Large Bowel

Question 1

What does the large bowel consist of? (5)

Answer 1

Colon
Caecum
Appendix
Rectum
Anal canal

Question 2

How long is the large bowel and what is its diameter?

Answer 2

1.5m

6cm

Question 3

What is the caecum?

Answer 3

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

Question 4

what is the appendix?

Answer 4

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

Question 5

What are the principle functions of the large bowel?

Answer 5

the reabsorption of electrolytes & water, and the elimination of undigested food and waste.

Question 6

What structures can the colon be divided into?

Answer 6

Ascending colon

Transverse colon

Descending colon

Sigmoid colon

Question 7

What is the ascending colon?

Answer 7

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

Question 8

what is the transverse colon?

Answer 8

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).

Question 9

Where is the descending colon?

Answer 9

colon runs from the splenic flexure to the sigmoid colon.

Question 10

Where is the sigmoid colon?

Answer 10

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

Question 11

Describe the blood supply to the parts of colon?

Answer 11

Proximal T colon : middle colic artery

Distal third T colon : inferior mesenteric artery

• Reflects embryological division between the midgut and hindgut.

Region between the two is sensitive to ischemia.

Question 12

What does the middle colic artery branch off?

Answer 12

Superior mesenteric artery

Question 13

What are fatty tags? ( appendices epiploicae)

Answer 13

The peritoneum carries fatty tags

Purpose unknown : suggested it is protective against intra-abdominal infections

Question 14

What are taenia coli needed for?

Answer 14

These are a coat of muscle with 3 thick longitudinal bands

Large intestine motility

The gut wall is pouched in appearace

Question 15

What are solitary nodules?

Answer 15

Similar to peyer’s patches in s.bowel

• nodules of lymphoid tissue

Question 16

What are haustra?

Answer 16

The pouched ovoid segments caused by teania coli

Question 17

where does the colon absorb electrolytes and water?

Answer 17

Most in the 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 18

What is the Dilated distal portion of the alimentary canal?

Answer 18

Rectum

Question 19

How is the histology of the rectum different to the colon?

Answer 19

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

What muscles make up the anal canal?

Answer 20

Terminal portion is anal canal.

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

Question 21

Describe 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

Why does the large intestine have a smaller surface area than small?

Answer 22

Has no villi

So mucosa appear smooth

Question 23

Describe the mucosal organisation of the large bowel?

Answer 23

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

Where are goblet cells most found in the large bowel?

Answer 24

Higher number of goblet cells than small bowel

In cypts than along the surface

Increases 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 secretion?

Answer 25

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

Question 26

Does the glycolcalyx have digestive enzymes?

Answer 26

Digestive enzymes * the surface of microvilli is covered with glycocalyx 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 27

Describe the muscular layers in the large bowel?

Answer 27

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. Shorter than circular muscle layer, ovoid segments called haustra- can contract individually. Apart from rectum and anal canal- substantial and continuous.

Question 28

How do contractions change across large bowel?

Answer 28

Proximal colon : antipropulsive patterns to retain chyme Transverse and decesnding colon : localised segmental contractions of haustral muscle cause back anf forth mixing Short propulsive movements every 30 mins

Question 29

What mass movement occurs?

Answer 29

Peristaltic wave Can propel 33-75% of large intestine in seconds Fibrous foo promotes rapid transport

Question 30

Describe the innervation to the ascending and transverse column?

Answer 30

Parasympathetic vagus nerve Sympathetic- lower thoracic and upper lumbar spinal cord. * More distal innervated by pelvic nerves.

Question 31

What innervation controls external anal sphincter?

Answer 31

somatic motor fibres in the pudendal nerves. Afferent sensory neurons detect pressure.

Question 32

What is Hirschsprung's disease?

Answer 32

Enteric nervous system also important- Hirschsprung’s disease (no enteric intramural ganglia). The absence of ganglion cells causes the muscles in the bowels to lose their ability to move stool through the intestine (peristalsis).

Question 33

What plexus is concentrated below taenia coli?

Answer 33

Myenteric plexus ganglia

Question 34

Describe paracrine control?

Answer 34

aldosterone promotes sodium and water absorption (synthesis of Na+ ion channel, Na+/K+ pump)

Question 35

What is the defecation reflex controlled by?

Answer 35

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

Question 36

Why is defecation a reflex of distension of the walls of the rectum?

Answer 36

Pressure receptors send signals via myenteric plexus to start peristaltic eaves down the sigmoid colon and rectum Internal and spincter inhibited Weak intrinsic signal increased by autonomic reflex. External anal sphincter under voluntary control. Urge resisted, sensation subsides

Question 37

Why are the last few cm of the rectum known as the social part?

Answer 37

Can distinguish between solid, liquid and gas. That perceptual ability is important in knowing what can be passed appropriately in what circumstance.

Question 38

What makes up faeces?

Answer 38

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

Question 39

What are the roles of intestinal flora? (7)

Answer 39

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 40

What are the most prevalent bacteria?

Answer 40

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

Question 41

What are Bifidobacteria?

Answer 41

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

Question 42

There are linked between the gut bacteria and?

Answer 42

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