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Stomach
Fo Lecture
The three functional parts of stomach are fundus,
corpus(body) and antrum. Gastric contents are isolated from
other parts of the gastrointestinal tract(GIT) by the lower
esophageal sphincter proximally and by the pylorus (pyloric
sphincter) distally. The antrum and pylorus are anatomically
continuous and respond to nervous control as a unit.
Motility:
Complex patterns of motility move food through the
stomach, where it can be broken down further by gastric
secretions and then propelled ito the small intestine. Only small
amount of food are digested or absorbed in the stomach. Gastric
motility serves three basic functions:
1- Storage: When food enters the stomach, the orad region
(primary the fundus) enlarges to accommodate the food.
2- Mixing: The presence of food in the stomach increase the
contractile activity of the stomach. The enhanced contractile
activity (a combination of peristalsis and retropulsion) mix the
food with stomach acid and enzymes, breaking it into smaller
and smaller pieces. When the food is mixed into a pasty
consistency, is called chime.
3- Emptying: When the chime is broken down into small
enough particles, it is propelled through the pyloric sphincter
into the intestine.
Types of motility:
1- Peristalsis: peristaltic contractions are initiated near the
fundal-corpus border and proceed caudally, producing a
peristaltic wave that propels the food toward the pylorus. The
force of peristaltic contractions is regulated by gastrin and
acetylcholine.
2- Retropulsion: is the back and forth movement of the chyme
caused by the forceful propulsion of food against the closed
pyloric sphincter. the forward and backward movement of the
chyme (caused by peristalsis and retropulsion ) breaks the
chyme into smaller and smaller pieces and mixes it with gastric
secretions present within the stomach.
Gastric emptying:
Occurs when the chyme is decomposed into small enough
pieces (typically less than 1 mm3) to fit through the pyloric
sphincter.
1- Each time the chyme is pushed against the pyloric sphincter,
a small amount (2-7 ml) may escape into the duodenum.
2- The amount of chyme passing through the pylorus depends
on the size of the particles. If the particles are too large, none of
the chyme will enter the duodenum.
3- Therefore, the rate of gastric emptying of solid depends on
the rate at which the chyme is broken down into small particles.
4- Liquids empty much faster than solids. The rate at which
liquids empty is proportional to pressure within the orad
stomach, which increases slowly during the digestive period.
Regulation of gastric emptying:
1- Local reflexes:
(a) Excitatory reflexes: initiated by expansion of the antrum,
are responsible for increasing motility. Although these reflexes
do not require the vagus nerve, vagotomy decreases the
magnitude and coordination of stomach contractions.
(b) Inhibitory reflexes: A variety of stimuli act on the
duodenum to initiate enterogastric reflexes that slow the rate of
gastric emptying.
2- Hormones: released from the stomach and intestine also
influence gastric motility. Gastrin :enhance gastric contraction.
Cholecystokinin (CCK) and secretin have a direct inhibitory
effect on gastric emptying.
3- Migrating motor complex (MMC): during the
interdigestive period. Any food left in the stomach is removed
by the MMC. The hormone motilin, which is release from the
duodenum ,increase the strength of MMC.