6 Digestion And Absorption In The GI Tract Flashcards
Two major routes for digestion
Cellular: from lumen through apical membrane, intracellular space of epithelial cells, and cross basolateral membrane into blood
Paracellular: between epithelial cells through lateral intracellular space through tight junctions
Tow types of digestive activity
Cavital (luminal): digestion resulting from the actions of enzymes secreted by the salivary glands, stomach, and pancreas
Membrane (contact): hydrolysis of enzymes synthesized by epithelial cells
Intestinal mucosa is ideal for its fx of ____ because its lumen is arranged in longitudinal folds called ____.
What increases the surface area and what do they contain?
Absorption
Folds of Kerckring
Villi and microvilli; digestive enzymes
Have tight junctions
Cell types in the intestinal epithelium and fx
Enterocytes: epithelial cells; digestion, absorption, secretion; susceptible to irradiation and chemo
Goblet cells: mucus secreting; provide physical, chemical, and immunological protection
Paneth cells: mucosal defense; secrete agates that destroy bacteria or produce and inflammatory response
Three ways substances pass through enterocytes
Pinocytosis: at the base of the microvilli; mechanism for protein uptake
Passive diffusion: through pores in the cell membrane, between cells; channels on both membranes
Membrane transporters: facilitated diffusion; active transport (can be cotransporters)
Pathway (barriers) of solutes moving across the the enterocytes from lumen to blood
Unstirred layer of fluid
Glycocalyx
Apical membrane with microvilli
Cytoplasm of cell
Basolateral membrane
Basement membrane
Wall of blood capillary/lymphatic vessel
Terminal ileum resection abolishes ____ and _____ absorption
Bile salt and vitamin B12
Name the enzyme in carb assimilation in the SI:
Starch-> maltose->glucose
Lactose->glucose and galactose
Sucrose-> glucose and fructose
Trehalose-> glucose
Alpha amylase (pancreatic secretions); maltase
Lactase (brush border enzyme)
Sucrase (brush border enzyme)
Trehalase (brush border enzyme)
Monosaccharides are absorbed into the enterocytes through ____.
Transports glucose and galactose
Transports glucose, galactose, and fructose
Transports fructose
All powered by ____.
Transport proteins
SGLT1 (usages Na concentration gradient)
GLUT2
GLUT5
Na/K ATPase
***know what happens if you don’t have a transporter; ex: lack GLUT5, build up of fructose in lumen
Failure in the ability to digest carbs
Brush border ____ enzyme is deficient or absent
Lactose remains in the lumen; holds water causing ____.
Lactose intolerance
Lactase
Osmotic diarrhea
D-Xylose test
D-xylose is a monosaccharide
Give a set dose to the pt that is normally easily absorbed by passive diffusion
Measure levels to see the absorptive capacity of the proximal SI to absorb sugars
***tests for malabsorption, but doesn’t tell you which sugar is the problem
Main digestive enzymes in protein assimilation:
Pepsinogen (from chief cells)-> pepsin
Trypsinogen (from pancreas)-> trypsin
Trypsin converts enzymes to ____.
Low pH of stomach
Enterokinase (from brush border of SI)
Proteases
There are separate ____ and ____ for each AA type including neutral, acidic, basic, and imino crossing the enterocyte (brush border)
AA co-transporters use ____.
Dipeptides and tripeptides co-transporters use ____.
Co-transporters and diffusion mechanisms
Na+
H+ (not the best choice; pancreatic secretions should be secreting HCO3 to neutralize the H+)
Deficiency of pancreatic enzymes; lack a protease (like trypsinogen)
Chronic pancreatitis and cystic fibrosis
Absence of trypsin
Leads to what?
Congenital trypsin absence
Absence of all pancreatic enzymes because trypsin catalyze the hydrolysis of those enzymes
Defect or absence of Na/AA cotransporters
Di-basic AA transporter absent from SI and kidney because of an intestinal deficiency causing AA to be secreted in the feces…what AA are absent?
