Small Intestine

Question 1

what is the small intestine the main site for?

Answer 1

digestion and absorption

Question 2

what are the 3 parts of the small intestine?

Answer 2

• duodenum – approx. 30 cm
• jejunum – approx. 3.5 m
• ileum – approx. 2.5 m

Question 3

what does the small intestine receive?

Answer 3

-chyme from stomach
(via the pyloric sphincter)
-pancreatic juice from pancreas 
-bile from gall bladder
(both via the sphincter of Oddi)

Question 4

what does the small intestine secrete?

Answer 4

intestinal juice

Question 5

what does the small intestine move?

Answer 5

Moves remaining residues to the large intestine via the ileocaecal valve

Question 6

what increases the surface area of the small intestine?

Answer 6

• circular folds (of Kerckring)
• villi
• microvilli (the brush border)

Question 7

what does the small intestine secrete into the blood from endocrine cells within thew mucosa?

Answer 7

• gastrin
• CCK
• secretin
• motilin
• GIP
• GLP
• Ghrelin

Question 8

what do these peptide hormones do?

Answer 8

-act on G-protein coupled receptors

Question 9

what are the control mechanisms of secretions in the small intestine?

Answer 9

Distension/irritation, gastrin, CCK, secretin, parasympathetic nerve activity (all enhance), sympathetic nerve activity (decreases)

Question 10

what does excessive activity of small intestine cause?

Answer 10

diarrhoea

Question 11

what is segmentation?

Answer 11

• “chopping” moves chyme back and forth – very vigorous after a meal (little / none between meals)
• Alternating contraction and relaxation of segments of circular muscle

Question 12

what initiates segmentation?

Answer 12

small intestine pacemaker cells causing the BER which is continuous. At threshold activates segmentation which in the duodenum is primarily due to distension by entering chyme

Question 13

what triggers segmentation in the empty ileum?

Answer 13

gastrin from the stomach (gastroileal reflex)

Question 14

why is the net movement aboral?

Answer 14

Duodenum has frequent segmentation contractions (12 per min), ileum has fewer (9 per min)

Question 15

what effect does parasympathetic and sympathetic activity have on strength of segmentation?

Answer 15

parasympathetic - enhanced

sympathetic - decreased

Question 16

what are the two activities that occur in the interdigestive ,or fasting state?

Answer 16

• a few localised contractions

- The migrating motor complex (MMC)

Question 17

Describe the migrating motor complex (MMC)?

Answer 17

• occurs between meals every 90-120mins
• strong peristaltic contraction passing length of the intestine (stomach ileocaecal valve)
• clears small intestine of debris, mucus and sloughed epithelial cells between meals – ‘housekeeper function’
• inhibited by feeding and vagal activity
• triggered by motilin, suppressed by gastrin and CCK

Question 18

what are the endocrine pancreatic secretions?

Answer 18

-insulin and glucagon - secreted to blood

Question 19

what are the exocrine pancreatic secretions?

Answer 19

digestive enzymes (acinar cells), aqueous NaHCO3- solution (duct cells) – secreted to the duodenum collectively as pancreatic juice

Question 20

what do pancreatic duct cells secrete?

Answer 20

secrete 1 – 2 litre of alkaline (HCO3- - rich) fluid into the duodenum per day

Question 21

what do duct cells do?

Answer 21

• Neutralises acidic chyme entering the duodenum :
• provides optimum pH for pancreatic enzyme function
• protects the mucosa from erosion by acid

Question 22

Describe duct cell secretions.

Answer 22

Sodium potassium ATPase – keeps intercellular sodium at a lower value- driving force for sodium to enter cells

• Carbonic anhydrase uses metabolic co2 and water to make carbonic acid (unstable and dissociates into proton and bicarbonate anion) – proton is coupled with sodium inward
• Bicarbonate is is exchanged with chloride, bicarbonate emerges in apical membrane in exchange for chloride but chloride can be recycled , using chloride channel
• Aqueous component of pancreatic juice- is full of bicarbonate
• By neutralizes acid, bicarbonate deactivates pepsin in the chyme protecting mucosa from erosin via acid and pepsin

Question 23

when are pancreatic enzymes released?

Answer 23

enzymes stored in zymogen granules and released in response to elevated [Ca2+]

Question 24

Name the proteases.

Answer 24

• trypsinogen
• chymotrypsinogen
• procarboxypeptidase A and B

Question 25

what state are the proteases in the granules?

Answer 25

inactive until they are released from acinar cells which activates trypsin which digests proteins

Question 26

what enzymes are active in the granules?

Answer 26

• amylases

- lipases

Question 27

Describe the 3 phases of the control of pancreatic secretion.

Answer 27

Cephalic – mediated by the vagal stimulation of mainly the acinar cells (20% total secretion)
Gastric – gastric distension evokes a vagovagal reflex resulting in parasympathetic stimulation of acinar and duct cells (5-10% total secretion)
Intestinal ( 70-80% of total secretion)