Physiology: Small Intestine and Exocrine Pancreas Flashcards
What is approximate length and diameter of small intestine?
Approx. 6-7m long and 3.5cm diameter
What are the 3 parts of the small intestine and their corresponding lengths?
- duodenum approx 30cm
- jejenum approx 3.5m
- ileum approx. 2.5m
What is succus entericus?
Intestinal juice secreted
What does motility of the small intestine cause?
- mixing of chyme with digestive juices - segmentation
- slow propulsion of food aborally - peristalsis
- movement of undigested residues to large intestine via the ileocaecal valve - migrating motor complex
How is the surface of the small intestine adapted for increased surface area?
- circular folds of Kerckring AKA plicae circularis
- villi
- microvilli - brush border
What is “chopping” in the small intestine?
It is the alternating contraction and relaxation of segments of circular muscle for chyme mixing
How is “chopping” in segmentation initiated?
It is initiated by small intestine pacemaker cells which cause the basic electrical rhythm which is continuous. When the BER reaches threshold segmentation is intiated
What causes threshold to activate segmentation in duodenum?
Distension caused by entering chyme
Which part of the small intestine has more frequent segmentation contractions ?
Duodenum - movement is slightly aboral but is slow for absorption
What enhances and decreases strength of segmentation contractions respectively?
Parasympathetic enhances
Sympathetic decreases
What triggers segmentation in the empty ileum?
Gastrin from the stomach (gastroileal reflex)
What is the gastroileal reflex?
Segmentation in the empty stomach by gastrin from the stomach
In what state does segmentation occur?
Digestive state
How does peristalsis occur in the interdigestive (fasting) state?
as a few localised contractions
What is the migrating motor complex? (MMC)
a strong peristaltic contraction that slowly passes along the length of the intestine (from stomach to ileocaecal valve)
How often does migrating motor complex occur?
every 90-120 mins between meals
What is the function of the MMC?
To clear debris, mucous and sloughed epithelial cells between meals
What inhibits the MMC?
Feeding and vagal activity
What stimulates MMC?
motilin
What suppresses MMC?
- gastrin
- CCK
Why do macrolides (e.g. erythromicin) sometimes cause GI disturbances?
They mimic the effect of motilin
What 7 peptide hormones are secreted by endocrine mucosal cells in the small intestine?
- gastrin
- secretin
- cholecystokinin (CCK)
- glucose-dependent insulinotropic peptide (GIP aka gastric inhibitory peptide)
- glucagon-like peptide 1 (GLP-1)
- motilin
- ghrelin
What receptors do peptide hormones act on?
G-protein coupled receptors
What cells secrete gastrin and where is it secreted?
- G cells
- gastric antrum (mainly) and duodenum
What 2 effects does gastrin have?
- stimulates H+ secretion by parietal cells
2. stimulates growth of gastric mucosa (trophic effect)
What cells secrete secretin and where is it secreted?
- S cells
- duodenum
What stimulates S cells to release secretin?
H+ and fatty acids in lumen
What effect does secretin have?
Promotes secretion of pancreatic and biliary HCO3-
What cells secrete cholecystokinin and where is it secreted?
- I cells
- duodenum and jejenum
What stimulates I cells to release CCK?
monoglycerides, free fatty acids, amino acids and small peptides in the lumen
What 4 effects does CCK have?
- inhibits gastric emptying
- causes pancreatic enzyme secretion
- stimulates relaxation of sphincter of Oddi and contraction of gall bladder to eject bile into duodenum
- potentiates the action of secretin
What cells secrete glucose-dependent insulinotropic peptide and where is it secreted?
- K cells
- duodenum and jejenum
What stimulates K cells to secrete glucose-dependent insulinotropic peptide?
fatty acids, amino acids and glucose in the lumen
What 2 effects does glucose-dependent insulinotropic peptide have?
- inhibit gastric emptying
2. stimulates release of insulin from pancreatic beta cells (incretin action)
What cells secrete glucagon-like peptide 1 and where is it secreted?
- L cells
- small intestine
What 3 effects does GLP-1 have?
- stimulates insulin secretion
- inhibits glucagon secretion from pancreatic alpha cells
- decreases gastric emptying and appetite
What is the effect of glucagon?
Glucagon causes the liver to convert stored glycogen into glucose
What cells secrete motilin and where is it secreted?
- M cells
- duodenum and jejenum
When is motilin secreted?
fasting state
What cells secrete ghrelin and where is it secreted?
- Gr cells
- gastric antrum, small intestine and other places (e.g. pancreas)
What is the effect of ghrelin?
stimulates appetite
What is the function of incretins?
they stimulate pancreatic beta cells in a feed-forward manner to stimulate insulin release
What control mechanisms enhance small intestinal juice secretion?
