Physiology of Digestion and Absorption Flashcards
What are the respective sizes of the duodenum, jejunum and ileum?
Duodenum - 30cm. Jejunum - 3.5m.
Ileum - 2.5m.
What substances does the small intestine receive and where do they come from?
Chyme from stomach, pancreatic juice from pancreas and bile from gall bladder.
What does the small intestine secrete?
Intestinal juice (succus entericus).
What hormone causes the ileocaecal valve to open?
Gastrin.
What are the 3 structures of the small intestine that make it well adapted for absorption?
Circular folds (of Kerckring), villi and microvilli (brush border).
What cells secrete gastrin, cholescystokinin (CKK), secretin and motilin and where are they located?
Gastrin - G cells of antrum and duodenum.
CCK - I cells of duodenum and jejunum.
Secretin - S cells of duodenum.
Motilin - M cells of duodenum and jejunum.
What are the 2 incretins?
Glucagon-like insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1).
What is another name for GIP?
Gastric inhibitory peptide.
Where are GIP and GLP-1 secreted from?
GIP - K cells of duodenum and jejunum.
GLP-1 - L cells of gut.
What are the functions of incretins?
Act upon B-cells of pancreas in feed-forward manner to stimulate the release of insulin.
What type of receptor do hormones secreted by the gut act on?
GPCRs.
How much succus entericus is secreted per day by the small intestine?
2 litres.
How does the composition of the succus entericus vary?
At different parts of the small intestine.
What are the control mechanisms of the small intestine secretions?
Distension/irritation, gastrin, CCK, secretin, parasympathetic stimulation (increases), sympathetic stimulation (decreases).
What are the 2 components of the succus entericus and their functions?
Mucus (protection/lubrication from goblet cells), aqueous salt (enzymatic digestions from crypts of Lieberkuhn).
Does the succus entericus have digestive enzymes?
No.
What transporter brings chloride into the enterocyte from the circulation?
Na+/K+/2Cl- co-transporter.
What channel does chloride enter the gut lumen through?
CFTR?
How does sodium and water enter the lumen after chloride?
Because of the osmotic gradient (goes around the cell rather than through it).
When does segmentation occur very vigorously?
After a meal.
Why is segmentation in the duodenum activated after a meal?
Due to distension by entering chyme.
What causes segmentation in the empty ileum?
Gastrin from the stomach (gastroileal reflex, feedforward mechanism).
What is the net movement in segmentation and why?
Aboral. Duodenal has 12 contractions per min, ileum has 9 per min.
When does the migrating motor complex (MMC) occur?
Between meals every 90-120 mins.
What is the MMC?
A strong peristaltic contraction passing the length of the small intestine (stomach to ileocaecal valve).
What is the purpose of the MMC?
Clears small intestine of debris, mucus and sloughed epithelial cells between meals.
What inhibits and triggers the MMC?
Inhibits: feeding and vagal activity, gastrin and CKK.
Triggers: motilin.
Why may macrolide antibiotics cause GI disturbances?
They may mimic the effect of motilin.
What does the exocrine part of the pancreas produce?
Digestive enzymes and aqueous sodium bicarbonate.
What cells produce digestive enzymes and bicarbonate in the pancreas?
Digestive enzymes - acinar cells. Bicarbonate - duct cells.
Describe the path of the pancreas from acni cells into the duodenum.
Acni cells to ducts which then combine into bigger ducts which flow into the main pancreatic duct. This then flows into the duodenum.
How much alkaline fluid does the pancreas secrete per day?
1-2 litres.
Why is the alkaline fluid important?
It neutralises acidic chyme which provides optimum pH for pancreatic enzyme function and protects the mucosa from erosion by acid.
When are enzymes stored in zymogen granules released in acinar cells?
When intracellular calcium concentration is elevated.
What causes an increase in calcium concentration in acinar cells?
Parasympathetic or hormonal stimulation.
Why are the proteases produced by the pancreas inactive?
They would autodigest the pancreas if they were active.
What enzyme in the mucosal cells of the duodenum activates trypsinogen (produced trypsin)?
Enterokinase.
What are the enzymes that the pancreas secretes?
Trypsinogen, chymotrypsinogen, procarboxypeptidase A and B, pancreatic amylase and pancreatic lipase.
What enzymes can trypsin activate?
Trypsinogen (autocatalysis), chymotripsinogen (produced chymotrypsin) and procarboxypeptidase A and B (produces carboxypeptidase A and B).
What are the 3 phases of the control of pancreatic secretion?
Cephalic (mediated by vagal stimulation of mainly the acinar cells [20% secretion]), gastric (distension evokes vagovagal reflex causing parasympathetic stimulation of acinar and duct cells [5-10% total secretion]), and intestinal.
In the intestinal phase, what does acid in the duodenal lumen cause?
Increase of secretin release from S cells, carried by blood to pancreatic duct cells, increased secretion of bicarbonate into duodenal lumen.
In the intestinal phase, what does fat and protein in the duodenal lumen cause?
Increased CCK release from I cells, carried by blood to pancreatic acinar cells, increases secretion of digestive enzymes into the duodenal lumen.
What are the main constituents of carbohydrates?
Starch, cellulose, glycogen, disaccharides.
What are the 2 types of digestion that occurs in the small intestine?
Luminal digestion - mediated by pancreatic enzymes.
Membrane digestion - mediated by enzymes on brush border of epithelial cells.
What membranes do things have to travel across to be absorbed?
The apical and basolateral.
What is assimilation?
The overall process of digestion and absorption.
Can di, tri or tetrapeptides be absorbed by the gut epithelium and where are they broken down?
Yes, in the enterocyte.
What types of sugars can be absorbed?
Only monosaccharides.
