CELLULAR
COMPOSITION OF STOMACH
Following are the Cells found in stomach:
·
Chief Cells
·
Parietal Cells
·
Foveolar Cells
·
Neuroendocrine Cells
PHASES OF
GASTRIC SECRETION
Secretion of gastric juice is a continuous process. Here’re its
phases:
·
Cephalic phase
·
Gastric phase
·
Intestinal phase.
·
Interdigestive phase
GASTRIC JUICE
Gastric juice contains 99.5% of water and 0.5% solids. Solids are
organic and inorganic substances. Gastric juice is a variable mixture of water,
hydrochloric acid, electrolytes (sodium, potassium, calcium, phosphate,
sulfate, and bicarbonate), and organic substances (mucus, pepsin, and
protein).
MOTOR FUNCTIONS OF STOMACH
The motor functions of the stomach are threefold:
1.
Storage of large quantities of food until the food
can be processed in the stomach, duodenum, and lower intestinal tract;
2.
Mixing of this food with gastric
secretions until it forms a semifluid mixture called chyme. There are 2
types of movements taking place in the stomach
I.
Mixing movements
II.
Propulsive movements
Mixing
& Propulsive Movements:
a.
Stretch of stomach wall causes
stimulus to be transmitted via the afferent nerve fibers to the enteric nervous
system
b.
The impulses cause the excitation of
the Auerbach’s plexus
c.
This leads to the generation of
spike potentials in the smooth muscles of the stomach
d.
Frequency of spike potential
increases causing the smooth muscles to generate AMP and contract in a rhythmic
manner (peristalsis)
e.
The stomach contracts in the oral to
the caudal direction
f.
The pyloric sphincter is closed
hence food does not pass through the sphincter and is pushed back up
g.
This way food is churned and mixed.
3. Slow emptying of the chyme from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.
STOMACH EMPTYING (PYLORIC PUMP)
Stomach emptying is a coordinated function by intense peristaltic contractions in the gastric/pyloric antrum. At the same time, the emptying is opposed by varying degrees of resistance to passage of chyme at the pylorus. Rate depends on pressure generated by antrum against pylorus resistance. Chyme = food in stomach which has been thoroughly mixed with stomach secretions.
FACTORS AFFECTING STOMACH EMPTYING
A. Factors That Promote
Gastric volume:
·
Increased food volume in stomach
promotes increased emptying
·
Antral distension stimulates
vasovagal excitatory reflexes leading to increased antral pump activity
Liquid vs solid food:
·
Clear fluids are empty rapidly (30
minutes). Solids stay in stomach longer (1-2 hours)
·
Pylorus is open enough for
H2O/fluids to empty with ease. Constriction of the pyloric sphincter to solids
until chyme is broken down into small particles and mixed to almost fluid
consistency
Types of food:
·
Protein empties fastest, followed by
carbohydrates. Fats take longest to empty
·
Note: high protein food especially
meat stimulate release of gastrin from antral mucosa
Hormonal factors:
·
Gastrin has mild to moderate
stimulatory effects on motor functions in the body of the stomach.
·
Enhances activity of pyloric pump
·
Motilin released by epithelium of
the small intestine enhances the strength of the migrating motor complex which
is a peristaltic wave that begins within the oesophagus and travels thru the
whole GIT every 60-90 min during the interdigestive period. Help empty
remaining food in stomach
Neural factor:
·
Parasympathetic innervation (via
vagus) stimulates motility
·
Local myenteric reflex
Drugs:
· Prokinetics e.g. cisapride, erythromycin metoclopramide
B. Factors That
Inhibit
Duodenal distension:
·
Results in inhibitory enterogastric
reflexes
·
Slow or even stop stomach emptying
if the volume of chyme in the duodenum becomes too much
Osmolarity of chyme:
·
Iso-osmotic gastric contents empty
faster than hyper or hypo-osmotic contents due to feedback inhibition produced
by duodenal chemoreceptors (hyper more inhibitory than hypo)
Types of food:
·
Fat and protein breakdown products
in the small intestine inhibit gastric emptying
Acid:
·
pH of chyme in the small intestine
of < 3.5-4 will activate reflexes to inhibit stomach emptying until duodenal
chyme can be neutralized by pancreatic and other secretions
Temperature:
·
Cold liquid empty more slowly
Hormones:
·
Cholecystokinin released from
duodenum in response to breakdown products of fat and protein digestion. Blocks
the stimulatory effects of gastrin on the antral smooth muscle
·
Secretin released from the duodenum
in response to acid, has a direct inhibitory effect on the gastric smooth
muscles
·
Others e.g. somatostatin, vasoactive
intestinal peptide (VIP), gastric inhibitory peptide (GIP)
Neural:
·
Sympathetic nerves (via the celiac
plexus) inhibits motility
Clinical factors:
·
Pregnancy delays gastric emptying
(progesterone)
·
Anxiety delays gastric emptying
·
Pain
·
Elderly
·
Disease states e.g. diabetes
mellitus (autonomic neuropathy), post-operative bowel surgery with resultant
ileus, high intra-abdominal pressure
Drugs:
·
e.g. opioids
Mechanical:
·
e.g. pyloric stenosis
HUNGER PANGS
These are uncomfortable sensations when blood glucose levels are
low or stomach remains empty for 12-24 hours and reach their peak in 3-4 days
and weaken subsequently. They are peristaltic waves that subsequently become
stronger and fuse together &they are intense in young healthy people.
Person feels mild pain its stomach’s pit. These are called as hunger pangs.