- distension/irritation
- gastrin
- CCK
- secretin
- parasympathetic nerve activity
What control mechanism decreases small intestinal juice secretion?
sympathetic nerve activity
What does small intestinal juice contain?
- no digestive enzymes
- mucous for protection + lubrication
- aqueous salts for enzymatic digestion (mostly from the crypts of Lieberkuhn)
What are the two endocrine pancreatic secretions and where are they secreted?
- glucagon
- insulin
- into the blood
What are the two exocrine pancreatic secretions, from what cells and where are they secreted?
- digestive enzymes from acinar cells
- aqueous NaHCO3- solution from duct cells
- secreted into duodenum as pancreatic juice
How much alkaline fluid do duct cells secrete each day?
1-2 litres
What is alkaline fluid rich in?
HCO3-
Why does alkaline fluid neutralise acidic chyme from the stomach?
- To provides optimum pH for pancreatic enzymes
- protects the mucosa from acidic erosion
What is significant in Cystic Fibrosis patients?
have reduced fluid secretions
What is the function of the Na+/HCO3- transporter in pancreatic duct cells?
It transports Na+ and HCO3- into the cell from the duct cell providing some of the HCO3- needed for secretion across the apical membrane to the lumen. (Na+ in and HCO3- in)
What is the function of the Na+/K+-ATPase in pancreatic duct cells?
Maintains the electrochemical gradient of Na+ and K+ ions (K+ in and Na+ out)
What is the function of the Na+/H+ exchanger in pancreatic duct cells?
Contributes to export of H+ ions liberated by H2CO3 dissociation (Na+ in and H+ out)
What is the function of the K+/H+-ATPase (proton pump) in pancreatic duct cells?
Contributes to H+ export liberated by H2CO3 dissociation (K+ in and H+ out)
What 4 transporters are located at the basolateral membrane of the duct cell?
- Na+/HCO3- transporter
- Na+/K+-ATPase
- Na+/H+ exchanger
- K+/H+-ATPase
What 2 transporters are located at the apical membrane facing the lumen?
- Cl-/HCO3- exchanger
2. CFTR -Chloride channel
what is the function of the Cl-/HCO3- exchanger in pancreatic duct cells?
HCO3- secretion into the lumen (HCO3- out and Cl- in)
What is the function of the CFTR in pancreatic duct cells?
Provides an importation proportion of the luminal Cl- needed for Cl-/HCO3- exchange
How are CFTR channels activated?
Via second messenger systems by secretin
List 5 enzymes secreted by pancreatic acinar cells and state whether they are active/inactive when released.
- trypsinogen (inactive)
- chymotrypsinogen (inactive)
- procarboxypeptidase A + B (inactive)
- pancreatic amylase (active)
- pancreatic lipase (active)
What enzyme catalyses trypsinogen in the duodenum?
enterokinase
What cells release enterokinase in the duodenum?
mucosal cells
Which enzyme autocatalyses itself once it is active?
Trypsin
What enzymes does trypsin catalyse?
- trypsinogen (itself) to trypsin
- chymotrypsinogen to chymotrypsin
- procarboxypeptidase A + B to carboxypeptidase A + B
What 3 phases are involved in control of pancreatic secretion and what percentage is each of total pancreatic secretion?
- cephalic 20%
- gastric 5-10%
- intestinal 70-80%
How is the cephalic phase in pancreatic secretion mediated?
By vagal stimulation of mainly the acinar cells
How is the gastric phase of pancreatic secretion triggered?
Gastric distension evokes a vagovagal reflex resulting in parasympathetic stimulation of the duct cells and acinar celsl
In what two ways is the intestinal phase of pancreatic secretion triggered?
- acid in the duodenal lumen -> S cells release secretin -> secretin travels to pancreatic duct cells through blood -> aqueous NaHCO3 solution secreted and neutralises acid
- fat and protein in the duodenal lumen -> I cells release CCK -> CCK travels to pancreatic acinar cells through blood -> digestive enzymes secreted into lumen to digest fat and protein
What does effect does acid in the duodenal lumen have on pancreatic secretion?
It triggers the intestinal pancreatic secretion phase because it triggers S cells to release secretin which trigger pancreatic duct cells to secrete aqueous NaHCO3 solution
What effect does fat and proteins in the duodenal lumen have on pancreatic secretion?
It triggers the intestinal pancreatic secretion phase because it triggers I cells to release CCK which triggers pancreatic acinar cells to secrete digestive enzymes
What is the difference between luminal and membrane digestion in the small intestines?
- luminal digestion is mediated by pancreatic enzymes secreted into the lumen
- membrane digestion is mediated by enzymes at the brush border of epithelial cells
