GI digestion and absorption

Question 1

blood supply to intestinal villus

Answer 1

Each villus has n arteriole which divides into capillaries beneath epithelial cells. Each villus has a venule and a lacteal too

Question 2

Where is protein digested

Answer 2

stomach, intestine, Brush border, intracellular peptidases

Question 3

protein digestion in stomach

Answer 3

pepsin breaks down into small peptides.

Question 4

protein digestion in small intestine

Answer 4

pancreatic proteases trypsin, chymotrypsin, carboxypeptidase, elastase break down into oligopeptides, di/tri peptides, amino acids.

Question 5

protein digestion in brush border

Answer 5

•Peptidases break down oligopeptides into amino acids, dipeptides, tripeptides

Question 6

protein digestion intracellularly

Answer 6

•Peptidases in the enterocyte can break down di/tri-peptides into amino acids

Question 7

safeguard to control activation of proteases

Answer 7

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

Question 8

Protein absorption mechanisms

Answer 8

1. 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)

Question 9

Which lipids are essential fatty acids

Answer 9

Linoleic acid which is converted to arachidonic acid and alpha-linolenic acid (all are fatty acids)

Question 10

Where are bile acids made

Answer 10

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

Question 11

Bile recycline

Answer 11

•Bile is recycled during a meal by uptake in the distal ileum – enterohepatic circulation

Question 12

How do bile salts facilitate absorption of fats

Answer 12

formation of micelles

Question 13

summarize lipid digestion steps

Answer 13

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

Question 14

How are fat soluble vitamins absorbed

Answer 14

absorbed along the length of the small intestines and are carried in micelles and form chylomicrons similar to dietary lipids.

Question 15

how are water soluble vitamins absorbed

Answer 15

•enter the enterocyte by simple diffusion (biotin, folic acid) or via specific transporters (e.g. Vit B12)

Question 16

Compare absorption and secretion at the intestinal crypts vs villi

Answer 16

•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

Question 17

Describe absorption of water in intestines

Answer 17

follows movement of solutes- paracellular and/or transcellular. Colon has lowest paracellular permeability b/c water movement is linked to transcellular ion movement

Question 18

Where does the majority of secretion and absorption of fluids occur

Answer 18

secretion: stomach. Absorption: jejunum

Question 19

Where/how is sodium absorbed

Answer 19

•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)

Question 20

Function of carbonic anhydrase

Answer 20

converts CO2 and water to H2CO3, which is then broken down to H ion and HCO3-

Question 21

Chloride absorption

Answer 21

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

Question 22

Potassium absorption

Answer 22

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.

Question 23

Calcium and magnesium absorption

Answer 23

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

Question 24

Vitamin D function

Answer 24

Synthesized in skin or absorbed by intestine. Stimulates uptake of Calcium by increasing Ca binding proteins and Ca ATPase molecules

Question 25

Iron absorption

Answer 25

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

Question 26

Causes of diarrhea

Answer 26

motility disorders or osmotic diarrhea (celiac disease, lactose intolerance) or secretory diarrhea (ie. cholera)

Question 27

How does cholera cause secretory diarrhea

Answer 27

•Increases cAMP levels in cells and this in turn activates the CF chloride channel, (and thus water) on the luminal surface

Question 28

How does oral rehydration therapy work

Answer 28

•antibiotics plus KHCO3 to prevent hypokalemia and metabolic acidosis, glucose (or amino acids) with NaCl to facilitate the absorption of electrolytes and water

Question 29

chronic pancreatitis

Answer 29

deficiency of pancreatic enzymes

Question 30

Cysteinuria

Answer 30

genetic absence/defect of the Na+-amino acid transporters.

Question 31

Hartnup disease

Answer 31

genetic absence/defect of the neutral amino acid transporter

Question 32

steatorrhea

Answer 32

excessive loss of fat in stool due to fat malabsorption disorders such as liver disease (bile salt deficiency), pancreatic insufficiency, or weight loss meds

Question 33

pernicious anemia

Answer 33

impaired absorption of B12