GI: Introduction Flashcards
What are the implications of the external environment?
- You have to ingest food, digest food, absorb food and egest what not needed
- Stop toxin/infection entering
- Very thin epithelium
- Need water in gut lumen for chemical reactions and cant lose to external world
What are the waste products that are not ingested in the gut?
- Bilirubin
- Cholesterol
What are the areas of mechanical disruptions in the GI tract?
- Mouth/Teeth
- Stomach
What are the muscular actions of the stomach?
- Vigorous contractions of the stomach cause food to be liquefied.
- Upper area create basal tone (tonic)
- Lower area has powerful peristaltic contractions that effectively grind food and mix stomach contents. Every 20 seconds proximal to distal
- Has additional inner oblique layer of muscle
How does the stomach resist rise in intraluminal pressure?
Eat quicker that digest
- Stomach can distend due to rugae (temporary folds)
- Receptive relaxation occurs to allow food to enter stomach without raising intragastric pressure to much and prevents reflux of stomach content during swallow
- Vagally stimulated relaxation
What is the purpose of the colon?
- Contents are only evacuated several times a day
- Acts as a temporary storage
- Gastrocolic reflex
- Mass movements to rectum which is normally empty
- Final water absorption
- Final electrolyte absorption
What are the contents of the stomach for chemical digestion?
- Acid
- Pepsin
What are the defences of the GI tract?
- Saliva
- HCl
- Liver (kupffer cells)
- Peyer’s Patches (Lymphoid follicles, Submucosa, mainly in terminal ileum)
What are broad functions of the oesophagus?
- Rapid transport of bolus to stomach through thorax
- Upper oesophageal sphincter prevents air from entering GI tract
- Lower oesophageal sphincter prevents reflux into the oesaphagus
What are broad functions of the the stomach?
- Storage facility
- Produce chyme
- Infections control (HCL)
- Secrete intrinsic factor (Vit B12)
What are broad functions of the the stomach?
- Storage facility (receptive relaxation)
- Produce chyme
- Infections control (HCL)
- Secrete intrinsic factor (Vit B12)
What are the broad functions of the duodenum?
- Start of small intestine
- Neutralisation/osmotic stabilisation of chyme (HCO3 secretions)
- Digestion wrapping up (pancreatic secretions, bile)
What are broad functions of Jejunum/ileum?
- Final digestion
- Nutrient absorption mainly in the jejunum
- Water/electrolyte absorption mainly in ileum
- Bile recirculation in ileum
- B12 absorption in the terminal ileum
What is the structure of the peritoneum?
- Parietal peritoneum in contact with abdomen
- Visceral in contact with organs
- Space between parietal and visceral peritoneum with fluid
How is the gut controlled?
- Autonomic nervous system
- Enteric nervous system
- Hormones and paracrine secretions
What presynaptic nerves formed by the sympathetic nervous system to supply the Gut?
- Greater splanchnic nerve (T5-T9)
- Lesser splanchnic nerve (T10-T11)
- Least (T12)
What is the purpose of the splanchnic nerves?
-Synapse with pre-vertebral ganglia
(Coeliac, Renal, superior mesenteric, inferior mesenteric and others)
-Mainly innervate blood vessels
-Generally inhibits GI function
-Post ganglionic fibres extend to myenteric and submucosal plexus and release norepinephrine
-Reduces motility
What are the nerves from the parasymapthic nervous system to the gut?
- Vagus nerve
- Pelvic splachnic nerves (S2-S4)
What does the right and left vagus become in the gut?
- Right vagus becomes posterior vagal trunk
- Left vagus becomes anterior vagal trunk
What is the functions of the parasymapathetic system on the gut?
- Pre ganglionic fibres (long) synapse in walls of the viscera
- Post ganglionic fibres (short) release Acetylcholine and peptides (GIP and VIP)
- Innervate smooth muscle/endocrine and secretory
Which parts of the GI tract does the Pelvic nerve innervate?
-Transverse colon to Anal canal
Which parts of the GI tract does the vagus nerve innervate?
-Oesaphagus to Transverse colon
What are the features of the enteric nervous system?
- Divisions of the nervous system
- Can operate completely independently
- Exists from the oesophagus to anus
- It has 2 main plexuses: Submucosal and Myenteric
What is another name for the submucosal plexus?
-Meissner’s Plexus
What is another name for the myenteric plexus?
-Auerbach’s Plexus
What is the function of the submucosal and myenteric plexuses?
Submucosal
- Secretions
- Blood flow
Myenteric
-Motility
What is the route that hormones take in the GI tract?
- Released from endocrine cells
- Pass into portal circulation
- Pass through the liver
- Enter systemic circulation to end up close to their release point
How is H+ production inhibited?
Somatostatin
- Released by D cells in antrum of stomach and pancreas
- Stimulated by H+ (low pH) on stomach lumen. Food is buffer so when it leaves the stomach pH drops
- Inhibits G cells
- Stomach distension reduces due to reduced vagal activity
- Inhibits histamine release
What is the action of Gastrin in the gut?
- Acts on G cells in antrum of stomach
- Increase gastric acid secretion
What is the action of cholecystokinin?
- I cells in duodenum and jejunum
- Increases pancreatic/gallbladder secretions
What is the control of the release of CCK?
- Stimulated by fat and protein
- Gall bladder contracts
- Pancreas stimulated
What is the action of secretin?
- Increases HCO3 from pancreas/gallbladder
- Decrease gastric acid secretion
What is the control of release of secretin?
- Release stimulated by H+ and fatty acids
- Released from S cells in the duodenum
What is the action of GIP?
- Increases insulin
- Decreases gastric acid secretion
What is the control of GIP release?
- Cells in the duodenum and jejunum
- Stimulated by sugars, amino acids and fatty acids
Why does appendicitis present with central abdominal pain initially?
- Visceral peritoneum involved
- Visceral afferents accompany sympathetic motor fibres back to spinal sensory ganglia
Why does pain localise to the Right iliac fossa (suprapubic region)?
- Involvement of the parietal peritoneum
- Due to somatic sensory pain
- Pain is localised
Where can visceral pain from foregut, midgut and handout structures be felt?
Foregut - Epigastric
Midgut - Periumbilical
Hindgut - Suprapubic/hypogastric
Which muscles in the GI tract are not smooth muscle and instead skeletal muscle?
- Pharynx
- Upper 1/3 of oesaphagus
- External anal sphincter (voluntary control)
What are the types of motility that occur in the GI tract?
Peristalsis
- Contraction proximal to contents and relaxation distal
- Propels contents in one direction
Segmentation
- Contraction splits contents then releaxes
- To and fro movement that mix contents
Mass movement
- Occurs in distal colon
- Rapid movement of contents into rectum
- Gastrocolic reflex