DIGESTION IN
INFANTS
In spite of immaturity of many of the classical digestive
mechanisms of the adult, the infant uses a number of compensatory systems to
achieve adequate digestion of nutrients. Thus, in the infant gastric
proteolysis is probably extremely limited, intestinal protein digestion is
adequate. Although starch supplements are better tolerated in breast-fed
infants, because of the compensation provided by human milk amylase, the infant
is able to digest lactose and short-chain glucose polymers with endogenous
brush border enzymes. Fat digestion is markedly aided by gastric lipase and, in
breast-fed infants, the bile salt-dependent lipase of human milk. Thus, in the
infant, gastric lipolysis is quantitatively much more significant than in
adults. The absorption of human milk whey proteins (and probably also cow milk
proteins) is probably associated more with the highly glycosylated form of
these proteins than with immaturity of neonatal digestive enzymes.
SOME CLINICAL CONDITIONS OF
STOMACH
Achalasia:
·
Achalasia is the best-defined
primary motility disorder and the only one with an established pathology.
·
The predominant neuropathologic
process of achalasia involves the loss of ganglion cells from the wall of the
esophagus, starting at the LES and developing proximally.
·
The degree of ganglion cell loss
parallels the disease duration such that, at 10 years, ganglion cells are
likely completely absent.
·
At the LES, the loss of inhibitory
nerves is demonstrated by loss of nitric oxide synthase and vasoactive
intestinal peptide (VIP) immunohistochemistry staining.
·
Variable amounts of inflammatory
cells have been described within the myenteric plexus along with the
disappearing nerves.
·
In the peristaltic esophageal body,
achalasia is characterized by a loss of intrinsic acetylcholine-containing
nerves.
·
Atrophic gastritis is a
process of chronic inflammation of
the gastric
mucosa of
the stomach, leading to a
loss of gastric
glandular cells and their eventual replacement by intestinal and fibrous
tissues. People with atrophic gastritis are also at increased risk for the
development of gastric adenocarcinoma
·
As a result, the stomach's secretion
of essential substances such as hydrochloric acid, pepsin, and intrinsic factor is
impaired, leading to digestive problems.
The most common are:
1.
Vitamin
B12 deficiency which results in a Megaloblastic
Anemia.
2.
Malabsorption of iron
leading to Iron Deficiency
Anemia.
·
It can be caused by persistent
infection with Helicobacter
pylori, or
can be autoimmune in
origin.
·
Those with the autoimmune version of
atrophic gastritis are statistically more likely to develop gastric carcinoma, Hashimoto's
thyroiditis, and achlorhydria.
·
There are basically two types of
gastritis:
1.
Type A gastritis primarily
affects the body/fundus of the stomach and is more common with pernicious
anemia.
2.
Type B gastritis primarily
affects the antrum, and is more
common with H. pylori infection.
Achlorhydria:
Decreased functioning of the parietal cells causes
Pernicious Anemia
Stomach contents, including acid, can travel backward up
the esophagus. There may be no symptoms, or reflux may cause heartburn or
coughing.
Gastroesophageal
reflux disease (GERD):
When symptoms of reflux become bothersome or occur
frequently, they’re called GERD. Infrequently, GERD can cause serious problems
of the esophagus.
Another name for stomach upset or indigestion. Dyspepsia
may be caused by almost any benign or serious condition that affects the
stomach.
Gastric ulcer
(stomach ulcer):
An erosion in the
lining of the stomach, often causing pain and/or bleeding. Gastric ulcers are
most often caused by NSAIDs or H. pylori infection.
Doctors consider ulcers in either the stomach or the
duodenum (the first part of the small intestine) peptic ulcer disease.
Gastric cancer is an uncommon form of cancer.
Adenocarcinoma and lymphoma make up most of the cases of stomach cancer.
Zollinger-Ellison
syndrome (ZES):
One or more tumors that secrete hormones that lead to
increased acid production. Severe GERD and peptic ulcer disease result from
this rare disorder.
Gastric varices:
In people with severe liver disease, veins in the stomach
may swell and bulge under increased pressure. Called varices, these veins are
at high risk for bleeding, although less so than esophageal varices are.
Gastritis, ulcers, or gastric cancers may bleed. Seeing
blood or black material in vomit or stool is usually a medical emergency.
Gastroparesis
(delayed gastric emptying):
Nerve damage from diabetes or other conditions may impair
the stomach’s muscle contractions. Nausea and vomiting are the usual symptoms.
DIAGNOSTIC
TECHNIQUES
Upper endoscopy
(esophagogastroduodenoscopy or EGD):
A flexible tube
with a camera on its end (endoscope) is inserted through the mouth. The
endoscope allows examination of the esophagus, stomach, and duodenum (the first
part of the small intestine).
Computed tomography (CT scan):
A CT scanner
uses X-rays and a computer to create images of the stomach and abdomen.
Using a
magnetic field, a scanner creates high-resolution images of the stomach and
abdomen.
Using a tube
through the nose into the esophagus, acid levels in the esophagus can be
monitored. This can help diagnose or change treatment for GERD.
After
swallowing barium, X-ray films of the esophagus and stomach are taken. This can
sometimes diagnose ulcers or other problems.
X-rays are
taken of the esophagus, stomach, and upper part of the small intestine.
Gastric
emptying study:
A test of how
rapidly food passes through the stomach. The food is labeled with a chemical
and viewed on a scanner.
Stomach biopsy:
During an
endoscopy, a doctor can take a small piece of stomach tissue for tests. This
can diagnose H. pylori infection, cancer, or other problems.
While most
people with H. pylori infection don't develop ulcers, simple blood or
stool tests can be done to check for infection in people with ulcers or to
verify that the infection is wiped out after treatment.
No comments:
Post a Comment