Download GIT Functions Of Stomach

Anatomically, the stomach is usually divided into
two major parts:
(1) the body
(2) the antrum.
Physiologically, it is more appropriately divided into
(1) the “orad” portion, comprising about the first
two thirds of the body
(2) the “caudad” portion, comprising the remainder
of the body plus the antrum
Basic electrical rhythm
• They are the slow,undulating changes in the
RMP of smooth muscle of GIT.
Intensity-5 to 15 mv
Frequency-3 to 12/min
3/min-In body of stomach
12/min-In duodenum
8 to 9/min-In terminal ileum
Causes of slow waves• Complex interactions among smooth muscle
cells and interstitial cells of Cajal.
• Interstitial cells undergo cyclic changes in
membrane potential due to unique ion
channels that periodically open and produce
inward current which generate slow wave
activity.
Peristaltic wave
• It is the basic propulsive movement of GIT and
is an inherent property of syncytial smooth
muscles.
• Stimulation at any point causes a contractile
ring to appear which moves forward and food
in front of ring also moves in forward direction
• Stimulus is distention of gut-stretching
stimulates enteric nervous system to cause
contraction 2 to 3 cm behind this point
Receptive relaxation of stomach
• When peristalsis appears, the gut relaxes
several centimeters in analward direction
which facilitates the propulsion of food in
forward direction.
Mechanism:-stretching – Vagovagal reflex –
impulses go to brain stem and back to
stomach - inhibitory neurons of the myenteric
plexus mediate this receptive relaxation
Motor Functions of the
Stomach
• Storage of large quantities of food
• Secretions of gastric glands
• Mixing of this food with gastric secretions
until it form Chyme
• Digestion
• Absorption
• Slow emptying of the chyme from the
stomach into the small intestine
Storage
Formation of concentric circles
Stretch
Vagovagal reflex
Decreased tone in the muscular wall of
stomach-Bulging of wall-Accomodates 0.8 to
1.5 litre of food in relaxed stomach
Secretion
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Lined by simple columnar epithelium
Mucus glands
Gastric (oxyntic) glands
Pyloric glands
• Four different types of cells make up the
gastric glands:
• Mucous cells
• Parietal cells
• Chief cells
• Endocrine cells
Mixing and Propulsion of Food
Appears once
every 15 to 20
seconds
Presence of
food in
stomach causes
weak peristaltic
constrictor
wave called
slow wave to
begin
Begin in mid to
upper portions
of stomach wall
and moves
towards antrum
Initiated by gut
basic electrical
rhythm
When constrictor
wave progresses
from body towards
antrum, becomes
more intense, some
are so intense that
they provide
peristaltic action
potential
It pushes the antral
contents under high
pressure towards
pylorus
It digs deeply into
the food contents
in the antrum
Opening of pylorus
is small and allows
only few ml or less
of antral contents
to empty into
duodenum
Also when
peristaltic wave
approaches
pylorus, pyloric
muscle contracts
which narrows the
pyloric opening
Resulting in
antral
contents to
squeeze
upstream
towards body
of stomach
Retropulsion
results in
mixing of food
Peristaltic
Wave
Chyme
• It is the murky semifluid or paste that results
from thorough mixing of food with gastric
secretions
Fluidity depends on:1. Relative amount of food
2. Water
3. Stomach secretion
4. Degree of digestion
Hunger Contractions
• Rhythmical peristaltic contractions in the body
of the stomach
• Strong tetanic- lasts for 2 to 3 minutes
• Intensity is increased
– young healthy individuals
– Low BSR
• Hunger pangs
Digestion
• Digestion of carbohydrates- starts in mouth
by enzyme ptyalin – only 5% of starch will be
hydrolyzed in mouth
• Digestion continues in fundus and body of
stomach for almost 1 hour
• Then activity of salivary amylase is blocked by
acid
• 30 to 40% of starch is hydrolyzed to form
maltose
• Digestion of proteins- Pepsin is most active at
pH 2 to 3 and inactive at pH 5.HCl is secreted
at pH of 0.8
• It digests protein collagen- the major
constituent of intercellular connective tissue
of meats
Absorption
• Stomach is the poor absorptive area of GIT b/c
it lacks typical villus type of absorptive
membrane and presence of tight junctions
b/w epithelial cells
• Highly lipid soluble substances like alcohol and
aspirin are absorbed in small quantity
Intrinsic factor
• Secreted by the parietal cells
• Necessary for absorption of vitamin B12 in
ileum
Stomach Emptying
• Promoted by intense peristaltic contractions in
the stomach antrum.
• Emptying is opposed by varying degrees of
resistance to passage of chyme at the pylorus.
Pyloric Pump
• Peristaltic waves, in addition to causing mixing
in the stomach, also provide a pumping action
called the “pyloric pump” and it forces several
milliliters of chyme into the duodenum
• Mostly weak contractions- cause mixing.
• (20% of the time the food stays in the
stomach) strong intense contractions very
tight ring like constrictions which squeeze the
contents of stomach- cause stomach emptying
Role of the Pylorus in Controlling
Stomach Emptying
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Distal opening- pylorus
Increased thickness of circular muscles
Tonically contracted
Pyloric sphincter
The degree of constriction  nervoushumoral reflex signals from stomach &
duodenum
• Allow passage of water and other fluids