Absorption Flashcards
Where is iron absorbed?
In the duodenum and proximal jejunum
Where are carbohydrates absorbed
What enzymes are required
Efficiently by the distal duodenum
Pancreatic Amylase
Glucoamylase at mucosa
Lactase at brush border
Sucroismomaltose
What are the mono and disaccharides?
Which are reducing substances?
How are they absorbed into cells
Lactose = Glucose and Galactose - via Lactase
Sucrose = Glucose and Fructose via. Sucrase isomaltase
Galactose (Sodium and energy dependent process) Active Transport - principle including in ORS - SGLT1 transporter
Glucose (Sodium and energy dependent process)
Fructose (Facilitated diffusion) via GLUT 5
THEN excreted into capillaryies by GLUT 2
Which enzymes are required for protein absoprtion
PEPSINOGEN/PEPSIN in STOMACH
PANCREATIC ENZYMES Trypsin Chymotrypsin Carboxypeptidase Elastase
These cut proteins into individual amino acids as well as oligopeptides
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Secreted as pro-enzymes
Activated by mucosal enzyme enterokinase
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Brush border has peptidases to break oligopeptides into dipeptides, tripeptides and amino acids
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Then absorbed into cell directly as individual/diepeptide/tripeptide via separate noncompeitive carriers
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All normally absorbed in upper half of small intenstitine
How long is the small intestine?
270 cm at birth, adult length 450-550 cm by 4 yrs of age
How often does the intestinal mucosa renew?
Every 4-5 days, but delayed in malnourised childnen or infants.
Cells of villi originate in crypts then become functional as they migrate
How does fat absorption occur?
In two phases
Phase 1:
Breakdown - lipase and colipase split triglycerides into monoglycerides and free-fatty acids - these are emulsified by bile salts, forming micelles (along wiht phospho lipids). Then absorbed (DIFFUSES, NO ACTIVE TRANSPORT REQUIRED)into enterocyte, re-esterified, packaged into chylomicrons and taken by intestinal lymphatics to the thoracic duct.
OR
If medium chain fats - absorbed directly by enterocyteand transported to liver via the portal system - INDEPENDENT of BILE SALTS
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In the infant - facilitated by lingual and gastric lipases, and bile-salt stimulated lipase in human milk
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2-3 % of pancreatic exocrine function ONLY required to achieve normal fat absoprtion (95%)
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Neonates are relative fat malabsorbers (only absorb 85%)
What faecal fat investigations exist?
What do they indicate?
Fat Crystals
- Loss of surface area with which to absorb fats
Fat Globules
- indicates failure of digestion of fats - therefore cholestatic liver disease with inadequate bile salts OR pancreatitis with
What diseases effect fat digestion or absoprtion?
Decreased pancreatic enzymes in cystic fibrosis
Poor bile salt production and then micelle formation in cholestatic liver disease
Loss of absorptive surface area in coeliac disease
Abnormal chylomicron formation in abetalipoproteinemia
Intenstitinal lymphangectasia affects transport of chylomicrons
How big is the colon?
What does it do?
75-100 cm tube
3x strips of longitudinal taenia coli
Cause folding called haustra
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Absorbs water and electrolytes
Scavenges by-products of bacterial degradation of carbohydrates
What are the cells of the stomach?
What do they secrete?
Chief cells (Gastric chief Cells) - Secrete pepsinogen and gastric lipase
Parietal Cells (Oxyntic)
- Secrete HCl and IF
- Using Proton Pump (the H+/K+ ATPase) - active trasnport against steepest ion gradient in body
Regulated by GASTRIN
Regulated by HISTAMINE (stimulated) - so Ranitidine (a H2 blocker works)
Regulated by ACETYLCHOLINE
Glands that secrete mucus - sit in cardia of stomac (near heart)
G-Cells - gastrin cell, release gastrin (which increases histamine)
What does Intrinsic Factor do?
