Module 6: Section 5

Question 1

3 segments of S intestine

Answer 1

• duodenum
• jejunum
• ilium

Question 2

motility of small intestine

Answer 2

• by segmentation
• mixes and slowly propels chyme
• movement is NOT peristaltic
• occurs by localized concentric contractions (occur every few cm)
  – traps chyme in relaxed areas
• after a brief period of time, contracted area relaxes and ring like contractions appear in the previously relaxed areas
• chyme is moved back and forth for thorough mixing and is slowly moved forward

Question 3

what is segmentation in the empty ilium caused by?

Answer 3

gastrin

Question 4

gastroileal reflex

Answer 4

food triggers peristalsis in ileum and moves contents to S intestine to colon

Question 5

what is S intestine segmentation initiated by

Answer 5

pacemaker cells that generate a basic electrical rhythm (BER)
- similar to stomach
- If BER brings circular SM to threshold then contraction will occur

Question 6

BER time in duodenum and jejunum vs terminal ileum

Answer 6

12 /min
- ileum = 9/min

Question 7

what is excitability of SM enhanced by?

Answer 7

distention of S intestine, the hormone gastrin and extrinsic nerve activity

Question 8

when is segmentation absent?

Answer 8

between meals

Question 9

what do segmentation contractions get replaced by when most of the meal has been absorbed?

Answer 9

migrating motility complex

Question 10

migrating motility complex

Answer 10

weak peristaltic waves
- sustained for short distances then die out
- start in stomach and move to intestine
- purpose = to move remnants of previous meal, mucosal debris and bacteria forwards to colon

Question 11

purpose of small intestine secretions

Answer 11

lubrication and protection
- and provide water for hydrolytic reactions in dig syst

Question 11

luminal surface of small intestine

Answer 11

microvilli that form that brush border

Question 12

brush border

Answer 12

• lumen facing epithelial surface of cells
• tightly backed microvilli and hydrolytic enzymes
• present in cells that specialize in absorption
• has membrane bound enzymes that are categorized into 3 classes

Question 13

3 classes of membrane bound enzymes in brush border

Answer 13

1) enterokinases
- can convert pancreatic trypsinogen to trypsin
2) disaccharidases
- complete carb dig by hydrolyzing disaccharides into monosaccharides
3) aminopeptidases
- hydrolyze small peptide fragments into individual AA

Question 14

what parts of small intestine does most absorption happen

Answer 14

duodenum and jejunum
- little in ileum

Question 15

how surface area of small intestine is increased

Answer 15

• inside surface is not smooth
• inner surface has many circular folds (increases SA 3 fold)
• projecting from folded surfaces are villi (increase SA 10 fold)
• epithelial cells on villi have microvilli of brush border (increase SA 20 fold)

Question 16

how can sodium be absorbed in S intestine

Answer 16

actively and passively

Question 17

passive absorption of sodium in S intestine

Answer 17

• presence of appropriate electrochemical gradient can cause it to passively move down into interstitial fluid by moving between intestinal epithelial cells through leaky tight junctions

Question 18

active absorption of sodium in S intestine

Answer 18

• movement though cells requires active transport
• can passively cross luminal membrane but once inside epithelial cell it is actively pumped out across basoateral membrane into interstitial fluid and then diffuses into capillaries

Question 19

3 sources of exogenous proteins that need to be digested and absorbed

Answer 19

1) digestive enzymes are all proteins and need to be absorbed
2) proteins from epithelial mucosal cells have been sloughed off
3) plasma proteins that normally leak from capillaries into dig tract lumen

Question 20

how are carbs transported in S intestine

Answer 20

sodium monosaccharide cotransport or sodium independent facilitated diffusion

Question 21

how galactose is absorbed in S intestine

Answer 21

• transported first
• absorbed by secondary active transport
• once inside cell, it is moved down conc. gradient across basolateral membrane and into blood

Question 22

how glucose is absorbed in S intestine

Answer 22

• transported second
• very similar to galactose using secondary active transport
• once conc. inside, glucose moves down conc. gradient across basolateral membrane into blood

Question 23

how fructose is absorbed in S intestine

Answer 23

• transported last
• by facilitated diffusion

Question 24

how are dietary fats absorbed into S intestine

Answer 24

1) Bile salts emulsify fats and pancreatic lipases digest triglycerides into monoglyceride and fatty acids within fat droplets 2) when micelles reach luminal membrane of epithelial cells monoglycerides and fatty acids can diffuse from micelles to epithelial cells - micelles return to chyme - process repeats and bile salts are reabsorbed by active transport 3) once in epithelial cells, monoglycerides and free fatty acids convert back into triglycerides - trigylcerides attack and make lipid droplets and get coated with lipoprotein to make fat droplets water soluble -- called chylomicrons 4) chylomicrons leave endothelial cells by exocytosis - have masement membrane so can't directly enter so they are taken up by lymphatic system before delivered back to blood

Question 25

steps of iron absorption

Answer 25

1) amount absorbed depends on state -only a portion of iron is able to be absorbed 2) dietary iron absorbed into S intestine epithelial cells and is needed for RBC production, transferred into blood 3) if immediately needed to RBC production it is delivered to blood - then bound to transferrin for delivery to bone marrow 4) if not immediately used, it's stored in epithelial cell in a granular form (ferritin) 5) if iron levels in epithelial cells are too high it is secreted into interstitial lumen 6) ferritin is also dumped into lumen when epithelial cell is sloughed off within 3 days - all excess iron is excreted in feces

Question 26

what is more readily absorbed, ferrous iron or ferric iron?

Answer 26

ferrous

Question 27

3 main causes of diarrhea

Answer 27

1) excessive S intestine motility 2) excess of osmotically active particles 3) toxins from v choleraae and other microorganisms

Question 27

cause of diarrhoea:excessive S intestine motility

Answer 27

- arises from local irritation (bacterial or viral) or emotional stress - increased motility means not enough time for water absorption

Question 28

cause of diarrhoea:excess of osmotically active particles

Answer 28

- eg lactose intolerance where if undigested lactose exerted an osmotic effect to draw water into intestines

Question 29

cause of diarrhoea:toxins from v choleraae and other microorganisms

Answer 29

these toxins promote excessive fluid secretion by intestinal mucosa

Question 30

enzymes involved in digestion of proteins

Answer 30

- chymotrypsin - enterokinase - trypsin - aminopeptidase - carboxypeptidase

Question 31

enzymes involved in digestion of carbs

Answer 31

- amylase - disaccharides

Question 32

enzymes involved in digestion of fats

Answer 32

lipase