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Physiology CAM202 > Small Intestine & Digestion > Flashcards

Small Intestine & Digestion Flashcards

1
Q

How Long is the Small Intestine?

A

about 7 metres

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2
Q

What are the three parts of the SI and what are their functions?

A

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)

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3
Q

Describe the Intestinal phase of Digestion

A

• Release of chyme (low pH liquidized food) into the duodenum, inhibits gastric activity, stimulates antacid & chemical digestion

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4
Q

Describe the enterogastric reflex

A

• 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)

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5
Q

Describe the endocrine role in the SI

A

• 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

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6
Q

What is the role of cholecystokinin (CCK) in the SI?

A

• 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)

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7
Q

What is the role of secretin in the SI?

A

• 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

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8
Q

What is the role of Gastric Inhibitory Peptide (GIP) in the SI

A
  • 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
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9
Q

What is the role of Motilin in the SI?

A

• 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

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10
Q

Describe the hormones used to control pancreastic and Biliary Secretions

A 
•	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)
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11
Q

Describe how CCK and Secretin act in the pancreas

A

o CCK ⇒ enzyme secretion
o Secretin ⇒ secretion of alkaline pancreatic juice
o Endocrine portion pancreas delivers directly into blood

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12
Q

Describe how CCK and Secretin act in the Gall Bladder

A

• Biliary System: CCK & Secretin
o Contraction of gallbladder
o Relax hepatopancreatic sphincter (Oddi) ⇒ release bile

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13
Q

Describe the Exocrine secretions of the Pancreas

A

• 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

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14
Q

Describe the pancreatic alkaline secretions

A

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

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15
Q

Describe the regulation of pancreatic secretion (going through the three phases of digestion)

A

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

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16
Q

Describe SI motility

A 
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
17
Q

Describe the Chemical Digestion in the Mouth

A

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)

18
Q

Describe chemical digestion in the duodenum

A

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

19
Q

Describe pancreatic secretions

A
  • 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
20
Q

What activates trypsin?

A 
•	In SI, enzymes activate pancreatic trypsin
o	Enterokinase (from SI brush border): trypsinogen ⇒ trypsin
21
Q

What does trypsin activate?

A

• Trypsin then activates other proteases
o Chymotrypsinogen => chymotrypsin
o Proelastase => elastase
o Procarboxypeptidase => carboxypeptidase

22
Q

What do pancreatic lipase and pancreatic amylase break down?

A
  • 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
23
Q

Enzymes of the small intestine are:

A

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

24
Q

What is bile made up of?

A 
•	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%
25
Q

What do bile salts do?

A
  • Bile salts emulsify ingested fats (monoglycerides & fatty acids) to form micelles
  • Increases surface area of lipid-water interface, where lipases act
  • Enables absorption of fats in the ileum (across the unstirred border)
26
Q

Secretion/release of bile

A

Secretion/release of Bile
• Gall bladder stores & concentrates bile
• Hepatocytes synthesise & transport bile acids – these deconjugate & become the ionic form (bile salts) in the alkaline environment of the duodenum

27
Q

Enterohepatic circulation

A

• Recycling of bile salts – primary source of bile salts comes from reclamation from the blood stream into hepatocytes (enabled by specific receptors in the terminal ileum)
• Failure to resorb (eg removal of ileum) causes insufficiency of lipid digestion & osmotic problems in colon
o Ulcerative colitis etc

28
Q

Explain the sites of nutrient absorption

A 
•	Absorption sites
o	Small intestine (90%)
o	Stomach (7-8%)
o	Large intestine (2-3%)
•	Probably via bacteria
•	Small molecules absorbed directly, larger molecules require breaking down
29
Q

Explain the process of CHO digestion and absorption

A
  1. Polysaccharides cleaved by amylases
  2. Disaccharides broken down by epithelial cells into monosaccharides:
    a. Glucose
    b. Galactose
    c. Fructose
  3. Co-transport into cells (Na+ gradient) except:
    a. Facilitated diffusion for fructose absorption
    b. Carbohydrate which cant be broken down is called dietary fiber
30
Q

Explain the process of Protein Digestion & Absorption

A

Protein Digestion & Absorption

  1. Polypeptides digested by proteases:
    a. Amino acids
    b. Dipeptides
    c. Tripeptides
  2. Di & tri- peptides are broken down in to AAs in (and on) the epithelial cells
  3. Amino acids are transported by protein carriers (sodium co-transporters) into intestinal epithelial cells
  4. AAs move into blood
31
Q

Explain the process of lipid break down

A

Breakdown
• Digested by lipases in the presence of bile salts
• TG broken into monoglycerides & fatty acids
• Small fatty acids (<12 C) absorbed by diffusion into epithelial cells
• Large fatty acids + monoglycerides + cholesterol + bile salts = micelles (deliver digested fats to epithelial surface)
• Micelles get across the unstirred layer at the brush border, and fat can then be absorbed into the epithelial cell

32
Q

Explain the process of Lipid Digestion and Absorption

A

Digestion & Absorption

  1. Bile salts emulsify into droplets
  2. Lipases render into monoglycerides & fatty acids, which
  3. Form micelles with bile salts
  4. Move into epithelial cells (bile salts remain in GIT)
  5. Inside cells, monoglycerides & FA are resynthesized to triglycerides in smooth ER
  6. Triglycerides + cholesterol + lipoprotein = chylomicrons
  7. Diffuse into lacteals (lymph)
  8. In the blood, TG are broken town to FAs & glycerol which diffuse into body cells

Physiology CAM202 (3 decks)

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