Gastroenterology Physiology
Ingestion
Digestion (breakdown)
Absorption
Transport
Enterocyte = intestine cell
Enteroendocrine cells: K, L, I, G, enterochromaffin, N, S
Gastrointestinal tract = gut (fore, mid, hind)
Intestine = bowel (small, large), enteric is adj.
4 layers from stomach to colon
Gastric mucosa:
Small intestine
Nutrients
Carbohydrates (hydrated carbon)
Monosaccarides (all are reducing)
Glucose (absorbed by Na cotransport)
Galactose (uses glucose transported)
Fructose (absorbed by facilitative transport)
Disaccharides (have to be digested to monosaccharides, all digestible have glucose base + 1,)
Lactose (Glucose and Galactose): in milk
Sucrose (Glucose and Fructose): in plants
Lactulose (Galactose and Fructose) non-absorbable! hence laxative
Maltose (2 glucose): in starch
cleaved by lactase, sucrase, maltase respectively AT the brush border
Africans and asians have a natural decrease in lactase from 4yo > lactose intolerance as adults
Polysaccaride
Glycogen: glucose
Starch (amylose and amylopectin): broken down by pancreatic amylase to glucose, di/oligosaccarides
These are broken down by glucoamylase
Diasaccaride broken down by lactase, sucrase, maltase at brush border
Fats
Ingestion is mainly triglycerides
Lipase and colipase digest to monoglycerides and fatty acids (CF affects this)
Bile salts bind fatty acids forming micelles (Cholestasis affects this)
(medium chain triglycerides absorbed independent of bile so can feed this to bypass, also done to treat chylothorax)
absorption (coeliac disease affects)
reesterified to chylomicrons in enterocyte (abetalipoproteinemia affects)
Chylomicrons travel in lymph (intestinal lyphangiectasia affects)
long chain polyunsaturated fatty acids
Proteins
Proenzymes secreted from pancreas
activated to enzymes by enterokinase (another enzyme) release from mucosa
Proteins hydrolysed to aminoacids and peptides by pancreatic enzymes: trypsin, chymotrypsin, elastase, and carboxypeptidases
separate transporters facilitate movement of aminoacids, dipeptides and triopeptide
(in the first week some intact proteins can sneak through 'leaky' junctions between enterocytes before they become tight ?role in abnormal immune responses)
Vitamins (Pentavit has A, B1/2/3/6, C, D)
Fat soluble (A, D, E, K)
Minerals
Macrominerals (>100mg/day or 0.01-1% of wt)
Location of absorption
small Intestine: most H2O, Na, K, Cl, passive paracellular Ca
first 1/2 small intestine: Carbohydrates, protein, and fat (inc fat sol vit) (lower half in reserve)
Duodenum: active Ca absorption (triggered by Vit D)
Duodenum and proximal jejunum: iron
Jejunum: disaccharides
Jej and ileum: Zn + water soluble vitamins
Ileum: Bile acid
Distal ileum: B12
Colon: extra H2O, electrolytes, byproducts of bacterial carbohydrate digestion, Vit K
diet-induced thermogenesis: alcohol > protein > carbohydrate > fat
Digestion (breakdown)
Absorption
Transport
Enterocyte = intestine cell
Enteroendocrine cells: K, L, I, G, enterochromaffin, N, S
Gastrointestinal tract = gut (fore, mid, hind)
Intestine = bowel (small, large), enteric is adj.
