16.4 Stomach Flashcards
1
Q
What is the stomach
A
It is a J-shaped sac-like chamber lying in between the esophagus and the small intestine
2
Q
What are the three sections of the stomach?
A
- Fundus
- Body
- Antrum
3
Q
Where is the fundus?
A
it is the part of the stomach that lies above the esophageal opening.
4
Q
Where is the body ?
A
It is the middle part of the stomach
5
Q
Both the fundus and the body have_________ smoth muscle layers
A
relatively thin
6
Q
Where is the antrum?
A
The lower part of the stomach
7
Q
Why is that the antrum has more heavier musculature than the fundus and body which relatively have thin smooth muscle?
A
For now let’s just say these differences play important roles in terms of gastric motility in these regions.
8
Q
Where is the pyloric sphincter located and what is its function?
A
it is the terminal portion of the stomach and it acts as a barrier between the stomach and the duodenum.
9
Q
Is the stomach ever totally empty?
A
no, never. if no food is present, there is fluid still present
10
Q
WHAT ARE THE THREE MAIN FUNCTIONS THE STOMACH PERFORMS?
A
- (most NB) It stores ingested food until it can be emptied into the small intestine at a rate optimal for digestion and absorption.
- It secretes HCl and enzymes to start up protein digestion
- Due to its mixing movements, the ingested food is pulverised and mixed with gastric secretions to produce chyme
11
Q
What is chyme?
A
is the semi-fluid, partially digested mixture of; food, stomach acid, and digestive enzymes that forms in the stomach and moves into the small intestine for further digestion.
12
Q
What must stomach contents be converted into before they are emptied into the small intestine?
A
chyme
13
Q
Gastric motility is quite __________
A
complex
14
Q
What are the four aspects of gastric motility?
A
- Filing
- storage
- mixing
- emptying
15
Q
Explain the filing aspect of gastric motility
A
This involves receptive relaxation which is mediated by the vagus nerve and allows the stomach to accommodate the meal with little change in intragastric pressure. The process occurs through the expansion of the stomach facilitated by deep folds in the stomach’s interior, which flatten out as the stomach relaxes with each mouthful.
16
Q
When emptying the stomach has a volume of about_____but can expand to about_________during a meal.
A
50mL, 1L(1000mL)
17
Q
What happens to the intragastric pressure when 1L of food is consumed?
A
It rises and there is discomfort due to the stomach being overdistended.
18
Q
Where does gastric storage take place?
A
In the body of the stomach
19
Q
What are the interstitial cells of Cajal?
A
They are a group of pacemaker cells located in the upper fundus and generate slow-wave potentials that move down the stomach toward the pyloric sphincter at a rate of 3mv/min. This BER of the stomach occurs continuously and may/may not result in contractions of the stomach’s circular smooth muscle layer.
20
Q
What does the stomach’s smooth muscle level of excitability determine?
A
It determines whether these slow-wave potentials will reach the threshold and generate action potentials, which then initiate peristaltic waves following BER
21
Q
What is basic electrical rhythm(BER)
A
Rhythmic patterns of spontaneous depolarization
22
Q
Once peristaltic waves are initiated what happens?
A
They move down from the fundu, body to the antrum and pyloric sphincter.
23
Q
Why are the peristaltic contractions in the fundus and body weak compared to the strong vigorous ones in the antrum?
A
This is due to the thicker smooth muscle at the antrum compared to the thin one at the fundus and body
24
Q
Where do weak mixing movements occur in the stomach?
A
The fundus and body
25
Where does strong mixing occur in the stomach?
the antrum
26
What part of the stomach typically contains only a pocket of gas rather than storing food?
The fundus
27
The strong peristaltic waves of the antrum mix food with gastric secretions to form?
chyme
28
What causes the pyloric sphincter to be almost typically, but not completely closed?
tonic contractions
29
What is the significance of tonic contraction of the pyloric sphincter ?
It allows water and other fluids to pass through into the duodenum easily but prevents large particles bigger than 2mm from passing through
30
What happens when peristaltic waves reach the pyloric sphincter?
