Large Bowel

Question 1

Describe anatomy of large bowel

Answer 1

Large bowel consists of the colon, caecum, appendix, rectum and anal canal. The caecum is a blind pouch just distal to the ileocecal valve- larger in herbivores. The appendix is a thin, finger-like extension of the caecum - not physiologically relevant in humans but recent evidence indicates otherwise. 1.5m long, 6cm diameter.

Question 2

What are the principal functions of the large bowel?

Answer 2

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

Question 3

What are the sections of the colon?

Answer 3

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 4

Describe blood supply of the parts of the colon

Answer 4

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 5

Describe structure of a transverse section of the colon

Answer 5

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). Structural or functional purpose of appendices epiploica unknown- suggested to have a protective function against intra-abdominal infections. Taenia coli necessary for large intestine motility. Nodules of lymphoid tissue are common in the walls of the distal small intestine (Peyer’s patches) and large intestine (solitary nodules).

Question 6

How are haustra formed?

Answer 6

Taenia Coli shorter than large intestine. Cause the formation of pouched ovoid segments called haustra (singular haustrum).

Question 7

What is the function of the colon and how is this carried out?

Answer 7

Colon absorbs electrolytes & water, more so in the proximal section. Na+ and Cl- absorbed by exchange mechanisms and ion channels, water follows by osmosis and K+ moves passively across lumen. Large intestine can reabsorb approx 4.5 litres water (usually 1.5 litres). Above this threshold diarrhoea.

Question 8

Describe location and structure of rectum

Answer 8

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 9

Describe structure of anal canal

Answer 9

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

Question 10

What 4 layers make up the wall of the large bowel?

Answer 10

Mucosa, Submucosa, Muscularis and Serosa. Mucosa consists of the epithelium, lamina propria and muscularis mucosae. Mucosa contains many glands and so does the submucosa. Submucosa contains many glands as well as Meissner’s plexus and lymph nodes. Muscularis consists of circular muscle and longitudinal muscle. Outermost layer is serosa.

Question 11

What cells are found in the mucosa?

Answer 11

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

Question 12

Describe mucosal organisation of large bowel

Answer 12

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 13

What is the purpose of goblet cells in the large bowel?

Answer 13

Higher no. of goblet cells than small bowel and 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. Goblet cell secretion stimulated by acetylcholine via parasympathetic and enteric NS.

Question 14

Describe abundance of cells in the mucosa of the large bowel

Answer 14

Villi are absent from large bowel. Enterocytes are still the dominant cells facing the gut lumen, but the mucus-secreting goblet cells 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 15

Describe structure and surroundings of microvilli

Answer 15

Microvilli (~0.5-1.5m high) make up the “brush border”. Several thousand microvilli per cell. Surface of microvilli covered with glycocalyx. This is a rich carbohydrate layer on apical membrane which serves as protection from digestional lumen yet allows for absorption. It traps a layer of water & mucous known as “unstirred layer” and hence regulates rate of absorption from intestinal lumen.

Question 16

Describe the muscle layers of the large bowel

Answer 16

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 17

How abundant are haustra?

Answer 17

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

Question 18

Describe motility in the different sections of the colon

Answer 18

Colonic contractions are 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 and increase in frequency following a meal.

Question 19

What is the mass movement of the colon?

Answer 19

Occurs 1-3 times daily and resembles peristaltic waves. 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 movement in the large bowel controlled?

Answer 20

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.
External anal sphincter controlled by somatic motor fibres in the pudendal nerves.
Afferent sensory neurons detect pressure.

Question 21

How does the enteric nervous system contribute to control of movement in large bowel?

Answer 21

Enteric nervous system also important- Hirschsprung’s disease (no enteric intramural ganglia). Myenteric plexus ganglia concentrated below taenia coli. Presence of food in stomach can stimulate mass movement although mechanism not fully understood. Also a degree of hormonal/paracrine control.

Question 22

How is defecation controlled?

Answer 22

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.

Question 23

Describe process of neural control during defecation

Answer 23

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. If urge resisted, sensation subsides.

Question 24

What is the social part of the rectum?

Answer 24

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 25

Describe composition of faeces

Answer 25

150g/day produced in adult. Two thirds water and solid contents contains cellulose, bacteria, cell debris, bile pigments, salts (K+). Bile pigments give colour while bacterial fermentation gives odour.

Question 26

Describe large bowel flora

Answer 26

All mammals have symbiotic relationships with their gut microbial community (microbiome). While stomach and small bowel have few bacteria and are protected, large bowel has many. These are essential to its normal function.

Question 27

What are the roles of the intestinal flora?

Answer 27

1. Synthesize and excrete vitamins e.g. Vitamin K- germ-free animals can have clotting problems.
2. Prevent colonization by pathogens by competing for attachment sites or for essential nutrients.
3. Antagonize other bacteria through the production of substances which inhibit or kill non-indigenous species.
4. 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.
5. Stimulate the development of certain tissues, including caecum and lymphatic tissues
6. Fibre (indigestible carbohydrate) can be broken down by colonic bacteria.
7. 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 28

What are the most prevalent gut bacteria?

Answer 28

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 29

What does recent research link gut bacteria to?

Answer 29

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

Question 30

What is the purpose of a faecal matter transplant?

Answer 30

Faecal matter transplant is a way of introducing healthy microbiome into individuals with compromised gut flora and replenish it.