GI: Functions of the stomach Flashcards
What are the functions of the stomach?
- short term storage (receptive relaxation prevents increased pressure and therefore prevents reflux)
- disrupts food through vigorous contractions
- digestion of proteins (acidic conditions activate proteases)
- disinfects
Describe the structure of the stomach?
Where the oesophagus enters the stomach is the cardia, with a small cardial notch leading to the fundus (top part)
Then there is the main body leading to the antrum and the pylorus.
There is a lesser and greater curve
What type of epithelia is present in the stomach?
From the lower oesophagus to the stomach there is an abrupt change of epithelia from stratified squamous to columnar.
What are the gastric pits?
Lots of little holes in the stomach wall which form the entrance to the gastric glands.
There are mucous, parietal, chief and G cells
Why is the stomach larger proximally than distally?
Accelerates the contents so liquid chyme ejected into duodenum
Separates the contents so that lumps are left behind
What does each cell type secrete? Parietal cells G cells Chief cells D cells Mucous cells
Parietal cells - HCL and intrinsic factor G cells - Gastrin Chief cells - pesinogen D cells - somatostatin Mucous cells - mucus
What are some stimulants of parietal cells? therefore causing increased HCL production
Gastrin, histamine, Ach
How is gastrin secretion controlled?
G cells are stimulated by peptides in the stomach and vagal stimulatin - Ach and gastrin-releasing peptide (GRP)
G cells are inhibited by a drop in pH (eg when food leaves the stomach) because low pH activates D cells which release somatostatin which inhibits G cells
Outline the production of HCL in the stomach
Water splits into OH- and H+
- the OH- combines with CO2 to form HCO3- which is moved into blood stream via HCO3- Cl- antiporter
- the H+ and Cl- from above are transported into the stomach lumen where they combine to form HCL
What are the 3 phases of digestion?
Cephalic, gastic and intestinal
Outline the cephalic phase of digestion
Accounts for 30% of HCl
Comprises smelling, tasting, chewing, swallowing
Parietal cells and G cells stimulated by vagus nerve
(anticipating food also increases gastric motility slightly)
Outline the gastric phase of digestion
Accounts for 60% of HCl
Distension of the stomach stimulates vagus which then stimulated parietal and G cells
Presence of AA stimulates G cells
Food acts as a buffer in the stomach so removed gastrin inhibition
Outline the intestinal phase of digestion
Accounts for 10% of HCl
Chyme initially stimulates gastrin secretion then this is soon overtaken by inhibition of G cells (reduced vagal stimulation) due to presence of lipids
How does the stomach protect itself from being digested?
- mucus and HCO3- which are released by surface mucus cells and neck cells in the gastric pits.
They form a thick alkaline layer which adheres to epithelia to keep it at a higher pH - high turnover of epithelial cells
- prostaglandins help maintain mucosal blood flow
What can breach stomach defences against acid?
Alcohol dissolves the mucus layer
H. pylori causes chronic gastritis
NSAIDs inhibit prostaglandin
