Digestion and Absorption process of GI Flashcards
Where does digestion start and end
begins in the stomach and ends in small intestine
Where does absorption take place
small intestine
2 main paths of absorption
cellular and paracellular
Cellular absorption
lumen –> apical membrane –> intestinal epithelial cell –> basolateral membrane –> bloof
Where are the transporters for cellular absorption
in membrane
Paracellular absorption
intestinal epithelial cell –> lateral intercellular space –> intestinal epithelial cell
2 types of digestive activity
cavital (luminal) digestion and membrane (contact) digestion
Cavital digestion
digestion from action of enzymes of salivary glands, stomach and pancreas
Membrane digestion
hydrolysis by enzymes of epithelial cells
Folds of Kerckring
longitudinal folds of small intestine
Function of villi and microvilli in small intestine
increase surface area
Where are the villi the longest and where are they the shortest
longest in duodenum and shorter in terminal ileum
Site of activity of digestive enzymes
microvillar surface
Barrier that allows some passage of nutrients, water, electrolytes
microvillar surface
Enterocytes
epithelial cells of intestine
Function of enterocytes
digestion, absorption, and secretion
Goblet cells
mucus-secreting cells
Function of goblet cells
physical, chemical and immunological protection
Paneth cells function
mucosal defense against infection and bacteria
What controls the flux of solutes and fluid between lumen and blood
enterocyte membrane
4 ways enterocytes move solutes and fluid
pinocytosis, passive diffusion, facilitated diffusion, and active trasport
Where does pinocytosis occur and whats its function
base of microvilli and used for uptake of protein
What movement takes place during absoprtion
transmural
Monosaccharides are absorbed by
enterocytes
Three end products of carbohydrate digestion
glucose, galactose, and fructose
What does lactose break into
glucose and galactose
What does sucrose break into
glucose and fructose
What breaks down starch
alpha amylase
What can starch break into
maltose, dextrins, maltotriose
Transport mechanisms for absorption of carbs
co-transport and facilitated diffusion
GLUT 2
gets glucose, galactose and fructose from intracellular into blood
SGLT1
takes glucose and galactose out of the lumen into intracellular
GLUT5
takes fructose from lumen to intracellular
What kind of transport is SGLT 1
secondary active transport
Where is the Na/K ATPase located on the cells for carb absorption
basolateral side
Lactose intolerance
failure to digest carb
Why does lactose intolerance occur
brush-border lactase is lacking
Result of lactose intolerance
osmotic diarrhea– undigested lactose remains in lumen and holds water
What happens to unabsorbed and undigested lactose
ferments into methane and hydrogen gas causing excess gas
How are proteins digested to absorbable forms
proteases in stomach and small intestine
Stomach protease
pepsinogen –> pepsin
Small intestine proteases
trypsinogen –> trypsin, chymotrypsinogen –> chymotrypsin, proelastase –> elastase, proccarboxypeptidase (A or B) –> carboxypeptidase (A or B)
Endopeptidases
pepsin, trypsin, chymotrypsin, elastase
Exopeptidases
carboxypeptidase A and B
What does protein breakdown into in the stomach
amino acids and oligopeptides
What does protein breakdown into in the small intestine
amino acids –> dipeptides and tripeptides, oligopeptides –> amino acids –> dipeptides and tripeptides
Function of trypsin
catalyzes hydrolysis of trypsinogen (autocatalysis)
Transport mechanisms for absorption of proteins
co-transport and facilitated diffusion
Luminal cotransporters for amino acids
Na+/amino acid cotransporter – one for each kind of amino acid - neutral, acidic, basic, and imino
How do dipeptides and tripeptides cross the luminal surface
H+/ Dipeptide or Tripeptide cotransporter
Basolateral (Blood) side of protein absorption
facilitated diffusion- one kind for each amino acid - neutral, acidic, basic, and imino
Disorders of protein assimilation
occur when there is a deficiency of pancreatic enzymes or transporter problem
Chronic pancreatitis and cystic fibrosis
deficiency of pancreatic enzymes
Congenital trypsin absence
absence of trypsin makes it seem like all pancreatic enzymes are gone
Cystinuria
genetic disorder where amino acids are secreted in the feces
Process of cystinuria
defect in or absence of Na+/amino acid co-transporter
Hartnup disease
can’t absorb neutral amino acids (tryptophan)
Genetics of Hartnup disease
recessive genetic disorder
Symptoms of Hartnup disease
diarrhea, red scaly skin, photosensitivity, short stature
Urine samples of Hartnup disease
abnormally high excretion of neutral amino acids (trypotophan) and their by products (serotonin)
Cystic fibrosis transmembrane conductance regulator (CTFR)
chloride ion channel
Mutation in CTFR
