Small Intestine Phase Flashcards
Why is the small intestine so important for digestion?
The small intestine is the most significant site for digestion and absorption of nutrients in the GI system.
What is the basis for the high digestion/absorption of nutrients in the small intestine?
Intense motility aids in the break down of food, and a large surface area of mucosal cells allows for massive absorption of nutrients.
What is the negative feedback that happens when food enters the duodenum?
As a meal enters the duodenum, it initiates feedback inhibition of gastric emptying - this allows for more digestion time in both compartments
Vagal regulation of gastric emptying relies on what sensory information?
the pH and hyperosmotic content of chyme stimulates the vagal response to the gastric emptying
What allows gastric emptying to start again?
Chyme moves further down the small intestine and the feedback inhibition is reduced
What two major factors contribute to the inhibition/slowing of the gastric emptying.
1) The presence of fat in the duodenum
2) The presence of low pH in the duodenum
What is the main stimulator of CCK release?
fatty acids in the duodenum
What is the main effects of CCK?
Release of bile from the gallbladder and digestive enzymes from the pancreas
What cells secrete CCK?
I-cells
What are the two main component of pancreatic secretions into the duodenum?
HCO3- and digestive enzymes
What important hormone for digestion is released by the endocrine pancreas?
Somatostatin (inhibitor of the G-cells of the stomach)
Where do the enzymatic components of the pancreas come from?
The acinar cells
Where does the aqueous component (HCO3-) of the pancreatic secretions come from?
The centroacinar cells
Describe the role of secretin in the regulation of pH in the duodenum.
At low pH, S-cells (of the duodenum) are stimulated to secrete secretin. Secretin then stimulates the secretion of HCO3- from the centroacinar cells of the pancreas.
An increase in pH inhibits further release of secretin
How does secretin cause the release of bicarbonate from duct cells?
Secretin, released from S-cells of the duodenum, binds to duct cells of the pancreas and stimulates cAMP production. This opens the CFTR Cl- channels and Cl- flows into the lumen of the pancreatic ducts.
This influx of Cl- drives the HCO3-/Cl- exchanger, and Cl- from the lumen is exchanged for HCO3-.
Other than secretion of HCO3- into the pancreatic ducts, what is the other net effect of secretin release?
Acidification of the blood. H+ is exchanged for Na+ on the basolateral membrane- this leads to net H+ absorption by the blood.
What is the problem with a defective CFTR channel, as in cystic fibrosis?
the bicarbonate secretory process is defective, as Cl- cannot leak out into the duct, and therefore Cl- cannot be exchanged for HCO3-.
This results in a decrease in a high concentration of precipitation of the acinar enzymes in the duct and destruction of the gland.
What cell types/where is CCK released?
CCK is released from I cells located in the small intestinal epithelium
What triggers the release of CCK?
Direct interaction with fatty acids, amino acids, CCK-releasing peptide (CCK-RP), or monitor peptide
What causes the release of CCK-RP?
Fatty acids or amino acids
Where is monitor peptide released from?
pancreatic acinar cells
How does CCK stimulate pancreatic acinar secretion?
1) binds to the CCK1 receptor on the acinar cells
2) Stimulates neural reflexes that leads to the release of ACh, GRP and VIP by pancreatic enteric neurons
Both of these processes leads to the phosphorylation of structural and regulatory proteins that allows for the fusion of granules containing digestive enzymes with the apical membrane of the acinar cells and secretion into the pancreatic ducts.
What is enterokinase?
Enterokinase (a brush border enzyme of the small intestine) cleaves trypsinogen –> trypsin (the active form)
Where are the pancreatic digestive enzymes converted by trypsin into their active form?
The duodenum.
What is trypsin?
Trypsin is the active form of trypsinogen.
Trypsin is the main enzyme responsible for cleaving pancreatic proenzymes into their active form.
Throughout the entire digestion process, when are pancreatic secretions first stimulated/increased?
During the cephalic phase via vagal and enteric nerves
What 4 factors increase pancreatic secretions during the intestinal phase?
Secretin release (from low pH) CCK secretion (from peptides, AAs, fatty acids) Enteropancreatic reflexes (from distention, hypertonicity) Vagovagal reflexes (from CCK sensory enteric neurons)
What is the major constituent of bile?
The majority of bile is made up of bile acids
What is the purpose of bile acids?
Bile acids are detergents and form micelles around fat to prevent them from globbing together
How is bile recycled?
Enterohepatic circulation: the majority of bile acids are recycled from the intestine back to the liver via the enterohepatic circulation.
How are conjugated bile acids reabsorbed?
Conjugated bile acids are reabsorbed by a symporter known as the apical sodium dependent bile acid transporter (ASBT)
Where do segmental contractions occur?
Small and large intestine
Where does do peristaltic contractions occur?
pharynx, esophagus, gastric antrum, small and large intestine
What two neurotransmitters are involved in orad contraction?
Ach and substance P
What two neurotransmitters are involved in caudad relaxation?
VIP and NO
What is the role of the hormone motilin?
Motilin is responsible for initiating periodic contractions that clear any remaining gastric or intestinal contents out into the colon. The movement that occurs is called the Migrating Motor Complex
What is secreted from Brunner’s glands (small intestinal glands)
mucus and bicarbonate
What is secreted from Lieberkuhn glands of the small intestine?
peptidases and enzymes that digest carbohydrates
What is secreted from the Paneth cells of the small intestine?
antimicrobial peptides and enzymes