Cystinuria
Cysteine, lysine, arginine, ornithine
Cannot absorb neutral AA
Symptoms resemble pellagra caused by a ____ deficiency
Hartnup disease
Niacin/vitamin B3
Sucrose, lactose, or glucose are are absorbed as glucose by _____.
Brush border enzymes
Proteins break down into substances that are more easily absorbed
Where?
Polypeptides broken down in ___.
Dipeptides, tripeptides, and AA
Mostly in SI
Stomach
Why is trypsinogen activated in the SI?
It is secreted by the pancreas; if it is activated in the pancreas, trypsinogen would be converted to trypsin and would begin to activate enzymes that would destroy the pancreas
Mutation of CFTR receptor
CFTR normally regulates the Cl channel on the apical membrane of ductal cells
Pancreatic problems: can’t absorb Cl so can’t secrete HCO3
Leads to pancreatitis
Cystic fibrosis
Lipids are ____, therefore must be solubilized in order to be transported
Insoluble
Enzymes of lipid assimilation:
Triglyceride->monoglyceride & 2 FA
Cholesterol ester->cholesterol & FA
Phospholipid-> lysolecithin & FA
Lingual, gastric, and pancreatic lipases
Cholesterol ester hydrolase
Phospholipase A2
What lipases initiate digestion of lipids in the stomach?
How?
Lingual and gastric lipases
Break down lipids into small droplets that are emulsified in the stomach by dietary proteins (no bile acids in stomach)
What role does CCK play in lipid assimilation?
When is it secreted?
Fx?
Allows more time for proper lipid digestion
Secreted when lipids first appear in the SI
Slows the rate of gastric emptying (more mixing)
Activates gallbladder secretions
What pancreatic enzymes are secreted into the SI to complete lipid digestion?
Pancreatic lipase
Colipase
Cholesterol ester hydrolase
Phospholipase A2
Enzymes activated by trypsin
Colipase
Phospholipase A2
Lipid digestion begins in the mouth with a ____ but most takes place in the ____.
Lingual lipase
SI
Catalyzes the production of cholesterol; hydrolyzes triglycerides to glycerol
Cholesterol ester hydrolase
Proenzyme activated by trypsin; displaces bile salts by binding to pancreatic lipase (pushes bile salts off fat droplets so lipase can do its job)
Coliapse
Proenzyme activated by trypsin
Phospholipase A2
Secreted as an active enzyme; inactivated by bile salts without pancreatic lipase
Pancreatic lipase
Five steps in lipid assimilation to make a chylomicron
Solubilization by micelles (packaging using bile salts)
Diffusion of micellar content across apical membrane
Re-esterification (put lipids back together; goes on continuously)
Chylomicron formation (packaged fat that leaves the cell)
Exocytosis of chylomicrons
Steps where lipid assimilation can go wrong
All of it can result in ____.
Pancreatic enzyme secretion
Bile acid secretion
Emulsification
Micelle formation
Diffusion of lipids into intestinal epithelial cells
Chylomicron formation
Transfer of chylomicrons into lymph
STEATORRHEA
Failure to secrete proper amounts of pancreatic enzymes
Pancreatic insufficiency
Gastrin-secreting tumor of the pancreas
Increased H+ secretion by gastric parietal cells
Overload of acid into duodenum
Zollinger-Ellison syndrome
(Improper acidity of duodenum because pancreas not working so can’t secrete HCO3; build up of H+; lipase can’t work in an acidic environment)
Impaired HCO3 and enzyme secretion
Pancreatitis
Bacteria deconjugate bile salts, impairing ____ formation.
Severe bacterial overgrowth damages the intestinal ____.
Two main causes are too little gastric secretion and SI dysmotility
Micelle
Mucosa
Small intestinal bacterial overgrowth (SIBO)
What interferes with the formation of micelles in the SI?
