Small Intestine - Structure and Function

Question 1

What is the total length of the small intestine?

Answer 1

~6 metres

range: 4.5 - 9 metres

Question 2

How long is the duodenum?

Answer 2

25cm

Question 3

What are the functions of duodenum?

Answer 3

Gastric acid neutralisation
Digestion
Iron absorption

Question 4

How long is the jejunum?

Answer 4

2.5m

Question 5

How much nutrient absorption occurs in the jejunum?

Answer 5

95%

Question 6

How long is the ileum?

Answer 6

3.5m

Question 7

What occurs in the ileum?

Answer 7

NaCl/H2O absorption - chyme dehydration

Question 8

How is the absorptive surface area enhanced?

Answer 8

By folds, villi and microvilli

Question 9

What is the structure of the small intestine like?

Answer 9

Screw-like structure - contents follow along it, helps with mixing and increases surface area

Question 10

What happens to villi in vegetarians?

Answer 10

Becomes flatter and leaf-like shape

Question 11

Where do epithelial cells form?

Answer 11

In the crypts, start as stem cells and move up the crypt and mature, travel to the top of the villus and die once they reach the top (~5 days)

Question 12

Why can gut cells get badly damaged by chemotherapy?

Answer 12

Chemotherapy targets fast dividing cells (tumours), gut cells are also fast dividing cells

Question 13

What makes of the villus?

Answer 13

Simple columnar epithelium
Lacteal for absorbing fats
Capillary network
Goblet cells for secreting mucus

Question 14

What does a villus cell absorb?

Answer 14

NaCl
Monosaccharides
Amino acids
Peptides
Fats
Vitamins
Minerals
Water

Question 15

What does a crypt cell secrete?

Answer 15

Cl

Water

Question 16

How much water does the small intestine secrete daily?

Answer 16

~1500ml

Question 17

Where does water secretion come from in small intestine?

Answer 17

Epithelial cells lining the crypts of Lieberkuhn

Question 18

How is water secreted from the crypts?

Answer 18

Secreted passively due to active secretion of chloride into intestinal lumen

Question 19

Why is water secretion important in digestion?

Answer 19

Maintains lumenal contents in liquid state
Promotes mixing of nutrients with digestive enzymes
Allows nutrients contact with absorbing surface
Dilutes and washes away potentially injurious substances

Question 20

What happens to water secreted by the crypts?

Answer 20

Reabsorbed by villi - when osmotic gradient formed by the transport of sugar and sodium through

Question 21

How does Cl get into the cells?

Answer 21

Na-K-Cl pump - Na and K are needed to pump in at the same time, Na pumped out by Na-K pump, K recycled by leaky K channel
Cl = -ve, inside of cell = -ve (Cl goes through cell out of channel) = osmotic gradient = WATER SECRETION

Question 22

What are the 2 distinct types of movement in intestinal motility?

Answer 22

Segmentation

Peristalsis

Question 23

What is segmentation?

Answer 23

Contration moves chyme into adjacent areas of relaxation
Relaxed areas contract = push chyme back
Result = thorough mixing of contents with digestive enzymes and brings chyme into contact with absorbing surface

Question 24

How is segmentation contraction initiated?

Answer 24

Depolarisation by pacemaker cells in longitudinal muscle layer
BER produces membrane potential = threshold = AP = contraction

Question 25

What determines the strength of contraction in segmentation?

Answer 25

AP frequency

Question 26

What determines frequency of segmentation?

Answer 26

Basic electrical rhythm

Question 27

What happens to BER as you move down intestine to the rectum?

Answer 27

BER decreases because segmentation produces slow migration of chyme towards large intestine = more chyme pushed down than up

Question 28

What does the parasympathetic nervous system (vagus) do to segmentation contraction?

Answer 28

Increases force of contraction

Question 29

What does the sympathetic nervous system do to segmentation contraction?

Answer 29

Decreases force of contraction

Question 30

What effect does the autonomic nervous system have on basic electrical rhythm?

Answer 30

No effect - since frequency does not change, only force

Question 31

When does peristalsis occur?

Answer 31

After absorption of nutrients, segmentation stops and peristalsis starts

Question 32

What is the migrating motility complex?

Answer 32

Pattern of peristaltic activity travelling down the small intestine (starting in gastric antrum)
1 MMC ends (terminal ileum) another begins

Question 33

What brings about the cessation of MMC and initiation of segmentation?

Answer 33

Arrival of food in the stomach

Question 34

What does MMC do?

Answer 34

Moves undigested material to L.I

Limits bacterial colonisation in S.I

Question 35

What is the hormone involved in initiation of MMC?

Answer 35

Motilin

Question 36

What are the 3 main things that occur when intestinal smooth muscle is distended? + what are they mediated by?

Answer 36

Muscle on oral side = contracts
Muscle on anal side = relaxes
Bolus moved into area of relaxation towards colon
Mediated by neurones in myenteric plexus

Question 37

What is the gastroileal reflex?

Answer 37

Gastric emptying leads to increased segmentation activity in the ileum

Question 38

What happens during the gastroileal reflex?

Answer 38

Opening of ileocaecal valve
Entry of chyme into L.I
Distension of colon
Reflex contraction of ileocaecal sphincter to prevent backflux into S.I