MOVEMENTS OF STOMACH
Activities of smooth
muscles of stomach increase during gastric digestion (when stomach is filled
with food) and when the stomach is empty.
Types of movements in
stomach
1. Hunger contractions
2. Receptive
relaxation
3. Peristalsis.
1. HUNGER
CONTRACTIONS
Hunger contractions
are the movements of empty stomach. These contractions are related to the
sensations of hunger. Hunger contractions are the peristaltic waves superimposed
over the contractions of gastric smooth muscle as a whole. This type of
peristaltic waves is different from the digestive peristaltic contractions. The
digestive peristaltic contractions usually occur in body and pyloric parts of
the stomach. But, peristaltic contractions of empty stomach involve the entire stomach.
Hunger contractions are of three types:
Type I Hunger Contractions
Type I hunger contractions
are the first contractions to appear in the empty stomach, when the tone of the
gastric muscles is
low. Each contraction lasts for about 20 seconds. The interval between
contractions is about 3 to 4 seconds. Tone of the muscles does not increase between
contractions. Pressure produced by these contractions is about 5 cm of H2O.
Type II Hunger Contractions
Type II hunger
contractions appear when the tone of stomach is stronger. Tone increases in
stomach if food intake is postponed, even after the appearance of the type I
contractions. Each of the type II contractions lasts for 20 seconds like type I
contractions. But the pause between the contractions is decreased. Pressure produced
by these contractions is 10 to 15 cm of H2O.
Type III Hunger Contractions
Type III hunger
contractions are like incomplete tetanus. These contractions appear when the
hunger becomes severe and the tone increases to a great extent. Type III hunger
contractions are rare in man as the food is taken usually before the appearance
of these contractions. These contractions last for 1 to 5 minutes. The pressure
produced by these contractions increases to 10 to 20 cm of H2O. When
the stomach is empty, the type I contractions occur first, followed by type II
contractions. If food intake
is still postponed,
then type III contractions appear and as soon as food is consumed, hunger
contractions
disappear.
2. RECEPTIVE
RELAXATION
Receptive relaxation
is the relaxation of the upper portion of the stomach when bolus enters the
stomach from esophagus. It involves the fundus and upper part of the body of
stomach. Its significance is to accommodate the food easily, without much
increase in pressure inside the stomach. This process is called accommodation of stomach.
3. PERISTALSIS
When food enters the
stomach, the peristaltic contraction or peristaltic wave appears with a
frequency of 3 per minute. It starts from the lower part of the body of stomach,
passes through the pylorus till the pyloric sphincter.
Initially, the
contraction appears as a slight indentation on the greater and lesser
curvatures and travels
towards pylorus. The contraction
becomes deeper while traveling. Finally, it ends with the constriction of
pyloric sphincter. Some of the waves disappear before reaching the sphincter.
Each peristaltic wave takes about one minute to travel from the point of origin
to the point of ending. This type of peristaltic contraction is called digestive peristalsis because
it is responsible for the grinding of food particles and mixing them with
gastric juice for digestive activities.
FILLING AND EMPTYING OF STOMACH
FILLING OF STOMACH
While taking food, it
arranges itself in the stomach in different layers. The first eaten food is
placed against the greater curvature in the fundus and body of the stomach. The
successive layers of food particles lie nearer, the lesser curvature, until the
last portion of food eaten lies near the upper end of lesser curvature,
adjacent to cardiac sphincter. The liquid remains near the lesser curvature and
flows towards the pyloric end of the stomach along a V-shaped groove. This
groove is formed by the smooth
muscle and it is
called magenstrasse.
But, if a large quantity of fluid is taken, it flows around the entire
food mass and is distributed over the interior part of stomach, between wall of
the stomach and food mass.
EMPTYING OF STOMACH
Gastric emptying is
the process by which the chime from stomach is emptied into intestine. Food
that is
swallowed enters the
stomach and remains there for about 3 hours. During this period, digestion
takes place. Partly digested food in stomach becomes the chyme.
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