TOP MidSem (A2) Flashcards
How do Ions & Water move through the Salivary Gland?
Salivon contains sodium, potassium, bicarbonate & chloride ions. HCO3- & K+ enter through channels, increasing concentration. Na+ & Cl- move out, decreased conc.
How much acid per day do parietal cells secrete?
roughly 2 L / day in the form of hydrochloric acid.
What is the function of acid in the stomach?
- kill bacteria
- aid digestion by solubilizing food
- important to establish the optimal pH (between 1.8-3.5 for the function of the digestive enzyme pepsin
What is the key protein for acid secretion?
H+/K+ ATPase (or proton pump), expressed on apical membrane of parietal cells. Uses energy derived from ATP hydrolysis to pump hydrogen ions into the lumen in exchange for potassium ions.
Describe the differences regarding H/K ATPase between the resting and acid-secreting states of parietal cells.
When the cell is resting (not stimulated), H+/K+ ATPases are located in tubulovesicles inside the cell. When the cell is stimulated, these vesicles fuse with the plasma membrane to form canaliculi, therefore increasing the surface area of the plasma membrane & the no. of proton pumps in the membrane.
Describe the Cephalic phase of acid secretion.
30% of acid secretion. Stimuli are things like the sight, smell, taste or thought of food; neural processing and activation of enteric neurons via PSNS preganglionic neurons (Vagus).
Release of ACh; positive influence on parietal, ECL and G cells.
Describe the Gastric phase of acid secretion.
50-60% of acid secretion. Primary factor is food present in stomach, which stimulates acid secretion, in 3 different ways:
- mechanoreceptors (stretch): activates neural reflex to stimulate acid secretion
- peptides & amino acids: stimulate G cells (gastrin)
- food as a buffer: raises pH to remove the stimulus for somatostatin secretion
Describe the Intestinal phase of acid secretion.
10-20% of acid secretion. Activation of negative feedback mechanisms reduce acid secretion once chyme enters the duodenum; prevents chyme from becoming too acidic.
Via both neuronal & hormonal reflexes.
“Enterogastrones”: hormones that inhibit stomach processes (e.g. acid secretion). CCK, secretin, GLP-1 and GIP act as enterogastrones.
What are some pharmacological treatments to excessive/lacking acid secretions in the gut?
Atropine - blocks actions of ACh
Ranitidine/Cimetidine - blocks H2 receptor
Omeprazole - blocks action of proton pump (most potent)
Pentagastrin - synthetic form of gastrin; stimulates CCK_B
Caffeine - blocks action of phosphodiesterase (cAMP breakdown)
If one has excess gastrin secretions, will they have…
- decreased acid secretion?
- damage to the intestinal mucosa?
- duodenal ulcer/s?
- steatorrhoea (fatty stools)?
- hypertrophy of gastric mucosa?
- vitamin B12 deficiency?
- NO: increased gastrin stimulates parietal cells indirectly to secrete HCl
2,3. YES: Increased acid production may overwhelm the bicarbonate rich secretions of the pancreas, liver and Brunner’s glands so contents remain acidic & erosive. - YES: Pancreatic lipases don’t work well in acidic environments and if the gastric contents aren’t neutralized in the duodenum the lipases may not be able to digest fats which then appear in the stools.
- YES: Gastrin effect on CCK2 receptors on parietal cells is trophic so would expect hypertrophy
- YES/NO/Debatable: Vitamin B12 deficiency occurs with decreased acid secretion, however the cobalamin/haptocorrin complex requires digestion by the pancreatic enzyes which may be inactivated in an acidic duodenum.
The cephalic phase of acid secretion control is:
a) mostly enteroendocrine mediated
b) vagally mediated
c) part vago-vagal and part enteroendocrine mediated
d) mediated by the enteric nervous system
b) vagally mediated
The cephalic phase of acid secretion is that stimulated by the thought, smell etc. of food and therefore can’t be due to stimulation of luminal receptors as no food has yet been consumed - therefore is mediated by the parasympathetic nervous system through the vagus.
Which of the following is NOT a function of saliva?
a) begins digestion of carbohydrate
b) facilitates absorption of glucose across the oral mucosa
c) facilitates speech
d) exerts an antibacterial effect
b) facilitates absorption of glucose across the oral mucosa.
glucose is absorbed in the SI - all others are functions of saliva.
