Large Bowel

Question 1

What does the large bowel consist of?

Answer 1

• colon
• caecum
• appendix
• rectum
• anal canal.

Question 2

What is the caecum?

Answer 2

a blind pouch just distal to the ileocecal valve- larger in herbivores

Question 3

What is the appendix?

Answer 3

a thin, finger-like extension of the caecum - not physiologically relevant in humans

Question 4

What are the principal functions of the large bowel?

Answer 4

1. the reabsorption of electrolytes & water

2. elimination of undigested food and waste

Question 5

What is the size of the large bowel?

Answer 5

1.5m long, 6cm diameter

Question 6

How does the ascending colon run?

Answer 6

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

Question 7

How does the traverse colon run?

Answer 7

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 lesser omentum (posterior side, mesocolon)

Question 8

How does the descending colon run?

Answer 8

from the splenic flexure to the sigmoid colon

Question 9

How does the sigmoid colon run?

Answer 9

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

Question 10

What is the proximal transverses colon supplied by?

Answer 10

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

Question 11

What is the distal third of transverse colon perfused by?

Answer 11

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

Question 12

What does the colon blood supply reflect?

Answer 12

• Reflects embryological division between the midgut and hindgut.
• Region between the two is sensitive to ischemia.

Question 13

What does the peritoneum carry?

Answer 13

• The peritoneum carries fatty tags (appendices epiploicae)

- The muscle coat has 3 thick longitudinal bands (taeniae coli); the gut wall is pouched in appearance (haustra)

Question 14

What is the purpose of the appendices?

Answer 14

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

Question 15

What is the taenia coli necessary fo?

Answer 15

Large intestine motility

Question 16

Where are nodules of lymphoid tissue?

Answer 16

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

Question 17

What are Haustra?

Answer 17

• Taenia Coli shorter than small intestine

* Cause the formation of pouched ovoid segments called haustra (singular haustrum).

Question 18

What is the function of the colon?

Answer 18

(Re) absorption
•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 19

Describe the rectum

Answer 19

• 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 20

Describe the anal canal

Answer 20

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

Question 21

What is the mucosal structure of the large bowel?

Answer 21

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

Question 22

What is the mucosal organisation like?

Answer 22

• Mucosa appears smooth at the gross level because it has no villi (smaller surface area 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 23

What are the goblet cells like in the large bowel?

Answer 23

• 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.

Question 24

Why is mucus needed?

Answer 24

• Mucus - facilitates the passage of the increasingly solid colonic contents and covers bacteria & particulate matter.
• Acetylcholine (parasympathetic and enteric nervous system) stimulates Goblet Cell secretion.

Question 25

What is the mucosal organisation like in the large bowel?

Answer 25

•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 26

Describe microvilli

Answer 26

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

Question 27

Describe Glycolyax

Answer 27

* 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 28

What are the muscle layers in the large bowel?

Answer 28

1. Like the small bowel, muscularis externa consists of an inner circular and outer longitudinal layer. 2. Circular muscles segmentally thickened. 3. Longitudinal layer concentrated in three bands- taenia coli 4. Between the taenia, longitudinal layer is thin 5. Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals.

Question 29

Describe muscle in the large bowel

Answer 29

* Shorter than circular muscle layer, ovoid segments called haustra- can contract individually. * Apart from rectum and anal canal- substantial and continuous. * Movement of large bowel more complicated than small intestine

Question 30

What is the motility of the large bowel?

Answer 30

* Colonic contractions- kneading process- minimally propulsive- 5-10cm/hr at most. * Promotes absorption of electrolytes and water. * Short propulsive movements every 30 mins. * Increase in frequency following a meal.

Question 31

What is motility like in the proximal colon?

Answer 31

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

Question 32

What is motility like in the transverse and descending colon?

Answer 32

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

Question 33

What is mass movement like in the large bowel?

Answer 33

* 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 34

What is the parasympathetic control in the large bowel?

Answer 34

- ascending colon and most of transverse colon innervated by vagus nerve - more distal innervated by pelvic nerves

Question 35

What is the sympathetic control in the large bowel?

Answer 35

lower thoracic and upper lumbar spinal cord (t8-L2)

Question 36

What is the external anal sphincter controlled by?

Answer 36

somatic motor fibres in the pudendal nerves (S2, S3 S4)

Question 37

Which neurones detect pressure?

Answer 37

Afferent sensory neurons detect pressure

Question 38

How is the enteric nervous system important in the large bowel?

Answer 38

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

Question 39

Where is the myenteric plexus ganglia concentrated?

Answer 39

below taenia coli

Question 40

What is the hormone control in the large bowel?

Answer 40

* 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 41

What happens during defecation?

Answer 41

1. Rectum filled with faeces by mass movement in the sigmoid colon. 2. Stores stool until convenient to void. 3. Defecation reflex controlled primarily by the sacral spinal cord- both reflex and voluntary actions. 4. Reflex to sudden distension of walls of rectum.

Question 42

What do pressure receptors do in defecation?

Answer 42

* 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.

Question 43

What is under voluntary control?

Answer 43

* External anal sphincter under voluntary control. | * Urge resisted, sensation subsides

Question 44

Describe the rectum

Answer 44

* Last few centimeters of the rectum known as the "social part" 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 45

How much faeces is produced?

Answer 45

•150g/day adult

Question 46

What makes up faeces?

Answer 46

* Two thirds water. * Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+). * Bile pigments give colour. * Bacterial fermentation gives odour.

Question 47

What is flora in the large bowel?

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. * Diverse, highly metabolically active community.

Question 48

How big is the microbiome?

Answer 48

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

Question 49

What is the role of intestinal flora?

Answer 49

1. Synthesise and excrete vitamins e.g. Vitamin K- germ-free animals can have clotting problems. 2. Prevent colonisation by pathogens by competing for attachment sites or for essential nutrients.  3. Antagonise other bacteria through the production of substances which inhibit or kill non-indigenous species. 4. Fibre (indigestible carbohydrate) can be broken down by colonic bacteria

Question 50

What can intestinal flora stimulate and produce?

Answer 50

1. 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. 2. Stimulate the development of certain tissues, including caecum and lymphatic tissues 3. 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 51

What are the types of normal flora?

Answer 51

- 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

Question 52

What are the links between gut bacteria and?

Answer 52

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