The pyloric sphincter closes tightly forcing larger particles to move back to the body of the stomach. This antral chyme will then be pushed again by the next peristaltic wave and tumbled back creating retropulsion. This occurs until the particles are broken down enough to be able to pass through the pyloric sphincter.
31
What factors largely control gastric emptying?
factors in the duodenum
32
The greater the excitability of the smooth muscle of the antrum.............
The more frequently BER generates action potentials, the stronger the antral peristaltic contractions, and the higher the emptying rate into the duodenum.
32
What is the driving force for gastric emptying and gastric mixing?
antral peristaltic contractions
33
What are the gastric factors that affect ga stric emptying ?
(amount of chyme in stomach)
1. stomach distension triggers increased gastric motility
2. fluidity of chyme
34
What are the 4 duodenal factors that affect gastric emptying?
( it can reduce the strength of antral peristaltic waves until it is ready to accommodate the incoming chyme.)
1. Fat is the most effective nutrient in delaying gastric emptying because its digestion and absorption take the longest and occur only in the small intestine. Triglycerides in the stomach trigger the release of CCK which inhibits antral contractions and induces pyloric sphincter contraction until the small intestine can accommodate more fat.
2. Acid- highly acidic chyme from the stomach enters the duodenum where it will need to be neutralised by sodium bicarbonate released by the pancreas to prevent damage to the duodenal mucosa and inactivation of pancreatic enzymes. The presence of unneutralised acidic chyme triggers the release of secretin which slows down gastric emptying until neutralisation occurs.
3. hypertonicity- Digestion of proteins and carbs in the duodenum increases the osmolarity of duodenal contents because of the presence of amino acids and glucose molecules. High osmolarity causes intestinal distension and circulatory disturbances due to decreased plasma volumes so gastric emptying will be slowed down so absorption rates can catch up with digestion rates.
4. distension- This is caused by excessive chyme in the small intestine. This causes the slowing down of gastric emptying until the small intestine has managed the chyme and is ready to accommodate more.
35
The four duodenal factors that influence gastric emptying activate duodenal receptors triggering __________ and ___________ responses to slow down the rate of gastric emptying.
neural, hormonal
36
explain the enterogastric reflex of the neural response.
the response is mediated through the intrinsic plexus (short reflex) and the autonomic nerve( long reflex) collectively called the enterogastric reflex.
37
Explain the hormonal response that slows down gastric emptying
this involves the release of hormones called enterogastrones from the small intestine mucosa into the blood. These hormones then inhibit antral contractions to slow down gastric emptying.
38
What are the two enterogastrones released by the small intestine mucosa?
Secretin and cholecystokinin (CCK)
39
True or flase: emotions can influence gastric motility
true! this is done by autonomic nerves.
40
What is the scientific name for vomiting?
Emesis
41
define emesis
It is the forceful expulsion of gastric contents from the stomach up the esophagus and out the mouth.
42
what is the major cause of emesis?
The contraction of the diaphragm and abdominal muscles
43
What center coordinates vomiting?
The vomiting center in the brain stem medulla.
44
What is a sign of intracranial pressure?
After you bump your head and you vomit.
45
What are the 5 causes of vomiting?
1. elevated intracranial pressure- eg by cerebral hemmorage
2. Tactile stimulation- eg inserting your finger in your mouth to touch your throat
3. irritation or distension- of the stomach and or small intestines
4. motion sickness- eg moving car (acceleration/rotation of head causes dizziness)
Chemical agents- eg alcohol
5. Psychological factors- eg anxiety, nauseating sensory stimuli, emotional factors.
46
What are the 2 effects of vomiting?
1. Reduction in plasma volume due to loss of fluids that are usually reabsorbed (causing dehydration and circulation issues)
2. Metabolic alkalosis
47
define metabolic alkolosis
It is a condition characterised by the blood becoming alkaline and having a high pH caused by pH disturbances caused by loss of stomach acidic contents such as vomiting.
48
How much gastric juice is approximately secreted by the stomach daily?
2L
49
Where exactly is gastric juice produced in the stomach?
