Session 5 - Function Of The Stomach Flashcards
What are the basic functions of the stomach?
Receive food - short term storage facility
Disrupt food - vigorous contractions of smooth muscle
Continue digestion - mainly of proteins
Disinfect
What cells are found in gastric pits?
Mucous cells
Parietal cells
Chief cells
G cells
Describe the action of smooth muscle in the upper and lower stomach.
Upper stomach:
- sustained contractions
- creates basal tone
Lower stomach:
- strong peristalsis mixes stomach contents
- coordinated movements (contractions every 20 seconds or so, proximal to distal)
The shape of the stomach is such that it is larger proximally than it is distally. What is the purpose of this?
Accelerates contents Separates contents (lumps left behind) Only liquid chyme ejected into duodenum
What mechanism allows food to be stored in the stomach without raising intra-gastric pressure.
Receptive relaxation:
- vagally mediated relaxation of stomach
- food can enter stomach without increasing pressure too much
- prevents reflux of stomach contents during swallowing
- rugae allow distension
How do the acidic conditions of the stomach help in digestion of proteins?
Acid denatures proteins
Activates proteases
Pepsinogen —> Pepsin
What products are secreted into the stomach lumen?
HCl
Intrinsic factor
Mucus/HCO3-
Pepsinogen (pepsin)
What do parietal cells secrete?
HCl and intrinsic factor
What do G cells secrete?
Gastrin
What do enterochromaffin like (ECL) cells secrete?
Histamine
What do chief cells secrete?
Pepsinogen
What do D cells secrete?
Somatostatin
Describe the general cell distribution across the stomach.
Cardia:
- predominantly mucus cells
Fundus/body:
- mucus cells
- parietal cells (HCl)
- chief cells (pepsinogen)
Pylorus:
- G cells (gastrin)
- D cells (somatostatin)
What things stimulate parietal cells to secrete HCl?
Gastrin (hormone released by G cells)
Histamine (paracrine signalling molecule, released by ECL cells)
ACh (neurotransmitter released by the vagus nerve)
What receptor does gastrin bind to?
CCK receptor
What stimulates G cells to release gastrin?
Peptides/amino acids in the stomach
Vagal stimulation - acetylcholine and gastrin-releasing peptide (GRP)
How does somatostatin inhibit acid secretion?
Somatostatin acts on G cells, inhibiting their release of gastrin. This therefore inhibits HCl release as there is less gastrin to act on parietal cells.
How is HCl production by parietal cells inhibited?
When food leaves the stomach pH drops (food acts as a buffer).
Low pH activates D cells, these release somatostatin which inhibits G cells and ECL cells (less gastrin and histamine).
Reduced stomach distension due to less food causes reduced vagal activity.
What are the phases of digestion?
Cephalic
Gastric
Intestinal
What happens in the cephalic stage of digestion?
During the cephalic stage of digestion, gastric secretion increases before food even enters the stomach. It results from smelling, tasting, chewing and swallowing causing parasympathetic stimulation to parietal cells and G cell via the vagus nerve. This ‘anticipation of food’ also causes a slight increase in gastric motility.
What happens in the gastric phase of digestion?
Distension of the stomach stimulates the vagus nerve
- this stimulates parietal cells and g cells
Presence of amino acids and small peptides
- stimulates G cells
Food acts as a buffer in the stomach
- removes inhibition on G cells
Enteric nervous system and gastrin cause strong smooth muscle contractions to occur
What happens in the intestinal stage of digestion?
Chyme initially stimulates gastrin secretion.
Soon overtaken by the inhibition of G cells
- presence of lipids activates the enterogastric reflex —> reduces vagal stimulation
- chyme stimulates CCK and secretin
In what ways does the stomach defend itself from digestion?
Mucus/HCO3- release Released by - surface mucus cells - neck cells in gastric glands Forms thick alkaline viscous layer that adheres to the epithelium, therefore the epithelial surface is kept at a higher pH.
High turnover of epithelial cells
- helps keep epithelia intact
Prostaglandins
- maintain mucosal blood flow, supplying the epithelium with nutrients
How do non-steroidal anti inflammatory drugs (NSAIDs) such as aspirin increase the risk of peptic ulcers?
NSAIDs inhibit prostaglandin release, reducing blood flow to gastric epithelium meaning fewer nutrients reach epithelial cells and there is a reduced turnover.
