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Flashcards in Intestinal Secretion and Absorption Deck (21)
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Brunner's glands

Tubular mucous glands located in duodenum that secrete large amounts of alkaline mucus
- tactile or irritating stimuli
- vagal stimulation
- in response to secretin
- inhibited by sympathetic stimulation


Crypts of Lieberkuhn

Entire surface of small intestine
- lie between intestinal villi
- crypts and villi both covered with: goblet/mucous cells, enterocytes


*Mechanism of water secretion

Cells (enterocytes) lining the crypts
- actively secrete chloride ions via chloride channels (cystic fibrosis transmembrane conductance regulator)
- actively secrete bicarb ions
- cause "electric drag" to draw sodium into lumen
- water followed by osmosis


*Digestive enzyme in secretions

Few enzymes free in secretions, enzymes expressed on or in enterocytes of villi
- several peptidases
- sucrase, maltase, isomaltase and lactase
- small amounts of intestinal lipase
- crypt stem cells provide replacements


*What is the turnover rate for crypt cells?

5 days to get from stem cells to the top
- villi tips are highly active and get turned over constantly


Regulation of secretion

Local stimuli (most important)
- enteric nervous reflexes
- secretion largely regulated by presence of chyme
Hormonal regulation
- secretion
- cholecystokinin


* Small intestinal absorption

Isosmotic water absorption
- water is transported by diffusion
- when chyme is dilute, water is absorbed
- when chyme is concentrated or hyperosmolar water is diffused back
- chyme is maintained isosmolar to the plasma


Absorption of ions

Active transport of sodium
- large amounts of sodium are lost with intestinal secretions
- large amounts of sodium are ingested
- intestine must absorb dietary sodium to prevent net loss


Sodium absorption

Na is actively transported out of cell by energy dependent processes
- results in low intracellular Na concentrations
- Na moves from lumen into cell down electric and concentration gradient
- water follows via osmosis


*Effects of aldosterone

In states of dehydration, large amounts of aldosterone are released
- aldosterone upregulates sodium absorption mechanisms
- particularly important in large intestine (colon absorbs about 90% of fluid in GIT)


Absorption of chloride

In proximal small intestine
- chloride absorption is rapid
- primarily diffusion along with sodium


Absorption of bicarb ions

To avoid losses due to duodenal and pancreatic secretions bicarb absorbed indirectly
- H ions secreted in exchange for Na
- H combines with bicarb to form carbonic acid
- carbonic acid dissociates to form water and CO2
- H2O remains in lumen and CO2 is rapidly absorbed into blood


Enterocytes in ileum and colon exchange _____ for _____

Bicarbonate secretion; chloride absorption



Vast majority absorbed as monosaccharides
- most abundant is glucose at 80% (final digestion product of starches)
- all monosaccharides absorbed by active transport


Absorption of glucose

Co-transported with active transport of Na
- fructose is not coupled to Na transported and will cross membranes via facilitated diffusion


Absorption of proteins

Absorbed as dipeptides, tripeptides, and amino acids
- energy provided by sodium co-transport
- some AA cross by facilitated diffusion


Absorption of fats

Digested to form monoglycerides and free FAs --> become dissolved within bile micelles
- micelles are transported to brush border and FFAs and monoglycerides diffuse freely thru enterocyte membrane
- taken up by ER to form new TGs
- packaged in chylomicrons


Large intestinal secretion and absorption

Up to liter or more of chyme enter colon each day
- water and electrolytes are absorbed (greater than 90% by volume)
- majority absorbed in proximal half
- distal half considered the storage colon


Electrolytes and water

Tight junctions between colonic enterocytes much tighter
- Na is absorbed against a much higher concentration gradient
- aldosterone enhances Na absorption capacity
- colon secretes large quantities of bicarb in exchange for chloride (neutralizes acidic bacterial by-products)
- water follows NaCl by osmosis


*Maximum absorptive capacity

When absorptive capacity is surpassed diarrhea may result
- 5-8 liters in humans


Bacterial action

Normal colonic bacterial flora provide some energy via digestion of cellulose
- other bacterial products: vitamin K, vitamin B-12, thiamin, riboflavin, gases