Flashcards in The Digestive System Deck (47):
Where is most of the digestive system located?
In the abdominal cavity:
- peritoneal cavity - surrounded by peritoneum
- retroperitoneal space - posterior to peritoneum
What is the difference between intraperitoneal and retroperitoneal organs?
Intraperitoneal organs = organs having a mesentery
- eg stomach, jejenum and ileum, transverse colon, liver and gallbladder.
Retroperitoneal organs = organs without a mesentery
- eg pancreas, duodenum, ascending and descending colons.
What are mesenteries?
Portions of peritoneum that suspend the gut and provide paths for blood vessels and nerves to viscera.
What are the 4 layers of the gut wall?
- mesothelium (simple squamous epithelium)
- thin layer of CT
2. Muscularis externa - 2 layers of smooth muscle:
- outer longitudinal layer
- inner circular layer
3. Submucoa - layer of CT
- muscularis mucosae
- lamina propria (CT containing mucosal glands and Peyer's patches)
What is digestion?
The conversion of what we eat by physical and chemical disruption into a solution that is:
- relatively sterile
- neutral in pH
- isotonic (same osmolarity as bodily tissues)
- from which we can absorb nutrients.
Which process occurs in the mouth?
Physical and chemical disruption of food:
- physical: action of teeth, tongue and muscles of mastication
- chemical: action of salivary amylase and lipase
By what is saliva produced?
Exocrine salivary glands:
What are the 4 functions of saliva?
1. Starts digestion (amylase and lipase)
2. Assists swallowing
3. Protects mouth (e.g. High calcium protects teeth)
4. Is bacteriostatic
What is food moving from the mouth to the oesophagus called?
What is the function of the oesophagus?
Rapid peristaltic transport of bolus to stomach.
What helps food move down the oesophagus rather than the trachea?
What control do we have over the oesophagus?
- Upper 1/3 under voluntary control (some striated skeletal muscle)
- Lower 2/3 under involuntary control (solely smooth muscle). Innervated via myenteric plexus and submucosal plexus.
What type of epithelium lines the oesophagus?
Stratified squamous non-keratinised... withstands abrasion.
What are the functions of the stomach?
1. Continued disruption
- physical: churning (3 muscle layers and mucosal rugae)
- chemical: acid (HCl) and proteolytic enzymes (e.g. Pepsin)
- eat faster than digest
- wall relaxes so that pressure doesn't rise (= receptive relaxation)
What does the combined action of acid, enzymes and agitation produce to be delivered to the duodenum?
Incompletely digested, hypertonic chyme.
From the stomach onwards, what type of epithelium lines the GI tract?
Simple columnar epithelium
What is special about the muscularis externa in the stomach?
Composed of 3 layers of SM: oblique, circular and longitudinal.
What structures does the muscosa in the stomach form?
- Shallow gastric pits: lined by surface mucous cells.
- Long, straight tubular gastric glands: extend downwards from the gastric pits towards the muscularis mucosae.
What is the function of surface mucus cells?
- Secrete mucus in response to distension, stomach content and acid secretion.
- Mucus is:
1. resistant to pepsin degradation
2. contains HCO3- ions which neutralise the effects of H+ ions
So protects the stomach lining from self-degradation.
What is the effect of alcohol/aspirin on the stomach?
Damages mucous cells.
But these are quickly replaced by mitosis in deeper cells in the neck of the gastric pit.
What is the function of parietal cells?
Secrete H+ ions into the lumen and HCO3- ions into nearby capillaries which move them to surface mucous cells.
What is the function of Chief cells?
Secrete pepsinogens which are converted into pepsins (once passed the mucus barrier to protect stomach) - partly hydrolyse proteins.
What is the function of enteroendocrine cells?
Include G cells which secrete gastrin- stimulates acid secretion from the wider gastric mucosa.
What are the 3 parts of the small intestine?
How long is the small intestine and what is its surface area?
- ~7 m
- >25 m2
What is the main function of the small intestine and how is it adapted for this process?
- Large surface area: plicae circularis, villi and microvilli
- Adequate contact time
- Good energy/blood supply and drainage (via hepatic portal vein)
What is the duodenum and what are its functions?
- 20-25 cm long, C-shaped, proximal portion of the small intestine that curves head of pancreas.
1. Dilution and neutralisation of acidic chyme
2. Completion of digestion
With which organs does the duodenum interact with to complete its functions?
How does the duodenum dilute and neutralise acidic chyme?
1. Walls contain Brunner's glands: secrete bicarbonate-rich mucus.
2. Pancreas and liver secrete alkali (e.g. In bile).
Dilute (hypertonic chyme to isotonic chyme):
1. Water is drawn in from extracellular fluid.
2. Bile released from liver contains water.
How do the pancreas, liver and duodenum function in completion of digestion?
Pancreas and duodenum secrete specific enzymes which act with bile from liver:
- cleave peptides to aa
- cleave polysaccharide to monosaccharides
- break down and re-form lipids
- break down nucleic acids
Where does most active absorption occur?
What is absorbed in each part of the small intestine?
- Duodenum: iron
- Jejenum: most of sugars, aa and fatty acids
- Ileum: vitB12, bile acids and remaining nutrients
What does the small intestine not absorb?
What structure does the mucosa and submucosa form in the jejenum?
Plicae circulares (circular folds projecting into gut lumen)
What are lacteals?
Dilated lymphatic vessel in small intestine which absorbs digested fats.
What is the large intestine composed of?
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
How long/large is the colon?
1.2 m long
6-9 cm wide
What is the function of the colon?
1. Water (and electrolyte) reabsorption (over 20hr transit) - content is semi-solid by end of large intestine.
2. Content awaits expulsion.
What type of lymphoid tissue does the colon's surface contain?
Where are most of the GI tract's bacteria located?
Colon (99% are anaerobic, most are lost in faeces)
What are bacteria in the colon involved in?
1. Synthesis of vitamins K and B12, thiamine and riboflavin
2. Breakdown of bile acids
3. Conversion of bilirubin to non-pigmented metabolites
How is bacterial invasion of the colonic wall and underlying tissue prevented?
Viscoelastic mucus gel separates luminal bacteria from epithelial surface.
Describe the neural control of the GI tract.
1. Somatic control (innervating striated muscle) during ingestion and (mouth and 1st 1/3 of oesophagus) and excretion (last sphincter of anus).
2. Autonomic nervous system controls the rest.
Describe the ANS innervation of the GI tract.
Post-ganglionic neurones form plexuses:
1. Myenteric plexus (in muscularis externa)
2. Submucosal plexus (in submucosa)
Describe the paracrine control of the GI tract.
Substances act locally:
- histamine - controls acid production in stomach
- vasoactive substances - affect blood flow in gut by dilating blood vessels
Describe the endocrine control of the GI tract.
Gut generally controlled by hormones it makes itself. Regulate:
- stomach acid secretion
- alkali secretion from liver and pancreas
- enzyme secretion