Large intestine structure and function

Question 1

how does the small intestine join with the large intestine

Answer 1

from the ileum - ileocaecal valve - caecum (blind sac) - vermiform (worm-like) appendix + lymph nodes = colon

Question 2

what are the four parts of the colon

Answer 2

ascending, transverse, descending, sigmoid

Question 3

what are the two muscle layers of the colon

Answer 3

circular - complete

longitudinal - incomplete

Question 4

what are the bands that run the entire length of the colon

Answer 4

teniae coli

Question 5

what does contraction of the teenier coli produce

Answer 5

pouches (HAUSTRA)

puckered appearance

Question 6

what is the mucosa of the colon comprised of

Answer 6

simple columnar epithelium - flat

Question 7

what enables lubrication of the colon for movement of faeces

Answer 7

large, straight crypts lined with large number of goblet cells

Question 8

what is the rectum

Answer 8

Straight, muscular tube (between end of sigmoid colon and anal canal)

Question 9

what is the mucosa of the rectum made from

Answer 9

simple columnar epithelium

Question 10

what is different abut the muscular external of the rectum compared to other areas of the GI tract

Answer 10

very thick

Question 11

what is the anal canal

Answer 11

the 2-3 cm between distal rectum and anus

Question 12

where is the muscular thicker in the anal canal than in the rectum

Answer 12

at the internal anal sphincter

Question 13

what is the external anal sphincter made from

Answer 13

skeletal muscle

Question 14

what kind of epithelium is found in the anal canal

Answer 14

starts as simple columnar - goes to stratified squamous

Question 15

what is the role of the colon

Answer 15

Actively transports sodium from lumen into blood - osmotic absorption of water - dehydration of chyme - creates solid faecal pellets

**no important absorption role in humans

Question 16

what does the long residence time of matter in the colon lead to

Answer 16

bacterial colonisation

Question 17

what does the colonic microflora help with

Answer 17

bacterial fermentation of undigested carbohydrates:

• short chain fatty acids (energy source)
• vitamin K (blood clotting)
• gas (flatus - nitrogen, CO2, hydrogen, methane, hydrogen sulphide)

Question 18

what normally keeps the anus closed

Answer 18

internal anal sphincter (smooth muscle under AUTOMATIC control)

external anal sphincter (skeletal muscle under VOLUNTARY control)

Question 19

what creates the urge to defaecate

Answer 19

following a meal - wave of intense contraction (mass movement contraction) - goes down the colon to the rectum

distension of the rectal wall produces by mass movement of faecal material into rectum - stimulates mechanoreceptors - creates the urge to defaecate

Question 20

what controls the defection reflex

Answer 20

parasympathetic control via pelvic splanchnic nerves (NOT SYMPATHETIC)

Question 21

what are the 3 stages of the defection reflex

Answer 21

1. contraction of rectum
2. relaxation of internal and contraction of external anal sphincters
3. increased peristaltic activity in colon - causes increased pressure on external anal sphincter - causes it to relax under VOLUNTARY control

= expulsion of faeces

Question 22

what control the voluntary delay of defaecation

Answer 22

descending pathways

Question 23

what leads to constipation

Answer 23

NO absorption of toxins from faecal material following long periods of retentions

Question 24

what are the symptoms of constipation

Answer 24

decreased frequency 
headaches
nausea
loss of appetite
abdominal distension (due to distension of rectum)

Question 25

what is diarrhoea

Answer 25

too frequent passage of faeces which are too liquid

Question 26

what can cause diarrhoea

Answer 26

pathogenic bacteria
protozoans
viruses
toxins
food

**major cause of death in children under 5 in developing countries

Question 27

give examples of enterotoxigenic bacteria

Answer 27

vibrio cholerae

escherichia coli

Question 28

how do enterotoxigenic bacteria cause diarrhoea

Answer 28

1. Produce protein enterotoxins which maximally turn on intestinal chloride secretion from crypt cells = increase H2O secretion
2. Act by elevating intracellular second messengers:
   - cAMP
   - cGMP
   - calcium
3. H2O secretion swamps absorptive capacity of villus cells  profuse watery diarrhoea (25 litres per day for cholera)

Question 29

why do people with cystic fibrosis not get diarrhoea

Answer 29

they cannot secrete water

Question 30

what is the treatment for secretory diarrhoea

Answer 30

enterotoxins DONT damage villus cells so give sodium/glucose solution - drive H2O absorption = rehydration

secretion still going = wash away infection

ORS - oral rehydration therapy