GI digestion and absorption Flashcards
blood supply to intestinal villus
Each villus has n arteriole which divides into capillaries beneath epithelial cells. Each villus has a venule and a lacteal too
Where is protein digested
stomach, intestine, Brush border, intracellular peptidases
protein digestion in stomach
pepsin breaks down into small peptides.
protein digestion in small intestine
pancreatic proteases trypsin, chymotrypsin, carboxypeptidase, elastase break down into oligopeptides, di/tri peptides, amino acids.
protein digestion in brush border
•Peptidases break down oligopeptides into amino acids, dipeptides, tripeptides
protein digestion intracellularly
•Peptidases in the enterocyte can break down di/tri-peptides into amino acids
safeguard to control activation of proteases
Trypsinogen is activated by enterocyte bound enterokinase into trypsin > trypsin activates chymotrypsinogen into chymotrypsin > chymotrypsin activates proelastase into elastase > elastase activates procarboxypeptidase A into carboxypeptidase A > CPA activates procarboxypeptidase B into carboxypeptidase B
Protein absorption mechanisms
- Sodium dependent co-transporters that utilize the N+/K+ ATPase gradient are the major route for the different classes of amino acids. Water follows. 2. Sodium independent transporters of amino acids. 3. Specific carriers for small peptides (di- and tri-) linked to H+ uptake (co-transporter; example is PEP T1). 4. Pinocytosis of small peptides by enterocytes (infants)
Which lipids are essential fatty acids
Linoleic acid which is converted to arachidonic acid and alpha-linolenic acid (all are fatty acids)
Where are bile acids made
Primary bile acids are produced in the liver from cholesterol – cholic acid & chenodeoxycholic acid. Secondary bile acids are formed by bacteria in the intestines & colon
Bile recycline
•Bile is recycled during a meal by uptake in the distal ileum – enterohepatic circulation
How do bile salts facilitate absorption of fats
formation of micelles
summarize lipid digestion steps
lingual and gastric lipase > pancreatic lipase (triglycerides into free fatty acids) > bile salts solubilize fat into micelles > free fatty acids transported into enterocytes > triglycerides resynthesized and chylomicrons formed > released into lacteal
How are fat soluble vitamins absorbed
absorbed along the length of the small intestines and are carried in micelles and form chylomicrons similar to dietary lipids.
how are water soluble vitamins absorbed
•enter the enterocyte by simple diffusion (biotin, folic acid) or via specific transporters (e.g. Vit B12)
Compare absorption and secretion at the intestinal crypts vs villi
•There is a net fluid secretion from cells in the intestinal crypts and a net fluid absorption from enterocytes on the villi. Villi surface area > crypt surface area
Describe absorption of water in intestines
follows movement of solutes- paracellular and/or transcellular. Colon has lowest paracellular permeability b/c water movement is linked to transcellular ion movement
Where does the majority of secretion and absorption of fluids occur
secretion: stomach. Absorption: jejunum
Where/how is sodium absorbed
•Absorbed all along the intestine, with most absorption in the jejunum (60-80%). Na/K ATPase gradient established. Mechanism via Na+/glucose & galactose or Na+/amino acid cotransport, NaCl cotransport, Na+/H+ exchange or passive diffusion via epithelial sodium channels (ENaC)
Function of carbonic anhydrase
converts CO2 and water to H2CO3, which is then broken down to H ion and HCO3-
Chloride absorption
Proximal intestines: passive due to loose TJs, offsets Na charge in intercellular space. Distal ileum and colon: Less leaky TJs, Cl/bicarb exchanger offsets acids produced by bacteria
Potassium absorption
passive. Paracellular movement in jejunum, transcellular in colon. K is normally high in cells due to Na/K pump, but a gradient is established as luminal water decreases on approach to the colon, with passive flux of K into cells.
Calcium and magnesium absorption
Ca++ and Mg++ compete for uptake by the cells. Ca++ enters enterocyte passively down its electrochemical gradient in proximal intestines. Ca ATPase pumps Ca out to the blood
Vitamin D function
Synthesized in skin or absorbed by intestine. Stimulates uptake of Calcium by increasing Ca binding proteins and Ca ATPase molecules
Iron absorption
regulated in proximal intestines. Transported as either heme or Fe. Two fates: binds to apoferritin to form ferritin that stays in the cell and is lost when the cell dies OR binds to transferrin (carrier protein), leaves the cell and goes into the blood
Causes of diarrhea
motility disorders or osmotic diarrhea (celiac disease, lactose intolerance) or secretory diarrhea (ie. cholera)
How does cholera cause secretory diarrhea
•Increases cAMP levels in cells and this in turn activates the CF chloride channel, (and thus water) on the luminal surface
How does oral rehydration therapy work
•antibiotics plus KHCO3 to prevent hypokalemia and metabolic acidosis, glucose (or amino acids) with NaCl to facilitate the absorption of electrolytes and water
chronic pancreatitis
deficiency of pancreatic enzymes
Cysteinuria
genetic absence/defect of the Na+-amino acid transporters.
Hartnup disease
genetic absence/defect of the neutral amino acid transporter
steatorrhea
excessive loss of fat in stool due to fat malabsorption disorders such as liver disease (bile salt deficiency), pancreatic insufficiency, or weight loss meds
pernicious anemia
impaired absorption of B12
