lecture 4- the small intestine

Question 1

Small intestine 3 segments

Answer 1

duodenum, jejunum, ileum

Question 2

Motility- segmentation

Answer 2

primary method of motility during digestion, contractions of the smooth muscle, areas alternatively contract and relax, mixes and propels chyme forward.

Question 3

Control of segmentation

Answer 3

initiated by pacemaker cells which produce the basic electrical rhythm. If smooth muscle cells reach threshold, then contraction occurs

Question 4

segmentation

Answer 4

is slight/absent between meals, and vigorous immediately after a meal.

Question 5

extrinsic nerves modify strength of contractions

Answer 5

increased by parasympathetic and decreased by sympathetic stimulation

Question 6

How can duodenum and ileum start the segment at same time?

Answer 6

The Gastroileal reflex: chyme in the stomach> gastrin secreted> gastrin acts on ileum smooth muscle> segmentation initiated while that section of intestine is empty

Question 7

Segmentation contractions

Answer 7

slowly propel chyme forward, allowing ample time for digestion and absorption to occur

Question 8

Propulsion

Answer 8

frequency of segmentation declines along length of S.I (faster pacemaker depolarisation in duodenum)

Question 9

Migrating Motility Complex (MMC)

Answer 9

after meal has been absorbed small weak repetitive peristaltic waves begin (segmentation ceases) 1.5 hours for waves to sweep remnants towards colon. Once wave reaches end, cycle begins again, to repeat until meal.

Question 10

Control of MMC

Answer 10

Between meals, S.I endocrine cells secrete motilin, stimulating peristalsis.

Question 11

Barrier between the S.I and L.I

Answer 11

Ileocaecal Juncture

Question 12

S.I secretions

Answer 12

exocrine gland cells in the S.I secrete succus entericus (aqueous salt and mucus solution)

Question 13

Role secretions

Answer 13

pancreatic enzymes need an alkaline & liquid environment to function optimally. HCO-3 is secreted into lumen in exchange for chloride (increased pH). CI is actively secreted. H2O maintains liquid chyme

Question 14

S.I Adaptations

Answer 14

All carbs, protein, fats & most electrolytes, vitamins & water are absorbed by the S.I. Most absorption occurs in the duodenum and jejunum. Ileum absorbs bile salts & Vit B12.

Question 15

S.I surface area

Answer 15

S.I has large surface area. each epithelial cell has thousands of microvilli, circular folds and villi

Question 16

Crypts of Lieberkuen (The epithelia of the villi extend down into the lamina propria where they form crypts)

Answer 16

water and electrolytes are secreted. within the crypts the mitotic activity of stem cells allows the entire lining to be replaced every 3 days.

Question 17

Enzymes within the brush-border membrane

Answer 17

enterokinase, disaccharidases, aminopeptidases

Question 18

Enterokinase

Answer 18

activates the proteolytic enzyme tripsinogen to trypsin

Question 19

Disaccharidases

Answer 19

maltase, sucrase, lactase digest disaccharides to monosaccharides

Question 20

Aminopeptidases

Answer 20

digest small peptide fragments into amino acids

Question 21

Transport mechanisms- Na+

Answer 21

Absorption of H2O, nutrients and electrolytes is linked to Na+ absorption

Question 22

Na+

Answer 22

is actively pumped out of cell and into interstitial fluid by Na+ -K+ ATPase pump in basolateral membrane (generates osmotic and electrical gradient for water and chloride to follow)

Question 23

Biochemical balance

Answer 23

juices that enter S.I are absorbed back into plasma (acid-base balance of body is not altered) HCI is neutralised by NaHCO3 forming Na+ + CI and H2CO3. Carbonic acid decomposes into CO2 + H20

Question 24

Diarrhoea

Answer 24

When secretion and absorption are not parallel diarrhoea can occur (and acid/base imbalance). Loss of fluids- dehydration, loss of nutrients, metabolic acidosis