Secreted by parietal cells
Absorbs B12 (Also called cyanocobalamin, hydroxyocobalamin, methycobalamin, adenosylcobalamin)
Failure (Through autoantiboides against parietal cells, or against IF or resection of XYZ - causes pernicious anaemia)
Pepsin frees B12 from food and is bound to carriers in stomach
Proteases in duodenum free B12 from carriers then it combines with IF
Terminal illeum of small bowel then absorb B12/IF complex
What structures are part of the foregut
Upper gastrointestinal tract
- Eosophagus
0 Stomach
- Duodenum until insertion of CBD
What structures are part of mid gut
Small bowel and large bowel
- from mid-duodenum until mid-transvers colon
What structures are part of hind gut
Mid transvere colon until upper anal canal
What is the daily nutritional requirement of iron
About 10mg intake
Only 10% is absorbed (1mg daily)
And a neonate has 0.5g of iron stores which need to increase to 5g by adult hood (4.5g = 4500mg = 1mg/day NET POSITIVE for 12 yrs)
Where is calcium absorbed
Duodenum and proximal jeujunum
Under influence of Vitamin D3 (1-25 dihydoxy vitamin D) - which induced calcium binding protein
What doesB 12 do?
Turns Homocystein (which is toxic) into Methionine Demethylates THF - THF is important for DNA synthesis
So deficiecny of B 12
Is high homocysteine
Low Methionine
Impaired THF -> Megaloblastic anaemia and neurology
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B12 is bound to transcobalamins (TC 1, TC II, TC III)
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Stored in liver (time to deficiency)
Hydrogen Breath Test in Gastroenterology?
Carbohydrate malabsoprtion - LOAD DEPENDENT - Fructose, Glucose, Galactose, Disaccharidase with Sucrose/
End point of any sugar that isn’t absorbed is colonic fermentation to H, CH4, CO2, Short Chain Fatty Acids - some absorbed - some excreted as acid diarrhoea
H2 breathed out 90 MINUTES post ingestion
False negative if not a hydrogen producer - So can use METHANE
LACTULOSE - non-digestable sugar - will guarantte H2 production IF fermenters
Early Peak then Late Peak = Bacterial OVERGROWTH
How much food do we eat?
Grams of CHO/Fat/Protein?
Well - we eat about 200-300 g/ CHO /day as adult - this is 50% energy and is half simple sugars/half starches
Protein is 0.75g/kg/day of high quality protein to maintain nitrogen balance, essential AAs, more for illness/ growth/preg/lactation
50g of protein for 70kg person
Therefore about 120-150 g fat to make up Energy
Why are medium chain triglycerides important?
Their magic is in being ~ 10 (6-12) carbon atoms long - they don’t required the lymphatic system and enter directly into portal vein bound to albumin
Don’t require lipase
Don’t require bile
Don’t require carnitine to get into mitochondria
But can’t be turned into the EFA (Essential Fatty Acids)
Gastro - In our population what is the commonest cause of severe cholestatic liver disease?
Failed Kasai after extrahepatic biliary atresia
- Good case for MCT fats
Which vitamin may be effected by PPIs?
B12 - Due to gastric acid being required to release B12 from food (where it is protected by R protein, which is then cleave by trypsin to bind with IF, and then absorbed in terminal illeum)
How much fluid is secreted into the GIT on the daily?
How is this controlled physiologically?
7L a day (cf. ingestion 1-2L)
cAMP drives chloride excretion from Cl channel
Vibrio Cholera - too much
Cystic Fibrosis - Thick/constipation/DIOS
What is the basis of eosphageal reflux?
In ALL age group - the transient relaxation of the lower eosophaegal sphincter (which is smooth muscle)
What does CCK do?
What does Secretin do?
Both release from small intestinal mucosa
CCK - Gall bladder contraction, pancreatic enzyme secretions
Secretin - Chloride and pancreatic HCO3 secretion from ducts