Gastric mucosal cells
50
What are the two areas that the gastric mucosa are divided into? Mention also which areas line the 3 sections of the stomach
1. Oxyntic mucosa- lining the body and fundus
2. Pyloric gland area (PGA)- lining the antrum
51
Where are the three types of gastric exocrine secretory cells found?
in the walls of the pits and glands in the oxyntic mucosa
52
What are the three gastric exocrine secretory cells?
1. Mucous cells
2. Chief cells
3. Parietal (oxyntic) cells
53
Where is the specific location and what is the secretion of the mucous cells?
Location: Line the gastric pits and gland entrances
Secretion: thin, water mucus
54
Where is the specific location and what is the secretion of chief cells?
Location: Deeper parts of the gastric glands
Secretion: Pepsinogen (precursor of pepsin)
55
Where is the specific location and what is the secretion of Parietal cells?
location: deeper parts of the gastric glands
secretion: HCl and intrinsic factor
56
What are the two cells of the gastric glands of the pyloric gland area?
1. Surface epithelial cells
2. Stem cells
57
Where is the specific location and what is the secretion of surface epithelial cells?
location: between gastric pits
secretion: sticky, alkaline mucus which forms a protective layer several millimetres thick over the mucosa
58
Where is the specific location and what is the secretion of stem cells?
location: gastric pits
secretion: Rapidly differentiate to produce new cells for the gastric mucosa. Daughter cells migrate to become surface epithelial cells or differentiate into chief cells or parietal cells, facilitating the complete replacement of the stomach mucosa approximately every three days.
59
What is the significance of gastric mucosal turnover?
To maintain the function and integrity of the gastric mucosa due to the constant exposure to the harsh acidic environment and digestive processes
60
What activates pepsinogen into pepsin?
HCl
61
When parietal cells are stimulated, what is the purpose of canaliculi (deep invaginations they form) along the luminal membrane?
they increase the surface area for transport proteins, which actively secrete HCl into the lumen of gastric pits.
62
hydrogen ions and chloride ions are actively secreted by separate ..........
pumps
63
Hydrogen ions are transported _________a steep concentration gradient making its concentration up to 3 million times higher in the lumen than in the blood
against
64
What are the four functions of HCl?
1. Activates pepsinogen
2. breaks down food
3. Protein denaturation
4. microorganism elimination
64
chloride ions are secreted by a__________ active transport mechanism against a smaller concentration gradient of 1,5 times
secondary
65
What initiates protein digestion?
Pepsinogen getting activated to pepsin
66
Where with the cytoplasm of chief cells is pepsinogen stored?
secretory vesicles called zymogen granules
67
pepsinogen is released into the gastric lumen by?
exocytosis
68
How exactly does HCl activate pepsinogen?
once pepsinogen has entered the gastric lumen, HCl cleaves a small fragment from the molecule converting it into pepsin
69
What ensures that pepsin levels increase rapidly?
because the pepsin that is available now acts on other pepsinogen molecules to produce more pepsin
70
what are the two functions of pepsin?
1. Protein digestion
2. Optimal environment
71
What is the significance of pepsin being in its inactive form until it is required for protein digestion?
pepsin only operates optimally in an acidic environment. so it is important that pepsinogen stored in the chief cells it is in the inactive form to prevent self-digestion of the chief cells' proteins.
72
What are the 3 function of the gastric mucosa mucus?
1. Mechanical protection: Lubrication
2. Protection against self-digestion: pepsin inhibition when pepsin contacts the stomach lining
3. Protection against acid injury: Alkaline nature, pH gradient
73
Intrinsic factor is essential for the absorption of?
Vitamin B12
74
What is the function of vitamin B12
it is essential for the normal functioning of the red blood cells(erythrocytes)
75
What kind of anemia occurs due to the absence of the intrinsic factor and why?
Pernicious anemia is caused by an autoimmune attack against parietal cells
76
What are the three endocrine and paracrine cells of the gastric glands? also state their locations and secretions
1. G cells: located in the pyloric gland area and secrete gastrin into the bloodstream
2. Enterochromaffin-like cells (ECL): are located among parietal cells and chief cells in the oxyntic mucosa and they secrete histamine
3. D cells: Located in the duodenum and minorly near the pylorus and they secrete somatostatin
77
Which three cells regulate the secretion of gastric digestive juices?