4 layers from stomach to colon
- mucosa
- submucosa
- muscularis
- serosa
Gastric mucosa:
- 85% pariental (oxyntic) cells- secrete HCl, pepsinogen, intrinsic factor (for B12), mucous
- Endocrine glands: secrete endocrine and paracrine peptides
- Pyloric glands of antrum: gastrin-secreting cells
- Chief cells
- G Cells
- Pepsin
Small intestine
- 2.7m at birth, 5m at 4y (adult length)
- Enterocyte mature as they migrate from crypts toward villi tip (5 days for villi to be entirely regenerated
- The apical surface of enterocytes lining the villi are covered with microvilli creating a brush border
- Brush border contains enzymes and transport mechanisms for monosaccarides, amino acids, dipeptides, tripeptides and fats
- S-cells of duodenum, stimulated by pH <4, secrete secretin hormone, initiates panreatic release of HCO3
- I- cells of duodenum, release cholecystokinin, initiating bile and pancreatic enzyme release
- 1m
- 3 longitudinal taenia coli
- absorbs H2O and electrolytes, by products of bacterial degradation of carbohydrates
- Distention of rectum causes reflex to evaculate, facilitated by voluntary relaxation of ext sphincter
Nutrients
Carbohydrates (hydrated carbon)
Monosaccarides (all are reducing)
Glucose (absorbed by Na cotransport)
Galactose (uses glucose transported)
Fructose (absorbed by facilitative transport)
Disaccharides (have to be digested to monosaccharides, all digestible have glucose base + 1,)
Lactose (Glucose and Galactose): in milk
Sucrose (Glucose and Fructose): in plants
Lactulose (Galactose and Fructose) non-absorbable! hence laxative
Maltose (2 glucose): in starch
cleaved by lactase, sucrase, maltase respectively AT the brush border
Africans and asians have a natural decrease in lactase from 4yo > lactose intolerance as adults
Polysaccaride
Glycogen: glucose
Starch (amylose and amylopectin): broken down by pancreatic amylase to glucose, di/oligosaccarides
These are broken down by glucoamylase
Diasaccaride broken down by lactase, sucrase, maltase at brush border
Fats
Ingestion is mainly triglycerides
Lipase and colipase digest to monoglycerides and fatty acids (CF affects this)
Bile salts bind fatty acids forming micelles (Cholestasis affects this)
(medium chain triglycerides absorbed independent of bile so can feed this to bypass, also done to treat chylothorax)
absorption (coeliac disease affects)
reesterified to chylomicrons in enterocyte (abetalipoproteinemia affects)
Chylomicrons travel in lymph (intestinal lyphangiectasia affects)
long chain polyunsaturated fatty acids
- Omega-3
- Linolenic acid (essential fatty acid precursor to Docosahexaenoic acid)
- Omega-6 fatty acid
- Linoleic acid (essential fatty acid precursor to Arachidonic acid)
Proteins
Proenzymes secreted from pancreas
activated to enzymes by enterokinase (another enzyme) release from mucosa
Proteins hydrolysed to aminoacids and peptides by pancreatic enzymes: trypsin, chymotrypsin, elastase, and carboxypeptidases
separate transporters facilitate movement of aminoacids, dipeptides and triopeptide
(in the first week some intact proteins can sneak through 'leaky' junctions between enterocytes before they become tight ?role in abnormal immune responses)
Vitamins (Pentavit has A, B1/2/3/6, C, D)
Fat soluble (A, D, E, K)
- A: Retinol/betacarotene
- D: Cholecalciferiol (D3 skin UVB + food/sup), ergocalciferol (D2 sup)
- E:
- K:
- B1: Thiamine
- B2: Riboflavin
- B3: Niacin
- B6: Pyridoxine
- B7: Biotin
- B9: Folate (in nature), folic acid (synthetic form): megoblastic anaemia
- B12 (cobalamin): megablastic anaemia
- C: ascorbic acid
Minerals
Macrominerals (>100mg/day or 0.01-1% of wt)
- Sodium, Potassium, Chloride, Calcium, Phosphate, Magnesium
- Iron, Zinc, Copper, Manganese, Fluoride
- Arsenic, Boron, Chromium, Iodine, Selenium, Silicon, Nickel, Vanadium
Location of absorption
small Intestine: most H2O, Na, K, Cl, passive paracellular Ca
first 1/2 small intestine: Carbohydrates, protein, and fat (inc fat sol vit) (lower half in reserve)
Duodenum: active Ca absorption (triggered by Vit D)
Duodenum and proximal jejunum: iron
Jejunum: disaccharides
Jej and ileum: Zn + water soluble vitamins
Ileum: Bile acid
Distal ileum: B12
Colon: extra H2O, electrolytes, byproducts of bacterial carbohydrate digestion, Vit K
diet-induced thermogenesis: alcohol > protein > carbohydrate > fat