Digestion and Absorption Process in GI tract Flashcards
What are the two main paths of nutrients, electrolytes and water from lumen to the blood?
cellular and paracellular
What are the two types of digestive activity ?
cavital (luminal) digestion: digestion from enzymes secreted by salivary glands, stomach, and pancreas
Membrane (contact) digestion: hydrolysis by enzymes synthesized by epithelial cells
The surface of the small intestine is arranged in longitudinal folds called…
folds of kerckring
Where are villi the longest in the small intestine?
duodenum
What are the functions of enterocytes? what are they susceptible to ?
they are epithelial cells that function in digestion, absorption and secretion
turnover rate: 3-6 days
Susceptible to irradiation and chemotherapy
What are the functions of goblet cells?
mucus-secreting cells that provide physical, chemical and immunologic protection
What is the function of paneth cells?
part of the mucosal defenses against infection
secrete agents that destroy bacteria or produce inflammatory responses
What mechanisms does the enterocyte used to control the flux of solutes and fluids between the lumen and blood?
pinocytosis: at base of microvilli (major mechanism of uptake of protein)
Passive diffusion: thru pores in cell membrane or intercellular spaces
Facilitated diffusion
active transport
What barriers must solute move across to get through enterocyte?
unstirred layer of fluid glycocalyx apical membrane cytoplasm basolateral membrane basement membrane wall of the blood capillary
The capacity of the intestine to adapt is key in several clinical scenarios including small bowel resection and bypass. When is adaptation limited?
If terminal ileum is resected, absorption of vit B112 and bile salts is abolished
In what form can carbs be absorbed?
only monosaccharides
glucose, galactose, and fructose
How is starch broken down?
amylase breaks it down to disaccharides then other enzymes like maltase and sucrase breaks it down to glucose
How is lactose and sucrose broken down?
Lactose by lactase to form glucose and galactose
sucrose by sucrase to form glucose and fructose
What transporters are on the lumenal side of epithelial cells that absorb carbs?
SGLT1: Symporter of Na and glucose/galactose - secondary active transport
GLUT 5: fructose - facilitated
What transporters are on the basolateral side of epithelial cells that help in the absorption of carbs?
Na/K ATPase
GLUT 2: does glucose, galactose, and fructose facilitated diffusion
What happens with lactose intolerance?
Brush-border lactase is deficient
Lactose remains in lumen and holds water, causes osmotic diarrhea
Fermented into methane and gas
what enzyme is released in the stomach to break down proteins?
pepsinogen
protein->amino acids and oligopeptides
What are the endopeptidases: break peptide bonds of nonterminal amino acid?
Pepsin, trypsin, chymotrypsin, elastase
What are the exopeptidases: break peptide bonds from end-pieces of terminal amino acids.?
Carboxypeptidase A
Carboxypeptidase B
What activates trypsinogen?
enteropeptidase in the brush border
What activates all endopeptidases and exopeptidases?
trypsin
What are the absorbable forms of proteins?
A.a, dipeptide and tripeptide
What transporters for a.a. can be found on the luminal side of Epithelial cells of the small intestine?
there are 4 separate cotransporters of Na/a.a.: one each for neutral, acidic, basic and imino aa.
Cotransporter for H/dipeptides/tripeptides
Na/H antiporter
What transporters for a.a. can be found on the basolateral side of epithelial cells of the small intestine?
Na/K ATPase
There are 4 separate facilitated diffusion mechanisms: one each for neutral, acidic, basic, and imino aa.
Peptidase inside the cell can break down dipeptides and tripeptides or they can diffuse through membrane
When do disorders of protein assimilation occur?
when there is a deficiency of pancreatic enzymes or a defect in transporters of the intestinal epithelial cells
- chronic pancreatitis and cystic fibrosis: enzyme deficiency
- trypsin absence
- cystinuria
- hartnup disease
What is cystinuria?
defect in or absence of Na- amino acid cotransporters: transporter for di-basic amino acids is absent from small intestine and kidney
Genetic disorder
a.a. secreted in feces
What is Hartnup disease?
inability to absorb neutral amino acids like tryptophan
Recessive genetic disorder
Symptoms similar to pellagra (deficiency of niacin)
Diarrhea, mood changes, neurologic problems, short, red rash, photosensitivity
Urine sample-high excretion of neutral aa and by-products (serotonin)
What is CFTR?
