Intestinal Secretion and Absorption

Question 1

Brunner’s glands

Answer 1

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

Question 2

Crypts of Lieberkuhn

Answer 2

Entire surface of small intestine

• lie between intestinal villi
• crypts and villi both covered with: goblet/mucous cells, enterocytes

Question 3

*Mechanism of water secretion

Answer 3

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

Question 4

*Digestive enzyme in secretions

Answer 4

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

Question 5

*What is the turnover rate for crypt cells?

Answer 5

5 days to get from stem cells to the top

- villi tips are highly active and get turned over constantly

Question 6

Regulation of secretion

Answer 6

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

Question 7

• Small intestinal absorption

Answer 7

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

Question 8

Absorption of ions

Answer 8

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

Question 9

Sodium absorption

Answer 9

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

Question 10

*Effects of aldosterone

Answer 10

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)

Question 11

Absorption of chloride

Answer 11

In proximal small intestine

• chloride absorption is rapid
• primarily diffusion along with sodium

Question 12

Absorption of bicarb ions

Answer 12

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

Question 13

Enterocytes in ileum and colon exchange _____ for _____

Answer 13

Bicarbonate secretion; chloride absorption

Question 14

Carbohydrates

Answer 14

Vast majority absorbed as monosaccharides

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

Question 15

Absorption of glucose

Answer 15

Co-transported with active transport of Na

- fructose is not coupled to Na transported and will cross membranes via facilitated diffusion

Question 16

Absorption of proteins

Answer 16

Absorbed as dipeptides, tripeptides, and amino acids

• energy provided by sodium co-transport
• some AA cross by facilitated diffusion

Question 17

Absorption of fats

Answer 17

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

Question 18

Large intestinal secretion and absorption

Answer 18

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

Question 19

Electrolytes and water

Answer 19

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

Question 20

*Maximum absorptive capacity

Answer 20

When absorptive capacity is surpassed diarrhea may result

- 5-8 liters in humans

Question 21

Bacterial action

Answer 21

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