Digestive System, Lecture 3

Question 1

Liver/Gallbladder Secretions - bile

Answer 1

• contains bicarbonate, cholesterol, bile pigments, phospholipids, organic waste, bile salts
• when not needed bile flow from production in liver to storage in gallbladder
• when needed bile flow stimulated by fatty acids in duodenum triggering release of hormone cholecystokinin (CCK)
  my notes:
• bile main role is in digestion of fats
  -CCK controlling flow of bile
• CCK can bind onto the cells in the gallbladder and increase their contraction that will force bile out of storage and can bind to cells in sphincter of oddi, that controls the opening from both common bile duct and pancreatic duct into the small intestine (so if you relax that it provides an opening into the duodenum)

Question 2

Liver/Gallbladder Secretions - bile components (2)

Answer 2

bicarbonate (minor way)
- small amount; helps neutralize acidic chyme coming out of stomach by gastric emptying (have to lower it down to a more alkaline level)
bile salts (do the fat digestion)
- enterohepatic circulation (circulation that puts bile into small intestine and then brings it back to liver) - release and recycling
◦ enter duodenum to aid in fat digestion
◦ reach ileum absorbed into portal vein to return to
liver (reabsorbed back into bloodstream, into
portal vein that delivers it to liver and can get
ready to do the cycle again)
◦ forms cyclic pathway between liver and
duodenum
◦ with 95% recycled only a small amount (5%) lost
in feces needs to be synthesized by liver to r
replace
- during fatty meal digestion entire bile salt content recycled several times

Question 3

Pancreatic Secretions - bicarbonate

Answer 3

• highly acidic chyme coming into duodenum from gastric emptying
• acid must be neutralized to create preferred alkaline small intestine environment
  bicarbonate:
• liver - bile (small amount)
• pancreas - pancreas juice
  pancreas duct cell (bicarbonate mainly comes from this)
• forms hydrogen and bicarbonate
• hydrogen into bloodstream / bicarbonate into pancreatic duct lumen
• bicarbonate flows in pancreatic juice to duodenum where it can neutralize acid coming from gastric emptying
• similarities to stomach parietal cell (something’s are just the opposite way around)

Question 4

Pancreatic Secretions - digestive enzymes

Answer 4

enzyme: action
trypsin, chymotrypsin: splits peptide bonds in proteins to peptide fragments
carboxypeptidase: splits terminal amino acids from end of protein
pancreatic lipase: splits triglycerides to fatty acids and monoglycerides
pancreatic amylase: splits polysaccharides to maltose

protein enzymes inactive with activation in duodenum:
trypsinogen -> trypsin activated by enterokinase (brush border enzyme)
chymotrypsinogen -> chymotrypsin activated by trypsin
procarboxypeptidase -> carboxypeptidase activated by trypsin

my notes:
- trypsin, chymotrypsin and carboxypetidase all break down proteins (protein digester)
- pancreatic lipase breaks down fats -> critical fat one cause it gets us to things we can actually absorb (fatty acids and monoglycerides)
- pancreatic amylase breaks down carbohydrates (poly to di (di mainly forming as maltose)
- secreted as inactive and then bond it to get active
- enterokinase is bound to wall to of GI tract (physically attached to brush border) - apical membrane (meaning trypsin has to go there and contact it)

Question 5

Pancreatic Secretions - hormonal regulation (secretin)

Answer 5

↑ acid from stomach, ↑ secretin secretion (small intestine), ↑ plasma secretin, ↑ bicarbonate secretion (pancreas), ↑ flow of bicarbonate into small intestine, neutralization of intestinal acid (small intestine)
- creating a more alkaline environment

Question 6

Pancreatic Secretions - hormonal regulation (CCK)

Answer 6

↑ intestinal fatty acids and amino acids, ↑ CCK secretion, ↑ plasma CCK, ↑ enzyme secretion (pancreas - acinar cells), ↑ flow of enzymes into small intestine, ↑ digestion and absorption of fats of proteins (small intestine)

Question 7

GI Tract - small intestine - mechanical digestion - segmentation

Answer 7

• mixing pancreas, liver, gallbladder, and intestinal secretions with stomach chyme
• propelling to large intestine
  segmentation - areas of contraction/relaxation:
• backward/forward motion (“massaging chyme”) for breakdown (larger to smaller molecules) and to bring in contact with absorptive surfaces (constricting in a spot and relaxing that same spot next time - alternating)
• rate: parasympathetic and gastrin - increase / sympathetic - decrease
  my notes:
• if we do not mix the enzymes with the physical secretions, we will not get a whole lot of breakdown
• the dotted lines are where it is pinched off (contracted) and the areas in between are where it is relaxed (alternating)

Question 8

GI Tract - small intestine - mechanical digestion - migrating myoelectric complex (MMC)

Answer 8

• after absorption mostly completed segmentation stops and MMC replaces
• begins in antrum moves down entire length of small intestine in steps
  ◦ peristaltic wave travels short distance and stops
  ◦ next wave starts roughly where last ended (short
  waves migrating down small intestine; taking ~2
  hours to reach large intestine)
  ◦ if chyme still in small intestine will return back to
  antrum and start again
• “sweeps” along so minimal left in small intestine (aids in preventing bacterial growth)

Question 9

Small Intestine - absorption (carbohydrates; CHO)

Answer 9

at this point:
digestion: polysaccharides to maltose
ingested disaccharides, monosaccharides
disaccharides to monosaccharides (glucose, galactose, fructose) by brush border enzymes on apical membrane
- sucrose (glucose – fructose) by sucrase
- lactose (glucose – galactose) by lactase
- maltose (glucose – glucose) by maltase
◦ all 3 of these are brush border enzymes - means
they are attached to lumen or wall of GI tract (not
free floating)
- cross apical membrane:
◦ facilitated diffusion glucose transporter or GLUT (
fructose)
◦ secondary active transporter involving sodium (
glucose or galactose) - cotransport with sodium
‣ not the three things together its one or the
other co-transporting with sodium (either
glucose/sodium or galactose/sodium)
- all cross basolateral membrane by facilitated diffusion GLUTs (same for all)
- most CHO digested/absorbed within proximal 20% of small intestine
- some CHO indigestible - soluble and insoluble fiber (carbohydrate elements not broken down into monosaccharrides)
- 14 different GLUT in human

Question 10

Small Intestine - absorption (proteins)

Answer 10

at this point:
- digestion - small peptides (made up 2-3 amino acids; AAs) and individual AAS
- ingested - individual AAs, some small peptides
- some small peptides to individual AAs by brush border enzymes on apical membrane
- cross apical membrane
◦ some small peptide (ones not broken down by
brush border enzymes) by secondary active
transporter involving hydrogen
◦ individual AAs by secondary active transporter
involving sodium
- small peptides that crossed apical membrane intact to individual AAs by peptidases inside cell
- all cross basolateral by facilitate diffusion amino acid transporters
- 20 AAs with different amino acid transporters (~10 in humans)