Large intestine

Question 1

What does the large intestine consist of

Answer 1

Consists of the colon, cecum, appendix, rectum and anal canal.

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

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

Question 2

what is the ileocecal valve?

Answer 2

It is the valve between the small intestine and the large intestine that prevents material from flowing back into the small intestine.

Question 3

What is the size of the colon and the function?

Answer 3

The principal functions of the colon are the reabsorption of electrolytes and water and the elimination of undigested food and waste.
Colon absorbs electrolytes and 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.

1.5m long, 6cm diameter.

Question 4

Describe the colon?

Answer 4

• The ascending colon is on the right side of the abdomen, runs from the cecum to the hepatic flexure.
• The 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).
• The descending colon runs from the splenic flexure to the sigmoid colon.
• Sigmoid (s-shaped) colon runs from descending colon to the rectum.

Question 5

What are the branches of the Superior mesenteric artery and what does it supply

Answer 5

1. Inferior pancreaticoduodenal artery: head of the pancreas and to the ascending and inferior parts of the duodenum
2. Ileocolic artery : supplies last part of ileum, cecum and appendix (distal loop)
3. Right colic artery: Ascending colon (distal loop)
4. Middle colic artery to the transverse colon ( distal loop)
5. jejunal and ileal arteries: branches to the ileum and jejunum

Question 6

what are the branches of the inferior mesenteric artery?

Answer 6

• Left colic artery: supplies descending colon
• Superior sigmoid branch
• Superior rectal artery

Question 7

State three features that are unique to the colon.

Answer 7

The peritoneum carries fatty tags (appendices epiploicae), and the muscle coat has 3 thick longitudinal bands (taeniae coli: Taenia coli necessary for large intestine motility, this is shorter than small intestine
); the gut wall is pouched in appearance (haustra:Cause the formation of pouched ovoid segments called haustra )
Appendices epiploicae
Taenia coli
Pocketed/segmented appearance - haustra

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

Question 8

Describe the features of the 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.

Terminal portion is anal canal. Surrounded by internal (circular muscle, not in your conscious control) and external (striated muscle, there is conscious control) anal sphincters.

The faeces rests on the transverse rectal folds, before you allow it to pass through the anus.

Question 9

What is found in the colon

Answer 9

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

Question 10

describe where goblet cells are and what stimulates their release?

Answer 10

• Higher no. of goblet cells than small intestine.
• More prevalent in the crypts than along the surface, no. increases distally towards rectum.
• The mucus facilitates the passage of the increasingly solid colonic contents, and covers bacteria and particulate matter.
• Acetylcholine (parasympathetic and enteric nervous system) stimulates Goblet Cell secretion.
• apical ends are packed with mucus-filled secretion granules

Question 11

What cells are NOT FOUND in the large intestine?

Answer 11

In the large intestine villi are absent.

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 intestine, new cells arise from crypt stem cells.

Crypts dominated by goblet cells.

No Paneth cells and enteroendocrine cells are rarer than in small intestine.

Question 12

What does the large intestine contain?

Answer 12

-does not contain Villi
-It has microvilli
-Microvilli (~0.5-1.5 micrometer high) make up the “brush border”.
There are several thousand microvilli per cell
The surface of the microvilli are covered with glycocalyx
Glycocalyx:
-rich carbohydrate layer on apical membrane
-protection from the digestional lumen, yet allows for absorption
-traps a layer of water & mucous known as the “unstirred layer” which regulates rate of absorption from intestinal lumen

Glycocalyx does not contain digestive enzymes.

Question 13

Describe the motility of the large intestine?

Answer 13

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 14

Describe mass movement ?

Answer 14

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 15

How is the large intestine controlled?

Answer 15

• 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.

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

Question 16

Describe the process in which it leads to defecation

Answer 16

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.

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, sensation subsides

Question 17

Describe the content of the faeces

Answer 17

150g/day adult.
Two thirds water. 
Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+). 
Bile pigments give colour. 
Bacterial fermentation gives odour.

Question 18

Describe the large intestine flora

Answer 18

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

Stomach and small intestine have few bacteria- protected.

Large intestine contains many, essential to normal function.

Diverse, highly metabolically active community.
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 19

what are the Roles of intestinal flora

Answer 19

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 to cross react with certain related pathogens, and thereby prevent infection or invasion.
Stimulate the development of certain tissues, including cecum 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 20

Name a bacterium that is thought to prevent colonisation by potential pathogens.

Answer 20

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

Question 21

State a region of the colon that is particularly vulnerable to ischaemia.

Answer 21

The area between the area that is perfused by the middle colic artery and the area perfused by the left colic artery.

Question 22

What are the most prevalent bacteria in the large intestine?

Answer 22

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