Lecture 18 - SI absorption (Freeman)

Question 1

sites of absorption

Answer 1

proximal small intestine - Ca, Fe, sugars, a.a., fats
distal small intestine - sugars, a.a., fats, bile salts (passively)
ileum - bile salts (atively) vitamin B12, water, electrolytes
cecum and colon - bacterial fermentation, absorption of water, electrolytes, VFAs

Question 2

anatomy of mucosa: SI and LI

Answer 2

includes crypts and villi

• SI have microvillie and villi
• colon has microvillie BUT NOOOO VILLI!! (therefore less absorption capabilities)
• both have crypts!
• 3 zones: zone of proliferation, zone of migration/differentiation, zone of exfoliation

Question 3

specialization in epithelial cells

Answer 3

crypts migrate up toward the tips of villi as they mature and gain digestive and absorptive functions

shortened villi: crypts cant travel far enough to mature and normal mucosal function is lost

cells have a short life span (<96 hours) and are lost to intestinal lumen

Question 4

epithelial transport mechanisms: gotta know 5 things

Answer 4

1. differences btwn epithelial and nonepithelial cells
2. improtance of Na transport mechanisms in teh transport of other substances like water and water soluble nutrients
3. driving forces for absorption
4. interactions between transport systems
5. concepts of concentrations in salt sol’ns

Question 5

differences between epithelial and nonepithelial cells

Answer 5

specialization in apical (faces lumen) and basement cell membranes (faces capillaries)

tight junctions between epithelial cells

Question 6

osmolality

Answer 6

concentation of ions expressed as millimoles/kg of water

Question 7

osmolarity

Answer 7

concentration of ions expressed as millimoles/liter of solution

Question 8

tonicity

Answer 8

refers to the concentrations inside and outside the cells

Question 9

transcellular transport (primary transport)

Answer 9

entry of Na: Na into lumen > Na in cell
this is favorable electrochemical gradient: no energy needed!

exit of Na: out of lumen into blood REQUIRES ENERGY
provided by Na-K/ATPase located on the basolateral membrane of cell - this keeps the intracellular concentrations of Na low (to favor Na entry from lumen)

Question 10

paracellular transport

Answer 10

after Na is pumped into the lumen on the capillary side, Na builds up and can “leak” back into the lumen between the cells (Cl and water will follow!) through tight junctions

Question 11

ion gradient hypothesis (secondary active transport)

Answer 11

energy necessary to drive flow into a cell against a concentration gradient is derived from coupling between transport of that solute with the transport of an ion (Na) down the concentration gradient

ex: Cl moves against EC gradient to “piggyback” with Na into cell. the Na pump on basolateral side drives this.

Question 12

things that are transported via secondary active transport

Answer 12

glucose, amino acids, B vitamins, Cl-, glucose, bile salts

Question 13

counter transport

Answer 13

involves exchange of one item for a similar item

like a cation for a cation (Na for H)

Question 14

charactersitics of isotonic fluid

Answer 14

NaCl is pumped in, creates high osmotic pressure, causes water to flow in, creates high hydrostatic pressure, this causes water ions to flow into blood

Question 15

which is more permeable jejnum or colon?

Answer 15

jejunum is more permeable than colon

Question 16

how do tight junctions control back leak?

Answer 16

thru the paracellular pathway (which is permeable to low molecular weight substrates and water)

Question 17

colon: tight or loose epithelium?

Answer 17

tight = water absorber

Question 18

small intestine: tight or loose epithelium?

Answer 18

loose !
consequences:
- ineffective in absorbing water and ions (back leaK)
- ineffecitve at maintaining a minute buildup of Na across epithelium
- lower transepithelial potential difference
- easier for Na pump to transport Na out of cell causes transcellular movement of Na to decrease along with a.a., sugars, and other nutrients that enter the cell thru a coupled Na-dependent system at the apical membrane

Question 19

solvent drag

Answer 19

a mechainsm by which water flow from lumen to blood thru the paracellular pathway draws low molecular weight substrates with it

Question 20

what weak electrolytes are absorbed from the colon?

Answer 20

VFAs: acetate, butytrate, propionate

negatively charged drugs

Question 21

normal ammonia absorption

Answer 21

ammonia is brought to liver from colon –> liver detoxifies ammonia via urea cycle –> converts ammonia to urea

Question 22

if ammonia absorption fails

Answer 22

happens in liver disease

ammonia accumulates in blood causes hyperammonia, can cause hepatic encephalopathy

can be managed by reducing colonic absorption of ammonia (give a synthetic disaccharaide which bacteria will break down and make H+ to combine with ammonia)

converts ammonia to ammonium

Question 23

ferrous iron (Fe++)

Answer 23

= absorbed

Question 24

ferric iron (Fe+++)

Answer 24

not absorbed

Question 25

where is iron mainly abosrbed

Answer 25

duodenum

Question 26

normal iron absorption

Answer 26

cells that absorb iron (ferritin), form iron-binding proteins that complex with ferrous iron entering the cell through an energy dependent brush border

Question 27

what happens when iron stores are low

Answer 27

little enters crypt cells from the blood –> migration to top of villi is low in iron binding protein –> Fe is absorbed from lumen at top

Question 28

what happens when iron stores are high

Answer 28

more enters crypt cells –> induces synthesis of Fe binding protein –> migration up villi –> iron is stored at tip and is eventually sloughed and excreted

Question 29

carbohydrate (CHO) absorption

Answer 29

starch digsted –> brush border –> starch digestion by amylase –> disaccharides –> monosaccharides –> absorbed

Question 30

fructose

Answer 30

absorbed with GLUT5 (not Na dependent)

Question 31

glucose and galactose

Answer 31

absorbed with SGLT1

Question 32

which are released with GLUT2 transporter?

Answer 32

fructose

glucose and galactose

Question 33

protein and a.a. absorption - peptides

Answer 33

absorbed as small di and tri peptides and mainly a.a.

a.a. are Na dependent

dipeptides and tripeptides are H dependent

glutamine a.a. - most important fuel source for SI epithelium

Question 34

fat absorption

Answer 34

micelar solubilization
consists of: bile salts, carbs, triglycerides
micelles diffuse thru unstirred layer
micelles are no absorbed they break apart and components are absorbed at brush border

absorption of fatty acids and monoglycerides

reesterification and fromation of cylomicrons

Question 35

where do chylomicrons go

Answer 35

lymphatics

Question 36

where do fat and carbs go?

Answer 36

blood!!

Question 37

calcium absorption

Answer 37

highest in duodenum

regulated by form of Vit D (1,25 dihydroxycholecaliferol) - stimulates syntehsis of Ca binding protein in duodenal enterocytes

Ca absorption involves: Diet (Vit D) –> liver –> kidney –> duodenal enterocyte

Question 38

viral diseases:

Answer 38

TCE coronoavirus - severe villi atrophy
rotavirus - villi damage to tips
parvovirus - attacks crypt cells –> collapse of villi

Question 39

adhesion of organisms to cells:

Answer 39

cryptosporidium - damages cell, causing villi blunting
E.coli - DOES NOT DAMAGE CELL or alter length - enterotoxin does!
salmonella - affects large intestine (produce enterotoxin –> diarrhea)