Week 3.9 - Small Intestine Structure and Function

Question 1

How long is the duodenum, jejunum and ileum?

Answer 1

25cm, 2.5M, 3.5M

Question 2

What is the duodenum responsible for?

Answer 2

gastric acid neutralisation
digestion
iron absorption

Question 3

What is the jejunum responsible for?

Answer 3

95% of nutrient absorption

Question 4

What is the ileum responsible for?

Answer 4

NaCl and H2O absorption and chyme dehydration

Question 5

What does the absorptive surface consist of?

Answer 5

folds (plica), villi, microvilli, crypts

Question 6

What are crypts responsible for?

Answer 6

• indentings further increase SA
• secrete water and Cl to lumen
• stem cells to regenerate epithelium

Question 7

What is the lifespan of small intestine epithelium?

Answer 7

3-5 days. regenerates by stem cells from crypts.

Question 8

What are villus responsible for absorbing?

Answer 8

• NaCl,
• monosaccharides,
• amino acids and dipeptides
• fats,
• vitamins,
• minerals,
• water

Question 9

How much water does the small intestine secrete per day?

Answer 9

1.5L normally

Question 10

Why is it important for water secretion in the intestine?

Answer 10

• to form liquidy chyme.
• enzymes work better in a fluid.
• aids nutrient presentation
• dilute toxins

Question 11

How does chloride enter intestinal lumen?

Answer 11

3 transporters.
- 1 allows 2 chloride in, 1 Na in and 1 K in.
- a K+ pump out.
- a NaK pump
all help move chloride in. then it exits via CFTR

Question 12

How does cystic fibrosis affect water absorption in lumen?

Answer 12

significantly - CFTR transporter is vital for chloride movement which drives water movement.

Question 13

What is segmentation?

Answer 13

occurs during meals to digest food. allows food to stay close to border for better absorption. contraction and relaxation at same time to cycle food.

Question 14

What generates segmentation?

Answer 14

longitudinal pacemaker cells. create basic electrical rhythm. decreases down towards rectal direction to avoid chyme moving too fast into colon.

Question 15

what autonomic control is on segmentation?

Answer 15

parasympathetic vagal nerve increases contraction, sympathetic decreases.
has no effect on BER as that is controlled by pacemaker cells in longitudinal muscles

Question 16

What is peristalsis?

Answer 16

follows absorption of nutrients, segmentation stops and peristalsis starts. contraction on oral side and relaxation on anal side to push bolus down

Question 17

What controls peristalsis?

Answer 17

MMC - migrating motility complex. hormone motilin initiates MMC.

Question 18

Where does peristalsis MMC begin and end?

Answer 18

gastric antrum to bottom of small intestine.

Question 19

What causes peristalsis to stop?

Answer 19

stops when food arrives in stomach and segmentation begins again.

Question 20

What are aims of MMC?

Answer 20

• move undigested food revenue out small intestine
• eliminate bacterial colonisation of small intestine by removing unabsorbed food/nutrients.
• prevents bacteria entering small intestine from large by creating one way path

Question 21

What nervous system mediates the movement in intestine?

Answer 21

myenteric plexus between longitudinal and circular layers in muscularis externa.

Question 22

What is the gastro-ileal complex?

Answer 22

• food enters stomach
• stretch receptors trigger segmentation and the opening of ileocaecal valve and intensifies MMC allowing chyme to enter large intestine, and distending the colon.
• local reflex of ileocaecal sphincter closes the valve.

Question 23

What is mostly absorbed in the terminal ileum?

Answer 23

bile salts and vitamin B12