Large bowel

Question 1

What are the 5 portions of the large intestine

Answer 1

Colon, caecum, appendix, rectum and anal canal

Question 2

What is the caecum?

Answer 2

The caecum is a blind pouch just distal to the ileocecal valve- larger in herbivores.

Question 3

What is the appendix?

Answer 3

The appendix is a thin, finger-like extension of the caecum - not physiologically relevant in humans.

Question 4

Dimensions of the large bowel

Answer 4

1.5m long
6cm diameter

Question 5

4 portions of the colon

Answer 5

Question 6

What is the blood supply to the colon like?

Answer 6

The proximal transverse colon is supplied with blood by the middle colic artery (branch of the superior mesenteric artery).

Distal third of transverse colon is perfused by the inferior mesenteric artery.

Reflects embryological division between the midgut and hindgut.

Region between the two is sensitive to ischemia.

Question 7

What is the name of and how many layers of longitudinal muscle surround the gut wall and what are they required for?

Answer 7

Taenia coli-3

Necessary for gut motility

Question 8

What lymphoid structures are present in the small and large intestine?

Answer 8

Small intestine (Peyer’s patches)
Large intestine (Solitary nodules)

Question 9

What are haustra?

Answer 9

Pouches in the side of the colon wall

Question 9

What is theorised to be the function of appendices epiploica

Answer 9

Thought to protect against intra-abdominal infections

Question 10

What causes haustra?

Answer 10

Taenia coli-longitudinal muscle

Question 11

Colon functions

Answer 11

Colon absorbs electrolytes & water

More in proximal colon.

Na+ and Cl- absorbed by exchange mechanisms and ion channels.

Water follows by osmosis.

K+ moves passively into lumen.

Large intestine can reabsorb approx 4.5 litres water (usually 1.5 litres). Above this threshold diarrhoea

Question 12

How is the rectum different to the colon histologically?

Answer 12

Rectal folds in its submucosa and the absence of taenia coli in its muscularis externa

Question 13

Describe the sphincter/muscle formation around the anal canal

Answer 13

Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters.

Question 14

Outline the histology of the large bowel mucosa

Answer 14

Like the small intestine
Enterocytes and goblet cells are abundant.
Abundant crypts
Stem cells are found in the crypts.

Question 14

Why does the large bowel appear grossly smooth?

Answer 14

Has no villi-Smaller S.A than small bowel

Question 15

How is water absorbed in the large bowel

Answer 15

Enterocytes have short, irregular microvilli - primarily concerned with resorption of salts.

(Water is absorbed as it passively follows the electrolytes, resulting in more solid gut contents)

Question 16

How is mucus production different in the large bowel compared to the small bowel

Answer 16

Large bowel contains many more goblet cells
-Stimulated by Ach(PS NS+Enteric NS)

Question 17

What is the glycocalyx and what is its function?

Answer 17

Rich carbohydrate layer on apical membrane
Serves as protection from digestional lumen yet allows for absorption.
Traps a layer of water & mucous known as “unstirred layer”
Regulates rate of absorption from intestinal lumen

Question 18

Describe the layout of the large bowel muscles

Answer 18

Like the small bowel, muscularis externa consists of an inner circular and outer longitudinal layer.

Circular muscles segmentally thickened.

Longitudinal layer concentrated in three bands- taenia coli

Between the taenia, longitudinal layer is thin

Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals.

Question 19

Generally, is motility more complex in the small or large intestine?

Answer 19

Large bowel movement is more complex

Question 20

What is the difference between segmenting and mass contractions in the large intestine

Answer 20

Segmenting:
Promotes absorption of electrolytes and water.

In the proximal colon, ‘antipropulsive’ patterns dominate to retain chyme.

In transverse and descending colon, localised segmental contractions of circular muscle called Haustral contractions cause back and forth mixing.

Short propulsive movements every 30 mins.

Increase in frequency following a meal.

Mass:
1-3 times daily- mass movement- resembles peristaltic wave.

Can propel contents 1/3-3/4 of length of large intestine in few seconds.

Food that contains fibre (indigestible material) promotes rapid transport through colon)

Question 20

How is the large bowel parasympathetically innervated?

Answer 20

Ascending colon and most of transverse colon innervated by vagus nerve. More distal innervated by pelvic nerves.

Question 21

How is the large bowel sympathetically innervated?

Answer 21

Lower thoracic and upper lumbar spinal cord.

Question 22

What fibres are the internal and external anal sphincter controlled by?

Answer 22

Somatic motor fibres in the pudendal nerves.

Question 23

What kind of neurones detect pressure in the lower GI tract?

Answer 23

Afferent sensory neurons

Question 24

How is the large bowel generally controlled?

Answer 24

Myenteric plexus ganglia concentrated below taenia coli

Presence of food in stomach can stimulate mass movement- hormonal? Neural?

Hormonal/paracrine control. e.g. aldosterone promotes sodium and water absorption (synthesis of Na+ ion channel, Na+/K+ pump)

Question 25

How is the rectum filled with faeces?

Answer 25

Mass movement in sigmoid colon

Question 26

How is the defecation reflex controlled?

Answer 26

Defecation reflex controlled primarily by the sacral spinal cord- both reflex and voluntary actions.

Question 26

How are the internal and external anal sphincter differently controlled?

Answer 26

External sphincter-Voluntarily controlled
Internal sphincter-Autonomically controlled

Question 27

What is the name of the last few centimeters of the rectum and what is its purpose?

Answer 27

Last few centimeters of the rectum known as the “social part” of the rectum

Can distinguish between solid, liquid and gas.

Question 28

Faeces:
How much is produced per day?
How much is water?
What forms the solid component?
What gives its colour?
What gives its odour?

Answer 28

150g/day adult.

Two thirds water.

Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+).

Bile pigments give colour.

Bacterial fermentation gives odour.

Question 29

What is the distribution of bacteria like in the GI tract?

Answer 29

Few in stomach and small intestine
Many in large bowel

Question 30

What 5 factors are intestinal flora thought to be connected with?

Answer 30

1) Drug metabolism
2) Insulin resistance
3) Bile acid metabolism
4) Lipid metabolism
5) Obesity

Question 30

What are the 7 major roles of intestinal flora?

Answer 30

Synthesize and excrete vitamins e.g. Vitamin K- germ-free animals can have clotting problems.

Prevent colonization by pathogens by competing for attachment sites or for essential nutrients.

Antagonize other bacteria through the production of substances which inhibit or kill non-indigenous species.

Stimulate the production of cross-reactive antibodies. Antibodies produced against components of the normal flora can crossreact with certain related pathogens, and thereby prevent infection or invasion.

Stimulate the development of certain tissues, including caecum and lymphatic tissues

Fibre (indigestible carbohydrate) can be broken down by colonic bacteria.

Produces short chain fatty acids which can regulate gut hormone release, or be absorbed to be used as an energy source or to influence functions such as food intake or insulin sensitivity directly.

Question 31

What are the 2 major types of normal gut flora?

Answer 31

Most prevalent bacteria are the Bacteroides- Gram-negative, anaerobic, non-spore forming bacteria. Implicated in the initiation colitis and colon cancer.

Bifidobacteria are Gram-positive, non-sporeforming, lactic acid bacteria. Have been described as “friendly” bacteria. Thought to prevent colonization by potential pathogens.