Deficiency of bile salts
Ileal resection
Small intestinal bacterial overgrowth (SIBO)
Total bile salt pool is reduced because of loss of enterohepatic recirculating of bile salts
Ileal resection
Reduced microvilli surface area
Lipid absorption impaired due to loss of microvilli (steatorrhea)
Nutritional deficiencies of folate and a vitamin B12
Diarrhea
What do you treat with?
Tropical Sprue
Tetracycline, folate
Autoimmune disorder leading to destruction of SI villi and hyperplasia of intestinal crypts
Malabsorption related to deficiencies in folate, iron, calcium, vitamins A,B12,D
Symptoms of malabsorption like numbness, tingling of extremities
Treatment?
Celiac Sprue
Gluten-free diet
Forms of gut microbacteria:
Type of fiber (inulin)
Bacteria/microorganisms contacting foods
Others
Prebiotics
Probiotics
Live bacterial therapeutics
Roux-en-Y gastric bypass
Fecal microbiota transplant
What type of vitamins have the same absorption mechanisms as lipids?
What type are absorbed by Na dependent cotransport
Fat-soluble (if pt had steatorrhea, they are not absorbing lipids…also not absorbing fat-soluble vitamins)
A,D,E,K
Water-soluble: B,C
Vitamin B12 is a ____ vitamin that forms complexes with other proteins to be absorbed like ____.
R proteins (secreted in salivary juices)
Intrinsic factor
Transcobalamin
Functions of vitamin B12
Performs several metabolic functions-hydrogen acceptor coenzyme
Acts as a coenzyme for reducing ribonucleotides to deoxyribonucleotides-replication of genes
Co-factor in folate coenzyme recycling and nerve myelination
Deficiency in vitamin B12 causes _____ of the large nerve fibers of the spinal cord
Important in ____ synthesis in RBC
Demyelination
DNA
Failure of RBC maturation when vitamin B12 is deficient
Pernicious anemia
Macrocytic and megaloblastic are other vitamin B12 deficiency disorders
In vitamin B12 absorption, ____ and _____ have to get in the right place and get absorbed
____ bind to the vitamin B12 and pull it along
Intrinsic factor and R protein
Intrinsic factor receptors
In gastric bypass, exclusion of the stomach, duodenum, and proximal jejunum alters the absorption of ____.
Gastrectomy causes the loss of parietal cells, the source of ____.
Vitamin B12
Intrinsic factor
What causes the stomach to not produce enough intrinsic factor, leading to pernicious anemia?
Atrophic gastritis
Autoimmune metaplastic atrophic gastritis
Chronic inflammation of the stomach mucosa that leads to loss of parietal cells
………………….
Immune system attaches intrinsic factor protein or gastric parietal cells
Atrophic gastritis
Autoimmune metaplastic atrophic gastritis
Deficiency in vitamin D absorption caused by what?
Vitamin D is a ____ vitamin.
Inadequate calcium absorption
Rickets (children)
Osteomalacia (adults/can add vitamin D to Ca)
Fat soluble (malabsorption of fat, can have vitamin D deficiency)
Iron absorption occurs when the liver secretes ____ into bile ->enters duodenum->binds with free iron and ____ forming transferrin.
_____binds to receptors on membranes of intestinal epithelial cells, absorbed into cells by pinocytosis and later released.
Apotransferrin
Hemoglobin
Transferrin
What nutrients are absorbed in the proximal SI (duodenum)?
Fat, sugars, peptides, AA, iron, folate, Ca, water, electrolytes
What nutrients are absorbed in the middle SI (jejunum)?
Sugars, peptides, AA, Ca, water, electrolytes
What nutrients are absorbed in the distal SI (ileum)?
Bile acids, vitamin B12, water, electrolytes
What nutrients are being absorbed in the colon?
Water, electrolytes, medium-chain triglycerides, MCTs, Ca, AA
