Download L07 Physiology2 Dr Faisal DigestiveSystem

University of Jordan
Faculty of Medicine
Physiology || for Pharmacy
L07 –Dr. Faisal
Digestive-System
Note:
1) Make sure you understand everything, exams questions will be based on
understanding NOT memorizing alone
2) Anything between *** was not mentioned during the lecture (only for your
knowledge)
3) make sure you go through the slides for the Digestive system by
Dr.Faisal
----------------------------------------------------------------------------------------------------------Last lecture we talked about general functions of the digestive system &
started with the mouth mentioning its processes in the system
2) Esophagus
Is a collapsible muscular tube (25 cm long) that lies posterior to the
trachea. The esophagus begins at the inferior end of the laryngopharynx.
Deglutition (swallowing): the movement of food from the mouth into the
stomach
Mouth  Pharynx  Esophagus  Stomach
Deglutition occurs in 3 stages:
1) The voluntary stage food is passed to the oropharynx (analward)
Swallowing starts when food is forced to the back of the oral cavity and
into the oropharynx by the movement of the tongue upward and
backward
2) The pharyngeal stage  it is involuntary, the Uvula and the soft palate
moves upward and close the nasopharynx, then the larynx moves
upward and forward and the epiglottis closes off the opening to the
larynx. Once the upper esophageal sphincter relaxes the food moves
into the esophagus


The pharyngeal stage is very fast because at this stage we can’t
breathe so it only lasts about 2 seconds
Food stimulates receptors in the oropharynx, which send
impulses to the deglutition center in the medulla oblongata and
lower pons of the brain stem
3) The esophageal stage  begins once food enters the esophagus.
During this phase Peristalsis which is a progression of contractions
and relaxations of the circular and longitudinal layers of the muscularis
that pushes the food forward
 It takes about 5 seconds( actually it depends on the kind of food
or drink and it also depends on the gravity)
Note:
Mucus secreted by esophageal glands lubricates the bolus (food) and
reduces friction (facilitates movement)
 Q: What kind of movement and absorption do we have in pharynx and
the esophagus?
Movement:
Pharynx  contractions
Esophagus peristalsis
Absorption:
Pharynx no absorption
Esophagus  no absorption
4) Stomach
Is a J-shaped enlargement of the GI tract
As we mentioned before the stomach has an extra layer of muscles
rather than the circular and longitudinal layers which is the Oblique
Layer
The stomach has four main regions: the cardia, fundus, body and the
pyloric part
Cardia  surrounds the opening of the esophagus into the stomach
Fundus the rounded portion
Body  a large central portion in the stomach
Pyloric part  is divisible into 3 regions: pyloric antrum, pyloric canal
and pylorus which ends in the pyloric sphincter
Stomach movements:
There are two types of movements
1) Local (mixing and cutting food into smaller parts)
2) Propulsion: each peristaltic wave moves the gastric contents from
the body of the stomach down into the antrum
Stomach secretions (Gastric secretions):
Secretions of the stomach are around 2 Liters per day
That comes from the gastric glands
Mucous  it neutralizes the acids and forms a barrier to protect the
mucosa from acids
 acid contamination with mucosa leads to Ulcers ‫قرحة المعدة‬
For people who get nervous a lot we expect for them to have ulcer
Because of sympathetic stimulation  heart rate  secretions of
stomach  acids will attack the stomach’s inner surface (Mucosa) 
Ulcer
Although parietal cells secrete H+ and Cl- into the stomach lumen, the net
effect is secretion more HCl (against concentration gradient)
Concentration of H+ in the lumen s much higher than concentration of the
intercellular
That is due to Proton pumps powered by energy (ATP) actively transport
H+ into the lumen.
Also parietal cells secrete Intrinsic factor (needed for absorption of
vitamin B12 that is used in red blood cell formation, or erythropoiesis and
reabsorbed in the last part of the Ileum)
So when parietal cells don’t secrete the intrinsic factor  this will lead to
deficiency of B12 folate deficiency anemia
And must be treated with injections
Chief Cells secrete Pepsinogen (an endopeptidase which is secreted in
inactive form by the stomach wall and converted into the enzyme pepsin
by gastric acid)
The only protein-digesting enzyme in the stomach is pepsin (active at
pH=2)
Endopeptidase breaks the peptide bond from the middle
Exopeptidase  breaks the peptide bond from terminals (ends)
 Q: what keeps pepsin from digesting the protein in stomach cells along
with the food?
First Pepsin is secreted in it’s in active form (pepsinogen), pepsinogen is
not converted into active pepsin until it comes in contact with HCl secreted
by parietal cells.
G Cells secrete the hormone Gastrin  which is secreted directly to the
blood  stimulates all gastric activities (secretion of HCl &pepsinogen,
movement and absorption in addition to growth of cells)
As we mentioned in the previous lecture
Control of the GI-system:
1) Hormonal control
2) Neuronal control
There are phases of secretions:
1) Cephalic phase (Neural ONLY)
70% of total gastric acid secretion occurs during this phase.
The cephalic phase is the stage in which the stomach responds to the
mere sight, smell, taste, or thought of food. About 25% of total acid
secretion occurs before food enters the stomach. These sensory and
mental inputs converge on the hypothalamus, which relays signals to
the medulla oblongata. The Vagus nerve fibers from the medulla
stimulate the parasympathetic nervous system of the stomach which,
in turn, stimulates gastric secretion (via parietal and G cells.
2) Gastric Phase (Neural & hormonal)
25% of total gastric acid secretion occurs during this phase.
The gastric phase is a period in which swallowed food and semidigested protein (peptides and amino acids) activate gastric activity.
Ingested food stimulates gastric activity in two ways: by stretching the
stomach and by gastric contents stimulating receptors in the stomach
3) Intestinal phase (Neural & hormonal)  usually inhibitory
 Q: How it is inhibitory?
When food enters the intestine from the stomach  presence of acids in
duodenum  stimulates the secretion of the hormone secretin
Also
Presence of fats and partially digested proteins  stimulates the secretion
of cholecystokinin (CCK)
Both Secretin and CCK primarily stimulate the pancreas and gall bladder,
but also suppress gastric secretion and motility. The effect of this is that
gastrin secretion declines and the pyloric sphincter contracts tightly to limit
the admission of more food into the duodenum. This gives the duodenum
time to work on the food it has already received before being loaded with
more.
The enteroendocrine cells also secrete glucose dependent insulinotropic
peptide. Originally called gastric-inhibitory peptide, it is no longer
thought to have a significant effect on the stomach, but to be more
concerned with stimulating insulin secretion in preparation for processing
the nutrients about to be absorbed by the small intestine
Gastric Emptying contraction of the stomach & the lower
esophageal sphincter and relaxation of the pyloric sphincter
Factors that stimulates emptying  Gastric Factors
1) Stretching of the stomach
2) Partially digested proteins
3) Alcohol
4) Caffeine
5) Stimulation of the parasympathetic (Vagus nerve)
6) Gastrin
Factors that inhibit emptying Duodenal Factors
1) Extension of the duodenum (enterogastric reflex)
***the enterogastric reflex is one of the three extrinsic reflexes of the
gastrointestinal tract, the other two being the gastrocolic reflex and the
gastroileal reflex. The enterogastric reflex is stimulated in the
duodenum by a pH of 3-4 and in the stomach by a pH of 1.5***
2) Secretin  due to presence of acids in the duodenum
3) CCK due to presence of partially digested proteins and fats
Done by:
Rahaf Mihyar