Small Intestine Structure And Function

Question 1

What is the small intestine composed of

Answer 1

Duodenum —> Jejunum —> Ileum

Question 2

Total length of small intestine

Answer 2

~6m
(25cm duodenum, 2.5m jejunum, 3.5m ileum)

Question 3

Role of the duodenum

Answer 3

Neutralises gastric acid from stomach (digestive enzymes released here from pancreas via sphincter of oddi), digestion, iron absorption

Question 4

Role of jejunum

Answer 4

Nutrient absorption (95% of the absorption happens here)

Question 5

Role of ileum

Answer 5

Absorb salt and water (dehydration of the chyme from the stomach)

Gives large intestine better chance to dispose of waste material in solid manner

Question 6

How does the shape of the mucosa of the small intestine aid with mixing and churning of digestive enzymes

Answer 6

Folds in mucosa work like threads of a screw, as material passes down it follows these folds and is mixed

Question 7

What is on the surface of the folds of the mucosa in the small intestine

Answer 7

Villi

Question 8

What is at the base of the villi, how are these involved in the lifespan of villi

Answer 8

At base there is a blind ended sac called a crypt
In these crypts are stem cells which mature and move up into the endothelial cells
When reach the top they die and shed of into lumen (lifespan 5days)

Question 9

Why is the GI system so badly effected by chemotherapy

Answer 9

Surface of GI system constantly renewing itself, chemo targets rapidly dividing cells so affects GI system badly

Question 10

How does the structure of the villi aid with nutrient absorption

Answer 10

Massive surface area so good contact with foodstuffs

At core of each villus is blood supply (arterial and venous drainage) - lots of capillaries so nutrients don’t have to travel far

Middle of core is lacteal (lymphatic capillary) - lipids can’t get through capillaries so enter through lacteal, ultimately transported to thoracic duct and emptied into bloodstream at subclavian vein

Question 11

What do the villus cells absorb (8)

Answer 11

NaCl
Monosaccharides
Amino acids
Peptides
Fats
Vitamins
Minerals
Water

Question 12

What do the crypts secrete

Answer 12

Cl & water
(Water is secreted osmotically as a consequence of active secretion of Cl ~1500ml water/day)
NORMALLY ALL WATER ABSORBED BY CRYPTS IS REABSORBED BY VILLI

Question 13

Why is water secretion important for digestive processes (4)

Answer 13

Maintains Lumenal contents in liquid state
Promotes mixing of nutrients with digestive enzymes
Aids nutrient presentation to absorbing surface
Dilutes and washes away potentially injurious substances

Question 14

What are the two types of intestinal movement (motility)

Answer 14

Segmentation
Peristalsis

Question 15

When does segmentation occur

Answer 15

During a meal

Question 16

What is the mechanism of segmentation

Answer 16

Contraction moves chyme into adjacent area of relaxation
Relaxed areas then contract pushing chyme back
Provides thorough mixing of contents with digestive enzymes and brings chyme into contact with absorbing surface (allowing for absorbition)

Question 17

What initiates segmentation contractions (electrical)

Answer 17

Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer
The intestinal basic electrical rhythm (BER) produces oscillations in membrane potential —> threshold —> action potential —> contraction

Question 18

What determines the strength of contraction and frequency in segmentation

Answer 18

Strength: action potential frequency
Frequency: BER

Question 19

What happens BER as you move down intestine towards rectum

Answer 19

BER decreases
(segmentation produces slow migration of chyme towards large intestine)

Question 20

What is the effect of the parasympathetic and sympathetic nervous systems on contraction in segmentation

Answer 20

Parasympathetic (vagus) = increased contraction
Sympathetic = decreased contraction

(Autonomic has no effect on BER)

Question 21

What is the migrating motility complex

Answer 21

Pattern of peristaltic activity travelling down small intestine (starts in gastric antrum)
As one MMC ends (terminal ileum) another begins

Question 22

What does MMC act to do (2)

Answer 22

Move undigested material into large intestine
Limit bacterial colonisation of small intestine

Question 23

What stimulates the end of segmentation and start of peristalsis

Answer 23

Segmentation stops once nutrients absorbed stimulating start of peristalsis

Question 24

What stimulates the end of peristalsis and initiation of segmentation

Answer 24

Arrival of food in stomach

Question 25

What is the law of the intestine (if intestinal smooth muscle is distended) (3)

Answer 25

If intestinal smooth muscle is distended:
- muscle on the oral side of bolus contracts
- muscle on anal side of bolus relaxes
- bolus is moved into area of relaxation towards colon

Question 26

What neurones mediate the law of the intestine, what’s this reflex called

Answer 26

Neurones in the myenteric plexus (movement is ONLY mediated by neurones in the gut zone enteric nervous system - aren’t any parasympathetic or sympathetic nerves impacting this)

Gastroileal reflex

Question 27

What happens in the gastroileal reflex

Answer 27

Gastric emptying —> increased segmentation activity in ileum

—> opening of ileocaecal valve (sphincter)
—> entry of chyme into large intestine
—> distension of colon
—> reflex contraction of ileocaecal sphincter (prevents backflow into small intestine)