Small Intestine & Digestion Flashcards
How Long is the Small Intestine?
about 7 metres
What are the three parts of the SI and what are their functions?
Duodenum (“12 fingers”)
o Site of most digestion
o Presence of acids here activates Secretin which stimulates alkaline secretions from the gall bladder, pancreas – causes them to be delivered to the gut
o Presence of lipids stimulates Cholecystekinin – mobilises the gall bladders bile delivery (Fats) & panceas digestive enzymes
Jejunum (“empty”)
o Mainly absorptive region
Ileum (“flank”)
o Absorbs fats/oils, bile salts, B12, water
o Immune function (Peyer’s patches)
Describe the Intestinal phase of Digestion
• Release of chyme (low pH liquidized food) into the duodenum, inhibits gastric activity, stimulates antacid & chemical digestion
Describe the enterogastric reflex
• Neural: enterogastric reflex reduces gastrin production & stomach motility
o Triggers long reflex which stops vagus nerve secreting ACh & CRP onto stomach wall
o Extrinsic reflex stimulated by the presence of acid levels in the duodenum at a pH of 3-4, or in the stomach at a pH of 1.5
o Upon stimulation of the reflex, the release of gastrin from G cells in the antrum is shut off.
• This inhibits gastric motility & secretion of gastric acid (HCl)
Describe the endocrine role in the SI
• Endocrine: secretin, CCK & GIP inhibit stomach (structurally similar to gastrin so competitively inhibit), & stimulate pancreas
o Exocrine pancreas: gastrin, CCK, secretin
o Endocrine pancreas: insulin, glucagon, somatostatin
• 10% of gastric secretions are evoked
• 65% pancreatic secretions (taking over)
• Release of bile by gall bladder
What is the role of cholecystokinin (CCK) in the SI?
• Gall bladder (Related to gastrin; competitive, less potent)
• Lipids & CHOs in duodenum stimulate ⇒ I Cells ⇒ CCK (into blood stream)
Stimulates
o Gallbladder contraction (bile)
o Pancreatic enzyme (and bicarbonate) secretion
Inhibits
o Gastric emptying (competes with gastrin)
o Appetite (satiety effects of CNS receptors)
What is the role of secretin in the SI?
• Nature’s antacid: alkaline releaser
• Low pH in duodenum ⇒ S cells (duodenum) ⇒ secretin
• Elicits bicarbonate from pancreas, liver, Brunner’s glands of duodenum
o Releases hydrogen ions into the blood stream so blood stream & gut both become neutral (neutralizes alkaline tide)
• Also acts at VIP (vasoactive intestinal polypeptide) receptors to inhibit gastric acid/pepsin secretion
What is the role of Gastric Inhibitory Peptide (GIP) in the SI
- Member of secretin/glucagon group
- Sugars (carbs) & fats in duodenum ⇒ K cells ⇒ GIP release
- Tells pancreas to prepare for sugar load (pancrease beta cells ⇒ insulin release)
- Other actions similar to secretin & glucagon
- Inhibits gastric acid secretion/motility
- Inhibits liver glycolysis
What is the role of Motilin in the SI?
