LEC 5 Flashcards
Gastric factors that promote emptying
- The amount of chyme in the stomach.
- The degree of fluidity of the chyme
- Type of food ingested (present in the stomach)
The amount of chyme in the stomach
1.↑food volume in the stomach→↑emptying
from the stomach
2.tension in stomach wall >stimulus for peristalsis
The degree of fluidity of the chyme in the stomach
increase fluidity influences gastric emptying.
Type of food ingested (present in the stomach)
a) Carbohydrate
b. Protein
c. Fat
Type of food causes rapid gastric emptying
Carbohydrate
Type of food ingested causes slow gastric emptying
Protein
Type of food ingested causes slowest gastric emptying
Fat
The rate of emptying of a high-fat meal is
six hours
The rate of emptying of a protein and carbohydrate meal is
three hours.
high protein food especially meat stimulate release of
gastrin
Duodenal factors that inhibits stomach emptying
- The presence of any degree of irritation of the duodenal mucosa
- The degree of distension of the duodenum
- The degree of acidity of the duodenal chyme.
- The degree of osmolality of the chyme
- Type of food.
The degree of distension of the duodenum
Slow or even stop stomach emptying if the volume of chyme in the duodenum
becomes too much
The degree of acidity of the duodenal chyme
Acid un-neutralized acid in the duodenum inhibits further empty
↑ acid → ↓ emptying of acidic
gastric contents
enterogastric means
from duodenum to stomach
enterogastric inhibitory reflexes
sensitive to the presence of irritants and acids in the duodenal chyme
2.activated within as little as 30 seconds
the reflexes when the chyme in the duodenum falls below about 3.5 to 4
block release of acidic stomach contents
2.pancreatic enzyme
The rate of gastric emptying is fastest when the stomach contents are
isotonic
The rate of gastric emptying is slowed when the stomach contents
hypertonic or hypotonic
The hypo-osmolar chyme in duodenum causes
distention of osmo-receptor, which causes mild inhibition of gastric emptying
hyperosmolar chyme in duodenum causes
shrinkage of osmo-receptor, which causes marked inhibition of gastric emptying.
Type of food.
When glucose, fat, or amino meet the duodenal mucosa inhibitory mechanisms decrease the fundic pressure and thereby slow the gastric emptying of nutrients.
Hyper-tonicity:
Large volumes of water entering the intestine from the plasma lead to intestinal distension,
more importantly, circulatory disturbances ensue كلب علبى يترتبbecause of the reduction in plasma
volume.
To prevent these effects, gastric emptying is reflexly inhibited when the osmolarity of the duodenal
content starts to rise.
The possible mechanism of duodenal factors that inhibits stomach emptying
↓entro-gastric reflex):
• Hormonal feedback from the duodenum inhibits gastric emptying
slow or even stop stomach emptying
- multiple nervous reflexes
2. the volume of chyme
entro-gastric reflexes have two effects on stomach emptying
First, they strongly inhibit the “pyloric pump” propulsive contractions, and
Second, they increase the tone of the pyloric sphincter
entro-gastric reflexs are mediated by three routes
1.directly from the enteric nervous system in the gut wall
2.extrinsic nerves→ the pre-vertebral sympathetic ganglia →back through
inhibitory sympathetic nerve fibers to the stomach
3.vagus nerves→ brain stem→ ↓ the normal excitatory signals transmitted to the stomach through the vagi.
normal excitatory signals transmitted to the stomach through
vagi
Hormonal feedback from the duodenum inhibits gastric emptying
- CCK
- GIP
- secretin
The degree of fluidity of the chyme leaving the stomach depends on
food,
water, and
stomach secretions and on the
degree of digestion that has occurred
effect of CCK on gastric emptying
inhibition
CCK can inhibit gastric emptying when
a. excess quantities of chyme
b. acidic or fatty chyme,
Secretin released from the duodenum in response to
acid
Chyme
After food in the stomach has become thoroughly mixed with the stomach secretions, the resulting
mixture that passes down the gut
The appearance of chyme
murky semifluid or paste معجون.
Motility of Fasted Stomach
- Migrating Motor Complex (MMC)
2. Hunger Contractions
Periodic phasic contraction occurring at
regular and low frequency, once /6-20 min
Function of MMC
1) It clears stomach from indigestible residues.
2) It prevents duodeno-gastric reflux
Migrating Motor Complex (MMC) Mechanism
strength is increased by motilin hormone
Emotions can influence gastric motility:
- Sadness and fear
- Anger and aggression
- Intense pain
- Pregnancy
- Elderly
- Disease states e.g. diabetes mellitus (autonomic neuropathy), post -operative bowel surgery with resultant ileus
Sadness and fear tend to
decrease motility
Anger and aggression tend to
increase motility
Intense pain from any part of the body tends to
tends to inhibit motility> response of increased sympathetic activity
Progesterone effects on gastric emptying
a. relaxes smooth muscle
b. inhibits gastric smooth muscle
c. creating incompetence of the lower esophageal sphincter leading to gastroesophageal sphincter disease
effect of gastrin produced by the placenta
increases gastric motility and secretion which may explain Gastric acid
production during the third trimester
explain Gastric acid
production during the third trimester
gastrin is produced by the placenta
Elderly
decrease gastric secretion and emptying
the glands contains the following cells
Parietal (oxyntic) cells:
ii. Chief (peptic) cells:
iii. Neck mucous cells:
iv. Entrochromaffin (EC) cell:
v. G cell:
vi. D cell:
Parietal (oxyntic) cells: secret
1.hydrochloric acid and intrinsic factor.
