Small Intestine Structure & Function

Question 1

How long is the small intestine and its parts?

Answer 1

Total ~ 6metres
Duodenum - 25cm
Jejunum - 2.5m
Ileum - 3.5m

Question 2

What are the functions of the 3 sections of the small intestine?

Answer 2

Duodenum:

• Gastric Acid neutralization
• Digestion
• Iron Absorption

Jejunum:
- 95% of nutrient Absorption

Ileum:

• Reabsorbs Salt & so water
• Also bile salts which are then recycled back through the liver and used again

Question 3

How much water is reabsorbed int he gut?

Answer 3

Around 9 litres a day passes through but only 200ml reaches the faeces

Question 4

What are plicae circulares?

Answer 4

Fold in the small intestinal walls that forms a cork screw thread.
Liquid contents of the gut follow this thread helping in mixing and absorption.

Question 5

What is the surface area of the gut?

Answer 5

around 200m2 thanks to the plicae, villi and microvilli

Question 6

What are crypts of Lieberkuhn?

Answer 6

Glands in the intestinal epithelial lining extending don through the mucosa

Question 7

What is the function of mucosal crypts?

Answer 7

All mucosal cells start life at the base of these crypts as stem cells.
They undergo a process of differentiation and move up the crypt, changing as they go.
- These crypts secrete Cl- ions which takes water with it.
- They also secrete a lot of mucous from goblet cells
- Paneth cells near the base are involved in the innate immune system
- Plus several other functions/cell types`

Question 8

What is absorbed by intestinal villi?

Answer 8

NaCl
Monosaccharides
Amino acids & Peptides
Fats
vitamins & Minerals
Water

Question 9

What process of transport is responsible for most absorption in the gut?

Answer 9

Na Coupled Secondary Active Transport

Molecule enters epithelium coupled to Sodium using a conc. gradient produced by NaKATPase pumping sodium out the other side.

Question 10

What is the purpose of secreting water out the crypts?

Answer 10

• Maintains liquid state of luminal contents
• Promotes mixing of the nutrients and digestive enzymes
• Aids nutrient presentation to the absorbing surface
• Dilutes and washes away any potential dangerous substances.

Question 11

Water leaves the crypts by osmotic diffusion through the tight junctions, however Cl ions must be pumped out to drive this diffusion, how is this done?

Answer 11

Cl, Na and K move into the epithelial cell from the blood through a NaKCl co-transporter.

• Sodium is pumped back out by NaKATPase and K passes out through its own channel
• This turns the inside of the cell very -ve which causes the Cl ions to be pushed out the other side by the charge through CFTR channel

Question 12

What is CFTR and what regulates it?

Answer 12

Cystic Fibrosis Transmembrane Regulator.

• Adenylate Cyclase converts ATP -> cAMP
• cAMP activate Protein Kinase A
• Protein Kinase A stimulates CFTR

Question 13

What are the two types of intestinal motility?

Answer 13

Segmentation

Peristalsis

Question 14

Define Segmentation:

Answer 14

Short intestinal segments contract

• This forces chyme up and down into adjacent relaxed segments
• They then contract and push it back
• Slightly more is pushed down than up so there’s a net slow movement towards the colon

Question 15

What is the purpose of segmentation vs peristalsis?

Answer 15

Segmentation mixes the chyme and digestive enzymes + brings it into contact with the absorbing surface.

Peristalsis moves undigested material along and limits bacterial colonization.

Question 16

How is a segmentation contraction generated?

Answer 16

Rate of segmentation is determined by the BER

Strength of contraction is determined by rate of APs which in turn is determined by parasympathetic (Vagal) and hormonal signals.

It can be inhibited by the sympathetic stimulation throuh thoracic splanchnic nerves

Question 17

What are the thoracic splanchnic nerves and what do they supply?

Answer 17

Greater (T7-T9)
Lesser (T10-T11)
Least (T12)
They supply the gut viscera with sympathetic innervation

Question 18

How does BER vary?

Answer 18

It is unaffected by the ANS.

However it decreases from duodenum to rectum.

Question 19

When does the small intestine switch from segmentation to peristalsis and vice versa?

Answer 19

Arrival of food in the stomach turns off peristalsis and triggers segmentation.

When the nutrients are absorbed peristalsis begins again

Question 20

What hormone intiates peristalsis?

Answer 20

Motilin

Question 21

How does peristalsis occur?

Answer 21

A Migrating Motility Complex (MMC) is a pattern of peristaltic activity which travels down the small intesinte from gastric antrum -> Terminal ileum.

One begins as another ends.

Triggered by motilin and inhibited by food entry to the stomach

Question 22

What is the law of the intestine?

Answer 22

If smooth muscle is distended. The muscle on the oral end contracts and anal side relaxes moving the bolus towards the colon.

Mediated by neurons of the myenteric plexus. (i.e. totally devoid of external stimulus)

Question 23

What is the gastroileal reflex?

Answer 23

When the stomach starts emptying, segmentation begins, not just int he duodenum but all the way down in the ileum..

• -> Ileocaecal valve opens
• -> Chyme enters large intestine
• -> Colon distends
• -> Reflex contraction of ileocaecal sphincter

Prevents reflux into small intestine.

Question 24

What is BER?

Answer 24

Basic Electrical Rhythm