absorbing cells of villi.
3. After
food is absorbed, it travels to the liver via the portal system.
Major constituents absorbed by
the small intestine
1.
Water and
electrolytes
a. The
average daily volume of fluid secreted in the small intestine is approximately
12L. If this volume was not absorbed, death from dehydration would result
within 24 hours.
b. Actively
transported ions include Na+, Cl–, Fe2+, Ca2+,
HCO3–
c. Passively
transported ions include Li+, K+, Br–, I–
d. Urea
and other non–electrolyte are absorbed by means of solvent drag, a movement of
solute in conjunction with water flow.
2.
Calcium,
magnesium and phosphate
3.
Vitamins
a.
Lipid–soluble vitamins are
absorbed after being solubilized with lipid micelles. Once absorbed, these vitamins
are incorporated into chylomicrons and transported via the blood.
b.
Water–soluble vitamins are
absorbed via passive diffusion except for Vitamin B12, folic acid,
thiamine and ascorbic acid.
Major constituents secreted by
the small intestine
1.
Enzymes
a. Salivary
and pancreatic alpha amylase
b. Amylopectin
and glycogen
c. Alpha–limit
dextrin
d. Lactase
e. Isomaltase
f.
Sucrose and glucoamylase
g. Enterokinase
h. Trypsin
2.
Hormones
a.
Cholecystokinin
(CCK)
Functions of
CCK
(1) Regulation
of gallbladder contraction and gastric emptying
(2) Neurotransmitter
(3) Induces
satiety signals
(4) Pancreatic
enzyme secretion
(5) Relaxation
of sphincter of Oddi via an indirect neural mechanism.
Stimulants of
CCK release
(1) L–tryptophan,
phenylalanine, lysine
(2) Increase
serum calcium level
(3) Pancreatic
juice
(4) Partially
digested fats and proteins
Inhibitors of
CCK release
(1) Somatostatin
(2) Bile
acids
b.
Secretin
Function
(1) Stimulation
of pancreatic bicarbonate and water secretion
(2) Use
clinically in evaluating pancreatic function
Stimulants for
secretin release
(1) Duodenal
acidification, below pH 4.0, release of secretion is maximal and is
proportional to the amount of acid entering the duodenum.
(2) Presence
of fatty acids
c.
Gastric
inhibitory peptide (GIP)
(1) Inhibition
of gastric acid secretion
(2) Stimuli
in insulin release from beta cells of the pancreas and are primarily
responsible for the rapid metabolism of an oral glucose load. It has been thus
renamed glucose –dependent–insulinotropic peptide.
(3) Secretion
of fluid and electrolyte by the ileal mucosa
d.
Vasoactive
intestinal polypeptide (VIP)
Functions of
VIP
(1) Stimulates
pancreatic secretion
(2) Relaxation
of lower esophageal sphincter, gastric fundus and anal sphincter
(3) Gut
blood flow and penile erection
(4) Inhibits
pentagastrin and histamine–stimulated gastric acid and pepsin secretion
(5) Stimulates
lipolysis, glycogenolysis and secretion by the small intestine and pancreas
(6) Neurotransmitter
Stimulants of
VIP release
(1) G
proteins
(2) Activation
of adenyl cyclase
MISCELLANEOUS HORMONES OF GI
ETIOLOGY
1.
Somatostatin
a.
Functions
(1) Inhibits
release and action of gastrin, CCK, secretin, VIP, motilin, GIP, insulin,
glucagon.
(2) Inhibits
gut secretion, gastric acid, pancreatic enzymes, bicarbonate, intestinal fluid
and electrolytes.
(3) At
lower doses, it initiates migratory motor complex and increases the rate of
gastric emptying
(4) At
higher doses, it inhibits gut motility and gallbladder contraction
(5) Antitrophic
effects in normal tissues.
b.
Stimulants for
release
(1) Ingestion
of meal
c.
Inhibitor for
release
(1) Cholinergic
stimulation
(2) Antral
acidification
2.
Gastrin–releasing
polypeptide
a.
Function
(1) Stimulates
gastrin release from antral G cells.
(2) Stimulates
smooth muscle contraction in the gallbladder, duodenum and stomach
(3) In
brain, has a role in thermoregulation and pain perception
3.
Calcitonin gene–related
peptide
a.
Function
(1) Sensory
neurotransmitter
(2) Inhibits
gastric and pancreatic secretion
(3) Inhibits
smooth muscle contraction
(4) Stimulates
somatostatin and acetylcholine release
(5) Increases
mucosal blood flow
(6) Gastric
mucosal cytopectin
4.
Galanin
a.
Function
(1) Inhibition
of postprandial release of neurohumoral substances such as insulin,
neurotensin, somatostatin and pancreatic polypeptide.
(2) Inhibits
intestinal motility
5.
Substance P
a.
Functions
(1) Neurotransmitter
substance in small diameter, unmyelinated sensory C fibers and in motor neurons
of the myenteric plexus
(2) Smooth
muscle contraction
(3) Epithelial
secretion
(4) Vasodilation
6.
Enkephalins
a.
Function
(1) Inhibits
gut motility and mucosal secretion
(2) Stimulates
sphincter tone
(3) Inhibits
acetylcholine release from myenteric plexus neurons.
7.
Neurotensin
a.
Function
(1) Stimulates
pancreatic and bicarbonate secretion
(2) Stimulates
colonic motility
(3) Trophic
effects on gastric, small bowel and colonic mucosa
(4) Binds
to mast cells thus releases histamine
8.
Motilin
a.
Functions
(1) Induction
of myoelectric complexes in the antroduodenal region which propagate distally.
(2) Contraction
of duodenal ileal, colonic and gallbladder smooth muscle.
(3) Gallbladder
emptying
(4) Stimulates
pepsis, pancreatic and chloride secretion
9.
Pancreatic
polypeptide family
a.
Function
(1) Inhibits
pancreatic bicarbonate and protein secretion
(2) Relaxation
of gallbladder
(3) Inhibits
cholinergic neurotransmitter and activation of inhibitory G proteins.
(4) Vasoconstrictor
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