a regulated anion channel that is located at the apical surface of the duct cell that transports Cl ions
What is one of the first organs to fail in cystic fibrosis? Why?
pancreas
Loss of bicarb secretion (Cl/HCO3 antiporter)
ability to flush active enzymes out of duct loss
Recurrent acute and chronic pancreatitis
What breaks down lipids in the stomach and small intestine?
lipases
How must lipids be processed to be absorbed?
lipids need to be solubilized in micelles and transported to apical membrane of intestinal epithelial cells for absorption
Describe the intial digestion of lipids in the stomach?
lingual and gastric lipases
10% of TG hydrolyzed to glycerol and FA
Broken down to small droplets
droplets emulsified in stomach by dietary proteins (NO BILE IN STOMACH)
What allows for sufficient time for lipids to get digested properly?
CCK, slows rate of gastric emptying
released when lipids
What happens to pancreatic lipase once it enters the small intestine? When does it become efficient?
Pancreatic lipase secreted as active enzyme is inactivated by bile salts
Colipase binds to pancreatic lipase to make it more efficient to break down TG into free fatty acids and 2 monoglyceride
What is colipase and phospholipase A2 activated by?
trypsin
What is the role of cholesterole ester hydrolase?
catalyze the production of cholesterol
hydrolyzes TG to produce glycerol
What is the optimal pH for pancreatic lipase?
6
How are lipids processed in epithelial cell of the small intestine? What can an abnormality in any of these steps result in?
Pancreatic enzyme secretion and bile acid secretion -> emulsification->micelle formation
- solubilization by micelles
- diffusion of micellar content across apical memb.
- reesterification
- chylomicron formation
- exocytosis of chylomicron into lymph
Steatorrhea
Chol + FFA = ?
MG + FFA = ?
LysoPL + FFA = ?
And where do these reactions occur?
Chol E
TG
PL
Epithelial cell of small intestine
What leads to abetalipoproteinemia? Why?
No ApO B because there will be no absoprtion of dietary lipids
What is pancreatic insufficiency?
failure to secrete adequate amounts of pancreatic enzymes
What can cause deficiencies of bile salts?
Ileal resection interrupts the enterohepatic circulation of bile salts which makes the total bile salt pool reduced
Small intestinal bacterial overgrowth can cause bile salts to decojugate
both causes failure of micelle formation and subsquent fat malabsorption
What causes SIBO?
Decrease gastric acid secretion and small intestine dysmotility can cause an overgrowth of bacteria
What is tropical sprue? What is the treatment?
reduction in the # of intestinal epithelial cells which in turn reduces the microvillar surface area
Cause: unknown
Steatorrhea, nutritional deficiencies (folate and vit B12)
cramps, nausea, weight loss,
Tx: tetracycline and folate for 6 mo.
What is non-tropical sprue? Most common with what race and sex?
celiac sprue
autoimmune disorder
hereditary compoent
Abs against gluten lead to destruction of villi in small intestine and hyperplasia of intestinal crypts
Malapsorption: vit B12, folate, iron, Ca, Vit D and Vit A
Caucasians and european ancestry
Women
What are the fat-soluble vitamins? how are the absorbed?
A, D, E, K
same mechanism as lipids
What are the water-soluble vitamins? How are they absorbed?
B1, B2, B3, B12, C, biotin, folic acid, nicotinic acid and panthothenic acid
Most absorbed via Na-dependent cotransport in small bowel
B12 (cobalamin): forms complexes with other proteins to be absorbed
What does cobalamin form a complex with in order to be absorbed?
R proteins: secreted in salivary juices
- Remains complex until in duodenum and pancreatic proteases
Intrinsic factor: secreted by gastric parietal cells
- Carries through IF receptor in distal ileum
Trnascobalamin II: carries from mucosa into blood
What surgeries can disrupt the absorption of vit B12?
Gastrectomy: loss of parietal cells
Gastric bypass: exclusion of stomach, duodenum and proximal jejunum alters absorption
What is vitamin B12 anemia called? What are common causes?
pernicious anemia
- atrophic gastritis: loss of parietal cells with chronic inflammation
- Autoimmune response against IF or gastric parietal cells
What does sunlight change 7-dehydrocholesterol into?
Cholecalciferol - vitamin D3
What is needed to increase Gut Ca absorption, bone calcification and bone resorption?
1,25 Dihydroxy-D3
