Large Bowel

Question 1

What is the large bowel made of?

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

Label this diagram.

Answer 4

Question 5

What are the principal functions of the large bowel?

Answer 5

The principal functions of the large bowel are the reabsorption of electrolytes & water, and the elimination of undigested food and waste.

Question 6

How long and wide is the large bowel?

Answer 6

1.5m long
6cm diameter

Question 7

What are the colons?

Answer 7

Ascending colon is on the right side of the abdomen, runs from the caecum to the hepatic flexure (the turn of the colon by the liver).

Transverse colon runs from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen). Hangs off the stomach, attached by a wide band of tissue called the greater omentum (posterior side, mesocolon).

Descending colon runs from the splenic flexure to the sigmoid colon.

Sigmoid colon (s-shaped) colon runs from descending colon to the rectum.

Question 8

What is the blood supply like for the colon?

Answer 8

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 9

What is special about the peritoneum?

Answer 9

The peritoneum carries fatty tags (appendices epiploicae), and the muscle coat has 3 thick longitudinal bands (taeniae coli); the gut wall is pouched in appearance (haustra)

Question 10

What is the suggestive function of the appendix?

Answer 10

Structural or functional purpose of appendices epiploica unknown- suggested to have a protective function against intra-abdominal infections.

Question 11

Why is the taenia coli necessary?

Answer 11

large intestine motility

Question 12

Where are nodules of lymphoid tissue common?

Answer 12

Nodules of lymphoid tissue are common in the walls of the distal small intestine (Peyer’s patches) and large intestine (solitary nodules)

Question 13

What are taenia coli (google)?

Answer 13

Teniae coli are three bands of longitudinal smooth muscle on the colon surface.

Question 14

What are the haustra?

Answer 14

gut wall pouched in appearance

Question 15

How would you compare the length of the taenia Coli and the small intestine?

Answer 15

it is shorter this causes the formation of pouced ovoid segments (the haustra)

Question 16

Describe the (re)absorption function of the colon.

Answer 16

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.

Question 17

How much water can the large intestine reabsorb?

Answer 17

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

Question 18

What is the rectum?

Answer 18

Dilated distal portion of the alimentary canal

Histology similar to the colon, but distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa.

Question 19

What is the anal canal?

Answer 19

Terminal portion is anal canal. Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters.

Question 20

Diagram of the mucosal structure of the large bowel.

Answer 20

Question 21

What does the myenteric plexus do?

Answer 21

innervate muscularis layer

Question 22

Describe the large bowel mucosal structure?

Answer 22

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

Question 23

What is the mucosal organisation of the large bowel?

Answer 23

Mucosa appears smooth at the gross level because it has no villi (smaller SA than small bowel)

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

Crypts dominated by goblet cells

Question 24

Describe the goblet cells of the large bowel.

Answer 24

Higher no. of goblet cells than small bowel.

More prevalent in the crypts than along the surface, no.
increases distally towards rectum.

Apical ends are packed with mucus-filled secretion granules
awaiting release.

Mucus - facilitates the passage of the increasingly solid colonic
contents and covers bacteria & particulate matter.

Question 25

What stimulates goblet cell secretion?

Answer 25

Acetylcholine (parasympathetic and enteric nervous system) stimulates Goblet Cell secretion.

Question 26

Describe the microvilli in the large bowel.

Answer 26

Microvilli (~0.5-1.5μm high) make up the “brush border”.
Several thousand microvilli per cell
Surface of microvilli covered with glycocalyx

Question 27

Describe the mucosal organisation of the large bowel.

Answer 27

Villi are absent from large bowel

Enterocytes are still the dominant cells facing the gut lumen, but the mucus-secreting goblet cells (stained red) dominate the crypts.

As in the small bowel, new cells arise from crypt stem cells.

Crypts dominated by goblet cells

No Paneth cells & enteroendocrine cells are rarer than in small bowel

Glycocalyx does not contain digestive enzymes.

Question 28

What is glycocalyx?

Answer 28

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 29

Describe the muscle layer of the large bowel.

Answer 29

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 30

What can contract individually in the muscle layer of the large bowel and describe muscle layers of rectum and canal?

Answer 30

Shorter than circular muscle layer, ovoid segments called haustra- can contract individually.

Apart from rectum and anal canal- substantial and continuous muscle layers

Movement of large bowel more complicated than small intestine

Question 31

Describe the motility of the large bowel.

Answer 31

Colonic contractions- kneading process- minimally propulsive- 5-10cm/hr at most.

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.

Question 32

What is mass movement in the large bowel.

Answer 32

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 33

How is the large bowel controlled.

Answer 33

para and symp
somatic motor fibres (sphincter)
enteric nervous system

Question 34

What is controlled by para and symp in the large bowel?

Answer 34

Parasympathetic: ascending colon and most of transverse colon innervated by vagus nerve. More distal innervated by pelvic nerves.

Sympathetic-lower thoracic and upper lumbar spinal cord

Question 35

what controls the external anal sphincter?

Answer 35

External anal sphincter controlled by somatic motor fibres in the pudendal nerves.

Question 36

What detects pressure in the large bowel?

Answer 36

Afferent sensory neurons detect pressure

Question 37

How is the enteric NS important for large bowel control?

Answer 37

Enteric nervous system also important Hirschsprung’s disease (no enteric intramural ganglia).

Question 38

What can trigger mass movement?

Answer 38

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

Question 39

Where is the myenteric plexus ganglia?

Answer 39

concentrated below taenia coli

Question 40

What promotes sodium and water absorption?

Answer 40

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

Question 41

Describe defecation.

Answer 41

Rectum filled with faeces by mass movement in the sigmoid colon

Stores stool until convenient to void

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

pt. 2

Reflex to sudden distension of walls of rectum

Pressure receptors send signals via myenteric plexus to initiate peristaltic waves in descending, sigmoid colon and rectum. Internal anal sphincter inhibited.

Weak intrinsic signal augmented by autonomic reflex

External anal sphincter under voluntary control

Urge resisted, sensations ubsides

Question 42

What sphincter in rectum is under voluntary control?

Answer 42

external

Question 43

What is the social part of the rectum?

Answer 43

Last few centimeters of the rectum

Can distinguish between solid, liquid and gas

That perceptual ability is important in knowing what can be passed appropriately in what circumstance.

Question 44

How much faeces is made a day?

Answer 44

150g

Question 45

What is faeces made of?

Answer 45

2/3 water

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

Question 46

What give colour and smell to faeces?

Answer 46

bile pigment give colour
bacterial fermentation gives odour

Question 47

What is the large bowel flora?

Answer 47

All mammals have symbiotic relationships with their gut microbial community (microbiome)

Stomach and small bowel have few bacteria- protected.

Large bowel contains many

Essential to normal function

Question 48

How would you describe the flora of the large bowel?

Answer 48

Diverse, highly metabolically active community

The microbiome in an average human adult comprises approximately 1.5kg of live bacteria, with the active biomass equivalent to a major human organ

Question 49

What is the role of intestinal flora? (7)

Answer 49

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 cross react 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 50

Roles of intestinal flora diagram.

Answer 50

Question 51

With recent research there are links between gut bacteria and what? (5)

Answer 51

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

Question 52

What are the types of normal flora?

Answer 52

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

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