Session 4 Flashcards
Describe the route of hormones acting on the stomach
Peptides released from endocrine cells
Into portal circulation
Pass through liver
Enter systemic circulation
End up pretty close to where they were
Define paracrine. Give example for stomach:
Paracrine - Relating to or denoting a hormone which has effect only in the vicinity of the gland secreting it.
Peptides released by endocrine cells
Act in local environment
Diffuse short distances E.g. somatostatin
Define Neurocrine. Give an example for the stomach
Peptides released by neurones in the GI tract
Released after action potential
Example: Gastrin releasing peptide (GRP)
GRP is released from the postganglionic fibres of the vagus nerve.
Increases release of Gastrin from G cells
What are the two broad categories of gastrointestinal hormones?
Gastrin family:
- Gastrin
G cells in antrum of stomach
Increases gastric acid secretion
- Cholecystokinin (CCK)
I cells in duodenum and jejunum
Increases pancreatic/gallbladder secretions by causing gall bladder contraction and stimulates the pancreas.
CCK is stimulated by fat and protein entering into the duodenum.
Bile used for emulsification/breakdown of fat and then enzymes from pancreas to break down proteins.
Secretin family:
- Secretin
S cells in the duodenum
Stimulated by H+ and fatty acids
Increases HCO3 from pancreas/gallbladder
Decreases gastric acid secretion
Chyme is very acidic so needs to be neutralised.
- Gastric inhibitory polypeptide (GIP)
Cells in the duodenum and jejunum
Stimulated by sugars, amino acids and fatty acids
Increases insulin
Decreased gastric acid secretion
What are the basic functions of the stomach?
Receive food - Short term storage facility - We eat faster than we can digest so short term storage is needed - Folds called rugae allow expansion of the stomach during eating and then contraction to avoid feeling of distension when not eating.
Disrupt food - 3 layers of smooth muscle which contract vigorously. Gives food larger surface area.
Continue/commence digestion - Mainly proteins through enzymes
Disinfect - acidic conditions guard against pathogens.
Describe the basic structure of the stomach. Describe the lining of the stomach.
From lower oesophagus to stomach:
Abrupt transition of stratified squamous to columnar - This is about secretion
Mucosa/submucosa thrown into non-permanent folds called Rugae.
Lots of little holes on its surface called gastric pits
Epithelial cells cover surface and extend into pits/glands
Mucous cells - secrete mucus - line stomach.
In the gastric pits:
Parietal cells - produce H
Chief cells - protease release
G cells -gastrin release
Stomach smooth muscle:
Extra oblique layer of muscle allows more vigorous contractions in order to mix/grind contents then move them along
Upper stomach - wall thinner - musclular wall thickens as we move inferiorly in the stomach.
Has sustained contractions
Creates basal tone
Lower stomach
Strong peristalsis mixes stomach contents
Coordinated movements
Contractions every 20 seconds (or so)
Proximal to distal
Shape of stomach:
Larger (proximal) to smaller (distal)
Contents accelerates
Lumps left behind (separates contents)
Liquid chyme ejected into duodenum
3 times a minute
What do Brunners glands do?
From the stomach, although still columnar going into the duodenum, when we dump chime from the stomach into the duodenum, it is still very acidic and hyper osmotic, so the body needs to make it isotonic and more neutral. Brunners glands are important to allow water to enter the duodenum.
What is the blood supply and drainage of the stomach and other organs around it? describe the route (draw)
Coeliac trunk arises from abdominal aorta at around and from it has three main branches: the splenic artery (Looks very wiggly), left gastric artery and common hepatic artery.
Left gastric artery anastomoses with the right gastric artery. These work to supply the lesser curve of the stomach.
Splenic runs posterior to the stomach and from this branches the left gastroepiploic artery which; together with the right gastroepiploic artery which branches from the gastroduodenal artery, supplies the greater curve of the stomach
Common hepatic gives off the right gastric artery and then the gastroduodenal artery which runs posterior to the duodenum.
Gastroduodenal artery is a massive haemorrhage risk if a peptic ulcer perforates near it.
After Gastroduodenal artery branches from common hepatic artery, the common hepatic artery is renamed the proper hepatic artery.
All veins from gut drain into the portal vein via correspondingly named veins e.g. Lesser curve of stomach supplied by left and right gastric arteries and drains via left and right gastric veins into the portal vein.
How does stomach act as storage?
We consume food faster than we can digest so we need to store it. Achieved through receptive relaxation
Vagally mediated relaxation of stomach
Allows food to enter stomach without raising intra-gastric pressure too much
Prevents reflux of stomach contents during swallow
Gastric mucosal folds (rugae) allow distension
How is digestion continued in the stomach?
Acidic conditions:
◦ Denatures proteins to make surface area bigger so its more easily broken down.
