Alimentary Systems 1 - The Oesophagus and Stomach Flashcards
Define digestion
The process of breaking down macromolecules to allow absorption
Define absorption
The process of moving nutrients and water across a membrane
Define alimentary
Relating to nutrients or sustinance
List the components of the GI system, from superior to inferior
- Parotid gland
- Sublingual gland
- Submandibular gland
- Oesophagus
- Liver
- Gall bladder
- Stomach
- Pancreas
- Small Intestine
- Colon
- Appendix
- Rectum
Describe the layers in the basic gut wall
- Epithelium
- Lamina propria (loose connective tissue)
- Muscularis muscosae
- Submucosa (connective tissue containing nerve plexus)
- Muscularis (smooth muscle containing nerve plexus)
- Serosa/adventitia (connective tissue with/without epithelium)
At which spinal level does the oesophagus start and end
- Starts at C6
- Ends at T10 where it pierces the diaphragm
What is the function of the oesophagus, and how does its structure relate?
- Conduit for food, drink and swallowed secretions from pharynx to stomach
- Non keratinising (wear/tear lining due to extremes of temperature/texture)
- Lubrication (mucus secretion)
What are the sphincters of the oesophagus?
- Upper oesophageal sphincter is skeletal muscle, it opens when we swallow
- Lower oesophageal centre is skeletal and smooth muscle, the swallowing centre of the brain has a smaller effect on this muscle
- Both are tonically active
What muscles are present in the wall of the oesophagus?
- Circular muscles, which constrict like the shutter of a camera
- Longitudinal muscles, which are the outer muscular layer of the oesophagus
What is peristalsis in the oesophagus, and which muscles are involved?
Contraction of the circular muscle which moves the food bolus down the oesophagus
Name the two muscles in the superior oesophageal sphincter
- Constrictor pharyngeal medius - which has commonality with the circular muscle layer of the GI tract
- Constrictor pharynges inferior - which has commonality with the longitudinal muscle layer
Where is the gastro-oesophageal junction, and what is its significance?
- The diaphragm constricts the oesophagus as it transitions to the stomach
- Stratified squamous becomes simple columnar
- Prevents reflux
- Also called the z-line
Why are gastric folds present in the stomach, and what is another name for them?
- Also called rugae
- They allow the stomach to contract and become smaller when food has been digested and in between meals
List the functions of the stomach
- Break food down into smaller particles
- Hold food and release it at a controlled steady rate into the duodenum
- Kill parasites and harmful bacteria
List the regions of the stomach and what they produce.
- Cardiac and pyloric region produce mucus only. These regions are at the entry and exits to the stomach
- Body and fundus, the main parts of the stomach, produce mucus, HCl and pepsinogen
- The antrum produces gastrin
What volume of acid is produced in a day?
- 2L/day
What is the concentration of acid in the stomach?
150mM, 3 million times higher than that in the blood
How do mucins protect the epithelial surface?
- Act as a gel coating with HCO3- trapped. This can quickly neutralise any acid
- The pH at the epitelial surface is 6-7, compared with 1-2 in the lumen
Describe the function of peristalsis in the stomach, as well as its control.
- Propels chyme towards the colon, as well as mixing the chyme
- Controlled by the ANS
- Makes up 20% of stomach contraction
Describe the function of segmentation in the stomach, as well as its control
- Weaker more random contractions
- Thoroughly mix chyme with stomach secretions.
- Pushes fluid chyme towards the pyloric sphincter, and solid food towards the fundus
- Makes up 80% of stomach contractions
- Activated by stretch, involving the enteric NS
List the functions and structure of chief cells in the stomach
- Secrete pepsinogen
- Abundant RER and golgi packaging and modifying
- Masses of apical secretion granules
Describe the structure of resting parietal cells
- Many mitochondria
- Cytoplasmic tubulovesicles (containing H+/K+ ATPase)
- Internal canaliculi, which act as reservoirs
Describe the structure of secreting parietal cells
Tubulovesicles fuse with the membrane, and microvilli project into canaliculi
Describe the process that occurs in an activated parietal cell
- Increased activity of H+/K+ ATPase
- Carbonic anhydrase generates hydrogen ions and bicarbonate
- Sodium potassium pump pumps potassium ions into the blood for transport of H+ into the cannaliculi
- Bicarbonate ions and chloride ions are exchanged, and the chloride ions enter the cannaliculi
- HCl is released into the stomach lumen
How is pepsinogen cleaved to active pepsin?
By the low pH of the stomach
How does gastrin increase acid secretion from the stomach?
- Produced in the pyloric antrum
- Stimulates histamine release from chromaffin cells
- Histamine triggers acid production by the parietal cells
List the phases of gastric secretion
- Cephalic phase
- Gastric phase
- Intestinal phase
What is the cephalic phase of gastrin secretion and how is it controlled?
- Thought, smell, site and taste of food
- Controlled by vagus stimulation (ACh)
What is the gastric phase of gastrin secretion and how is it controlled?
- Triggered by stretch and chemo receptors due to food in stomach
- Controlled by vagus stimulation and ACh
What is the intestinal phase of gastrin secretion and how is it controlled?
- Gastrin release stimulated by high protein concentration in the duodenum (suggests stomach hasnt broken down chyme enough)
- Inhibitory if protein concentration is low, if the pH is less than 2 or if there is high lipid conc.
- Uses gastric inhibitory peptide, cholescystokinin and secretin to inhibit, as well as the enterogastric reflex
List two drugs used to reduce acid secretion in the stomach and their site of action
- Omeprazole acts on the K+/H+ ATPase in the cannaliculi membrane of the parietal cell
- Ranitidine acts on the histamine receptor of the parietal cell
List the stages of swallowing
Stage 0: The oral phase. Chewing and saliva help to prepare the bolus for swallowing. Both oesophageal sphincters are constricted.
Stage 1: The Pharyngeal phase. As the food bolus moves to the back of the pharynx the pharyngeal musculature helps to guide it towards the oesophagus. Both oesophageal sphincters open.
Stage 2: Upper oesophageal phase. The upper sphincter closes, and superior rings of circular muscle contract as inferior rings dilate. Sequential contractions of longitudinal muscle help guide the food down the gullet.
Stage 3: Lower oesophageal phase. As food passes through the lower sphincter that too closes, and the peristaltic wave continues to push food into the stomach.
What is the function of the G cells?
- These are enteroendocrine cells found at the bottom of the gastric pits.
- Produces gastrin
What is the function of the enterochromaffin-like cells?
- These cells are a type of neuroendocrine cell found deep in the gastric glands, usually in the vicinity of parietal cells.
- They secrete histamine which stimulates the secretion of acid from the parietal cells.
What is the function of D-cells?
- These enteroendocrine cells secrete somatostatin, which has a generally inhibitory effect on gastrointestinal function.
- Within the gastric gland somatostatin inhibits ECL production of histamine and parietal cell activity, both of which inhibit the secretion of hydrochloric acid.
What is the function of gastric stem cells?
- These pluripotent cells are capable of differentiating into all of the different cells of the stomach, under the influence of different factors.
- The development pathway varies for different parts of the stomach.
What drugs are used to treat acid reflux?
- Histamine 2 receptor blockers
- Proton pump inhibitors