A subject suffers diarrhoea after an intestinal perfusion with a 400mM Mannitol solution. Mannitol is not absorbed across the intestinal wall. The diarrhoea most likely develops because Mannitol:
a) decreases intestinal motility
b) causes an osmotic gradient for fluid secretion into the lumen
c) stimulates luminal receptors which increase cAMP levels
d) stimulates the insertion of chloride channels in the luminal membrane
b) causes an osmotic gradient for fluid secretion into the lumen
a) is incorrect as a decr. in intestinal motility normally incr’s (not decr’s) absorption and the presence of mannitol may incr. motility due to the distension caused as fluid moves into the gut in response to the osmotic load. C&D are mechanisms for incr’d fluid secretion in the intestines *e.g. in Cholera) but aren’t the mechanism used here. The hyperosmotic solution draws fluid into the intestines and increases fluid content of the stools leading to diarrhoea.
What is the major role of the hormone SECRETIN?
a) stimulates bicarbonate secretion from the pancreas
b) stimulates gastric acid secretion
c) stimulates contraction of the gall bladder
d) stimulates the production of digestive enzymes in the pancreas
a) stimulates bicarbonate secretion from the pancreas
Secretin is Nature’s antacid and stimulates the production of bicarbonate (not digestive enzymes) from the pancreas. It neutralizes the acidity of the chyme from the stomach, so doesn’t stimulate gastric acid secretion. CCK stimulates contraction of the gall bladder and release of digestive enzymes from the pancreas in response to duodenal amino-acids/proteins & fats.
Which of the following statements is INCORRECT?
a) pepsin is released from the chief cells in its inactive form thereby protecting these cells from being digested by the enzyme
b) secretion of bicarbonate ions from the surface epithelial and mucous neck cells neutralises acidic chyme before it enters the duodenum
c) renewal of the stomach mucosa is possible because of the presence of stem cells located in the gastric pits that are then able to form specialized epithelial cells
d) mucus and bicarbonate ion secretions from the gastric mucosa protect the lining of the stomach from degradation
b) secretion of bicarbonate ions from the surface epithelial and mucous neck cells neutralises acidic chyme before it enters the duodenum
All are correct except B. Acidic chyme leaves the stomach and is neutralized in the duodenum. It’s not neutralized in the stomach.
Pernicious anaemia can occur when:
a) the stomach has been bypassed or removed
b) the terminal ileum has been removed
c) there is a deficiency of intrinsic factor
d) all of the above occur
d) all of the above occur
The migrating myoelectric complex or MMC is:
a) initiated by the presence of food
b) thought to be initiated by a secretion of the M cells
c) thought to cause mass movements in the colon
d) part of the defaecation reflex
b) thought to be initiated by a secretion of the M cells
The MMC is thought to be mediated by Motilin which is secreted by the ‘M’ cells.
Which of the following breakfasts would be the slowest to empty from the stomach?
a) cereal and skim milk
b) fried eggs, bacon and hash browns
c) boiled egg, toast and juice
d) black coffee
b) fried eggs, bacon and hash browns
Solid meals with a high fat content will empty more slowly than liquid meals and those with a lower fat content.
Which of the following statements about parasympathetic innervation of the digestive system is INCORRECT?
a) parasympathetic stimulation is usually stimulatory to the digestive system
b) parasympathetic stimulation increases salivary, gastric, pancreatic and biliary secretion
c) parasympathetic innervation is part of the extrinsic nerve supply to the digestive tract
d) parasympathetic stimulation of the salivary glands produces a saliva rich in mucus
d) parasympathetic stimulation of the salivary glands produces a saliva rich in mucus
Both the sympathetic & parasympathetic nervous systems stimulate salivary secretion. SNS stimulation induces secretion of a thick mucous saliva while PSNS stimulation induces secretion of a high volume water saliva.
Fluid absorption in the jejunum occurs:
a) within the crypt cells of the epithelium
b) due to the secondary active transport of sodium and amino acids and sugars across the basolateral membrane of the cell
c) in response to the insertion of cAMP stimulated Chloride channels on the apical surface
d) to a far greater degree than fluid secretion
d) to a far greater degree than fluid secretion
The SI secretes the “succus entericus” - the “soup of the intestines” - in the order of ~1L/day. However the SI absorbs a far greater amount - ~7L/day. Absorption takes place in the VILLUS (not crypt) cells, primarily by the co-transport of sodium with sugars/amino acids across the LUMINAL (not basolateral) membrane. The insertion of chloride channels on the apical membrane is associated with SECRETION of fluid into the gut lumen (not absorption of fluid from the gut).