G cells, enterochromaffin-like cells, D cells
78
Explain the three functions of gastrin, histamine and somatostatin in regulating gastric digestive juices secretion.
1. Gastrin stimulates parietal cells and chief cells which promotes the release of HCl. It also stimulates ECL cells to secrete histamine which further stimulates parietal cells.
2. Histamine acts locally on parietal cells to enhance HCl secretion and potentiates the action of aCh and gastrin.
3. Somatostatin inhibits the secretion of HCl by acting on parietal, G, and ECL cells, providing negative feedback to reduce acid production
79
What releases Acetylcholine and what is its function?
It is released by the intrinsic nerve plexuses in response to short local reflexes and vagal stimulation. It stimulates parietal, chief, G, and ECL cells to increase HCl and pepsinogen secretions.
80
What is the effect of decreased parietal cells as we age?
less HCl, less pepsin, longer digestion, less energy
81
What three types of factors influence the rate of gastric secretion?
1. factors arising from before the food reaches the stomach
2. factors resulting from the food in the stomach
3. factors arising when the food has left the stomach and is now in the duodenum
82
What are the three phases of gastric secretion?
1. Cephalic
2. Gastric
3. Intestinal
83
Explain cephalic phase
This is where there is an increase in the secretion of HCl and pepsinogen due to the sensory input that occurs before the food is in the stomach. This increased gastric secretion is due to vagal stimulation in two ways.:
1. the vagus nerve stimulates the intrinsic nerve plexuses to promote increased secretion of ACh which leads to increased HCl and pepsinogen secretion
2. direct vagal stimulation of the G cells to induce gastrin secretion which in turn further enhances the secretion of pepsinogen and HCl, with the effect of HCl being potentiated by gastrin promoting the release of histamine
84
Explain the gastric phase
It begins when food enters the stomach. The stimuli acting on the stomach are protein, distension, caffeine and alcohol. These stimuli increase gastric secretion by overlapping efferent pathways.
85
Explain the intestinal phase
Whereas the other two phases are excitatory, this phase is inhibitory. The importance of the intestinal phase is to help shut off the flow of gastric juices and chyme begins to be emptied into the small intestine. Remember the function of somatostatin.
86
What are the three ways gastric secretion is reduced as food leaves the stomach?
1. The fact that the protein that caused increased gastric secretion is gradually being withdrawn from the stomach and into the duodenum
2. Because proteins are such great pH buffers, when food leaves the stomach the pH drops significantly. Somatostatin is released in response to the high gastric acidity and it decreases gastric secretion.
3. The same duodenal factors that inhibit gastric motility (fat, acid, hypertonicity, distension) are brought about by emptying stomach contents into the duodenum. The enterogastric reflex and enterogastrones both inhibit/ reduce antral contractions and suppress the gastric secretory cells. This inhibitory response is the intestinal phase of gastric secretion.
87
what three substances are immediately absorbed into the blood after entering the stomach?
caffeine, alcohol, apsirin
88
What protects the stomach lining against gastric secretions?
The gastric mucosa barrier
89
The lateral edges of the mucosal cells are joined by..................near the luminal borders
tight junctions
89
What causes peptiulcers?
it is caused by the gastric mucosa barrier being breached by acid which injures and damages the stomach lining.
90
Where are the three areas in the GIT where peptiulcers can occur?
The esophagus, Stomach and Duodenum
91
What are the components that make up the gastric mucosa barrier?
The mucus layer which entraps bicarbonate, tight junctions of the mucosal cells, impermeable luminal membranes, rapid mucosal turnover every 3 days.
92
In which parts of the stomach does carbohydrate digestion continue and protein digestion begin?
carbohydrate: body of stomach
protein: antrum of stomach
93
Only a small amount of alcohol is absorbed into the stomach while most of it is absorbed in the ............................ because of the ................
small intestine, greater surface area of the intestinal mucosa.
94
Why does alcohol get absorbed in the stomach?
because the substance is somewhat lipid soluble and gets absorbed by the lipid membranes of the surface epithelial cells and enters the blood via the submucosal capillaries.