Chief (peptic) cells secret
pepsinogen
Neck mucous cells: secret
mucous
Entrochromaffin (EC) cell
Histamine
G cell:
Gastrin
D cell
somatostatin
Oxyntic gland
in the body region; contains Chief cell, D cell and Parietal cell, Entrochromaffin cell, mucous neck cell
Pyloric gland
in the pyloric region; contains Entrochromaffin cell, D cell, and G cell
, mucous neck cell
The mucus is also secreted along with
HCO3
− by mucus cells on the
surface of epithelium between glands
The stomach absorbs
alcohol and aspirin but no food no water
Alcohol
lipid soluble>diffuse through the lipid membranes of the epithelial cells that line the stomach >can enter the blood through the sub-mucosal capillaries
Acetylsalicylic (aspirin).
In the highly acidic environment of the stomach lumen, weak acids are
lipid soluble, so they can be absorbed quickly by crossing the plasma membranes of the epithelial
cells that line the stomach
Mucous cells secrete
thin, watery mucus
Mucus serves as a
protective barrier against several forms of potential injury to the gastric mucosa
how Mucus serves as a protective barrier
1.lubricating properties→ no mechanical
injury.
2.protect the stomach wall from self-digestion
3.alkaline →protect against acid injury
how protect the stomach wall from self-digestion
pepsin is inhibited when it comes in
contact with the mucus layer coating the stomach lining. (However, mucus does not affect pepsin
activity in the lumen, where digestion of dietary protein proceeds without interference.
The cells of the gastric glands secrete about
2500mL
Gastric secretion includes
hydrochloric acid, pepsin, and intrinsic factor.
The substance intrinsic factor is a
49-KDa glycoprotein
The substance intrinsic factor is secreted by
the parietal cells
When parietal cells of the stomach are destroyed
- achlorhydria
2. pernicious anemia
achlorhydria
lack of stomach acid secretion
the Chief (peptic) cells of the oxyntic glands secreted
pepsinogen
pepsinogen + hydrochloric acid,=
active pepsin
pepsinogen molecular weight
42,500
pepsin molecular weight
35,000
Pepsin functions
proteolytic enzyme at pH 1.8 to
3.5)
2.Pepsinogen acts as a signal for release
gastrin and cholecystokinin.
pH of about 5
no proteolytic activity
Mucosal barrier
The mucus gel layer (extrinsic barrier) gastric mucosa (intrinsic barrier)
mucosal barrier function
protects gastric and duodenal mucosa against the damaging effect (auto-digestion) of gastric juice
The mucus gel layer visible layer
several millimeters thick over
the surface of the mucosa above gastric mucus membrane
The mucus gel layer composed of
mucus
Bicarbonate
Mucus is secreted by
neck cells of gastric gland and
surface mucosal cells
Mucus is
slimy” لزجmaterial that coats epithelial surfaces of stomach
Mucus is composed chiefly of
mucins and
inorganic salts
Mucins are
large, heavily glycosylated proteins
Mucins are located
on the surface of the stomach cavity
main components of the gastric mucosa epithelial mucus layer.
Mucins
Secretion of gastric mucin increases
i. by vagal stimulation
ii. by irritant foods.
iii. by some prostaglandins
Mucins structure
trefoil peptides
hh
Mucins form a barrier as apart of mucosal defense system that protects
gastric epithelial cells against chemical, enzymatic, microbial, and
mechanical damage.
The hydrochloric acid crosses this barrier in finger-like channels, leaving
the gel layer intact.
Bicarbonate secreted by
surface mucosal cells
Bicarbonate
is trapped in the mucus gel, so that a pH gradient is established that ranges
from pH 1.0 to 2.0 at luminal side to pH 6.0 to 7.0 at the surface of epithelial cells.
Prostaglandins stimulate mucus and bicarbonate secretion
luminal membranes of the gastric mucosal cells are almost impermeable to
H+→acid cannot penetrate into the cells and damage them
the gastric mucosal cells joined together near their luminal borders by
tight junctions →acid cannot diffuse between the cells from the lumen into the underlying submucosa.
gastric mucosa entire stomach lining is replaced every
three days
rapid mucosal turnover
cells are usually replaced before they are exposed to the wear and tear of harsh gastric conditions
The daughter cells that result from cell division
- migrate out of the pit to become surface epithelial cells
2. migrate down deeper to the gastric glands, where they differentiate into chief or parietal cells
When the barrier occasionally is broken
the gastric wall is injured by its acidic and
enzymatic contents
↓
an erosion, or peptic ulcer, of the stomach wall results.