◦ Activates proteases (kept inactive to avoid self digestion) acidic conditions activate them. Pepsinogen to Pepsin
◦ Disinfect stomach contents
What does the stomach secrete?
HCL - Gives acidic conditions
Intrinsic factor - involved in VitB12 absorption.
Mucus/HCO3^- - Mucus coats stomach surface, bicarbonate secreted into mucus makes mucus an alkaline protective layer.
Pepsinogen (pepsin) for protein breakdown
Histamine - stimulates gastric acid secretion Somatostatin - inhibits G cells
How are cell types distributed across the stomach
Cardia: Predominantly mucus secretion
Fundus/body: Mucus, HCL, pepsinogen
Pylorus: Gastrin, somatostatin
However, lots of crossover
How is HCL production controlled in the stomach?
Parietal cell - produces H+ - Also called oxyntic cell
Stimulated by:
◦ Gastrin - acts on CCK receptor
◦Histamine - acts on H2 receptor to amplify gastrin function
◦ Ach - Ach from Vagus nerve acts on muscarinic receptors
Control of Gastrin secretion
G cells located mainly in Antrum
G cell stimulated by:
◦ Peptides/amino acids in stomach lumen
◦ Vagal stimulation - Acetylcholine (thought/sight/smell of food)
◦ Gastrin-releasing peptide (GRP)
Inhibition of HCL production
Essentially inhibition of G cells
◦ When food leaves stomach pH drops as presence of food acts as a buffer for the acid
◦ Low pH activates D cells
◦ D cells release somatostatin
◦ Somatostatin inhibits G cells (and ECL cells)
◦ Stomach distension reduces from reduced vagal activity
Summarise control of stomach secretions
How is HCL produced in the parietal cells?
Pumps acid against large gradient through H+/K+ ATPase and the result is H+ is pumped into the lumen.
Starts off by splitting water inside the cell into OH- and H+. The OH- can combine with CO2 to make bicarbonate ions (HCO3-) through carbonic anhydrase.
So bicarbonate being pumped out of the cell through the basolateral membrane (in exchange for chloride ions) and H+ out of the apical membrane (chloride ions also leave the cell through chloride channels).
Alkaline tide - When we have a meal and we have to produce lots of acid we make the venous blood leaving the stomach more alkaline which goes down and is used later on to neutralise the acid later in the GI tract.
Describe the phases of digestion
Cephalic (30% of total HCL)
◦ Parasympathetic stimuli
◦ Smelling, tasting, chewing, swallowing
◦ Direct stimulation of parietal cell by vagus nerve
◦ Stimulation of G cells by vagus (GRP released) ‘Anticipating food’
◦ Also increases gastric motility (slightly)
Gastric (60% of total HCL)
◦ Distension of stomach stimulates vagus which then stimulates parietal cells and G cells
◦ Presence of amino acids and small peptides - Stimulate G cells
◦ Food acts as a buffer in stomach - removes inhibition on Gastrin production
Enteric NS and Gastrin cause strong smooth muscle contractions
Intestinal (10% of HCL production)
◦ Chyme initially stimulates Gastrin secretion
◦ Partially digested proteins detected in duodenum
◦ Short phase
◦ Soon overtaken by inhibition of G cells
◦ Presence of lipids activate enterogastric reflex - Reduces vagal stimulation
◦ Chyme stimulates CCK and secretin (help suppress secretion)
How does the stomach protect itself from digestion?
Mucin is secreted by the foveolar cells
- Forms viscous mucus layer that adheres to epithelium and helps prevent physical damage from food
- HCO3 ions are secreted into mucus layer to provide a (pH neutral) barrier against the stomach acid
- Rich blood supply to gastric mucosa can remove and buffer acid that has breached mucus layer
- Prostaglandins are released that promote the above processes
- NSAIDS decrease prostaglandin release
High turnover of epithelial cells
◦ Helps keep epithelia intact
What is GORD? Triggers? Symptoms? Consequences? Treatment?
Gastro-oesophageal reflux disease
Symptoms: Chest pain, acid taste in mouth, cough
Consequences of GORD: •Nothing •Oesophagitis •Strictures •Barrett’s oesophagus
Triggers include: • Obesity (increased intra-abdominal pressure) • Pregnancy (increased intra-abdominal pressure) • Hiatus hernia • LOS function • Delayed Gastric emptying
Treatment
◦Lifestyle modifications (smaller meals, don’t eat before bed, lose weight, prop self up a bit with pillows when sleeping)
◦Pharmacological: Antacids H2 antagonists Proton pump inhibitors
◦Surgery (fundoplication - It involves wrapping the fundus of the stomach around your lower oesophagus - can cause dysphagia so rare)
Describe the differences between duodenal ulcers and gastric ulcers
What are the different pathological features of gastritis?