associated with deficiency of pancreatic enzymes
What organ is the first to fail in cystic fibrosis
pancreas
How are lipids digested to absorbable forms
lipases in stomach and small intestine
Triglyceride breakdown into
via lingual, gastric, and pancreatic lipases into monoglyceride and 2 fatty acids
Cholesterol ester breakdown into
via cholesterol ester hydrolase into cholesterol and fatty acid
Phospholipid breakdown into
via phospholipase A2 into lysolechitin and fatty acid
Why lipid breakdown is complicated
lipids need to be solubilized in micelle and transported to apical membrane for absorption
Lingual and gastric lipases…
initiate lipid digestion in stomach
Where are lipid droplets emulsified
in stomach by dietary proteins – with NO bile acids
What enzyme in the stomach helps lipid digestion and how
CCK because it slows gastric emptying allowing more time for breakdown
Where does most lipid digestion occur
small intestine
Function of bile salts
emulsify fat
Pancreatic enzyme and lipid digestion
enzymes secreted into small intestine to complete digesiton
Pancreatic lipase
inactivated by bile salts
Colipase
secreted an inactive for and activated by trypsin and binds to pancreatic lipase to displace the bile salt
Cholesterol ester hydrolase
catalyze cholesterol production and hydrolyze triglycerides into glycerol
Phospholipase A2
proenzyme is activated by trypsin
Optimal pH for pancreatic lipase
6
Products of lipid digestion
micelles
Micelle structure (5)
bile salt, hydrophilic side, hydrophilic side, nonpolar lipid and phospholipid
5 steps of processing a lipid
- solubilization by micelles, 2.diffusion of micellar content across apical membrane, 3.reesterification, 4.chylomicron formation, 5.exocytosis of chylomicron
* *includes Apo B ** on chylomicron
No ApoB on chylomicron can lead to
abetalippoproteinemia and you can’t absorb dietary lipids
Abnormality of lipid assimilation
steatorrhea
Pancreatic insufficiency
failure to secrete adequate pancreatic enzymes
Function of pancreatic enzymes
regulate acidity of duodenum
Zollinger-Ellison syndrom
gastric secreting tumor causing H+ increase and overload of acid in duodenum
Pancreatitis
impaired HCO3 secretion and impaired enzyme secretion
Deficiency of bile salt
interferes with formation of micelles
Causes of bile salt deficiency
ileal resection and small intestinal bacterial overgrowth (SIBO)
Ileal resection and bile salt deficiency
interrupts enterohepatic circulation of bile salts and bile salt pool is reduced
Small intestinal bacterial overgrowth (SIBO)
bacteria deconjugate bile salts causing failure of micelle formation and fat malabsorption
Two main causes of small intestinal bacterial overgrowth
decrease in gastric acid secretion and small intestin dysmotility
Clinical presentation of SIBO
chronic diarrhea, weight loss, and malabsorption
Tropical sprue
reduction in number of intestinal epithelial cells which reduces microvillar surface area
Why is lipid absorption impaired in tropical sprue
because surface area for absorption is decreased
S/S of tropical sprue
steatorrhea, deficient in folate and B12, diarrhea
Treatment for tropical spure
tetracycline and folate
Non-tropical sprue (celiac sprue)
autoimmune disorder where antibodies develop against gluten
S/S of non-tropical sprue
malabsorption and deficiency in vitamin B12, folate, iron, calcium, vitamin D, and vitamin A
Prevalence of Non-tropical sprue
caucasian and european ancestry and woman
GI symptoms of celiac spure
abdominal pain, constipation, diarrhea, steatorrhea
Non Gi symptoms of celiac sprue (due to nutrient deficiency)
tingling in hands, itchy skin, easy bruising
Fat soluble vitamins
A, D, E, K
Mechanism of fat soluble absorption
same as lipids
Water soluble vitamins
B1,2,3,12, C, biotin, folic acid, nicotinic acid, and pantothenic acid
How are water soluble vitamins absorbed
Na+ dependent cotransport in small intestine
Vitamin B12 importance
complexes with other proteins– R protins, intrinsic factor, and transcoalbumin II
2 disruptions in absorption of vitamin B12
gastrectomy and gastric bypass
Gastrectomy
loss of parietal cells and intrinsic factor source
Gastric bypass
exclusion of stomach, duodenum, and proximal jejunum alters absorption of vitamin B12
Pernicious anemia
stomach doesn’t produce enough intrinsic factor and decrease in B12
Common causes of pernicious anemia
atrophic gastritis and autoimmune condition
Atrophic gastritis
chronic inflammation of stomach mucosa causing loss of parietal cells
Abnormalities of absorption of vitamin D
Rickets, osteomalacia due to inadequate Ca2+ absorption b/c deficient vitamin D
2 sources of D3 in vitamin D formation
Sunlight and Diet
Vitamin D effects of body (3)
increase in gut Ca2+ absorption, increase bone calcification and increase bone resorption
Enzyme that makes vitamin D usable to body
1 alpha hydroxylase
Usable form of vitamin D
1,25 Dihydroxy D3