• 22 amino acid peptide hormone
o Circulates during fasting periods
• Secreted by endocrine M cells of SI
• Released every 90 minutes during fasting, sweeping waves which keep gut “ticking over”
• Serum levels correlate with waves of GI motility – squeezing the empty gut clean
• Amplifies the migrating myoelectric complex – gut coordination during rest
Describe the hormones used to control pancreastic and Biliary Secretions
• Chyme entering duodenum stimulates enterogastric reflex (neural, long reflex) • Hormones (vagal & direct stimulation): o Increase cholecystokinin CCK (I Cells) o Increase secretin (S cells) o Decrease gastrin (G cells of stomach)
Describe how CCK and Secretin act in the pancreas
o CCK ⇒ enzyme secretion
o Secretin ⇒ secretion of alkaline pancreatic juice
o Endocrine portion pancreas delivers directly into blood
Describe how CCK and Secretin act in the Gall Bladder
• Biliary System: CCK & Secretin
o Contraction of gallbladder
o Relax hepatopancreatic sphincter (Oddi) ⇒ release bile
Describe the Exocrine secretions of the Pancreas
• Exocrine secretion
o Aqueous component (comes from duct cells lining pancreatic duct)
• Stimulated by Secretin
• Includes HCO3- & water
o Enzyme component (comes from pancreatic acinar cells)
• Stimulated by ACh (PSNS), and CCK
• Includes trypsin, chymotrypsin & carboxypolypeptidase
Describe the pancreatic alkaline secretions
Pancreatic Alkaline Secretions
• Fasting state, don’t secrete much
• Raises duodenal pH
• Secretin resembles ECF at low rates
• Secretin is the principal stimulant of duct cells (potentiated by CCK and ACh)
o Secretin is also a potent inhibitor of gastric acid secretion by Parietal cells & gastrin by G cells
• Increases secretion of HCO3-
o The amount of bicarb you secrete is regulated by feedback to match the amount of acid secreted by the stomach
• Accumulation of H+ in ECF (counters alkaline tide); exchange of Cl- and HCO3- in lumen
Describe the regulation of pancreatic secretion (going through the three phases of digestion)
Cephalic phase
o Sight & Smell
o Vagal
o 25% of pancreatic response
Gastric phase
o Only 5-10% of pancreatic response
o Distension of the stomach stimulates vagovagal reflexes – ACh
o Gastrin not an important pancreatic stimulant in humans
Intestinal phase
o 65% of pancreatic response
o CCK and secretin in response to acid chyme in duodenum
Describe SI motility
Peristalsis • Slow movement • Pressure gradient proximal => distal • Stimulated by gastrin, CCK, insulin • Inhibited by SNS (danger/stress) Segmentation • Dominant activity of SI • Circular muscle mixes chyme • As the digestion continues & you get more monomers this initiates peristaltic contraction to move intestinal contents along
Describe the Chemical Digestion in the Mouth
Digestion in mouth
• Lingual lipase (“fat breaker” from parotid and Ebner’s glands
• Saliva contains an amylase (starch breaker) enzyme
o Breaks polysaccharides at every second bond ⇒ disaccharides
o E.g. starch (a polysaccharide) ⇒ maltose (a disaccharide)
Describe chemical digestion in the duodenum
Duodenum
• Enzymes & acids are added to food to break down large molecules into smaller absorbable monomers or oligomers
• Nutrient intake can be classified into 3 categories, each with specific digestion & absorption mechanisms
o Carbohydrates (CHOs)
o Fats
o Proteins
• Dumped into hepatic portal vein
Describe pancreatic secretions
- Pancreatic enzymes are secreted as inactive pro-enzymes, then activated in small intestine by enterokinase on brush border
- Disruption of secretion process can cause premature activation of enzymes => degradation of cells of the pancreatic duct ⇒ pancreatitis
What activates trypsin?
• In SI, enzymes activate pancreatic trypsin o Enterokinase (from SI brush border): trypsinogen ⇒ trypsin
What does trypsin activate?
• Trypsin then activates other proteases
o Chymotrypsinogen => chymotrypsin
o Proelastase => elastase
o Procarboxypeptidase => carboxypeptidase
What do pancreatic lipase and pancreatic amylase break down?
- Pancreatic lipase: triglycerides ⇒ fatty acids, monoglycerides
- Pancreatic amylase: breaks up complex carbs
- Pancreatic juice is alkaline (pH 8) due to secreted ions (HCO3-, CL-, Na+, K+); this range is essential to enzyme function & micelle formation
Enzymes of the small intestine are:
Enzymes of the Small Intestine
• Secreted on mucosa, bound to membranes
• Deal with small peptides, disaccharides
• Enterokinase activates trypsin ⇒ others
• Disaccharidases (⇒ monosaccharides):
o Sucrase
o Lactase
o Maltases
• Peptidases
• SI wall is protected by alkaline mucous secreted by Brunner’s glands in GIT wall
What is bile made up of?
• Solubilises fats by breaking them up into droplets • Bile o Water 97% o Bile salts 1% o Bile pigment 0.2% o Cholesterol 0.06% o Other salts 0.7%
