Large bowel

Question 1

How is the large bowel structured?

Answer 1

Large bowel consists of colon, caecum, appendix, rectum and 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 principle functions of the large bowel?

Answer 4

Reabsorption of electrolytes and water

Elimination of undigested food and waste

Question 5

How long is the colon?

Answer 5

1.5m long, 6cm in diameter

Question 6

How is the colon structured?

Answer 6

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

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

Descending colon runs from splenic flexure to the sigmoid colon

Sigmoid colon runs from descending colon to the rectum

Question 7

What supples the colon with blood?

Answer 7

Proximal transverse colon is suppled with blood from middle colic artery
Distal third of transverse colon is perfused by inferior mesenteric artery
This division reflects the embryological division between the midgut and hindgut. Region between the 2 is sensitive to ischaemia

Question 8

What are features of the wall of the colon?

Answer 8

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

Structural or functional purpose of appendices epiploica is unknown (perhaps protective function against into-abdominal infections)

Taenia coli necessary for large intestine motility

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

Question 9

How are haustra formed?

Answer 9

Taenia coli are shorter than in small intestines

This causes the formation of pouched ovoid segments called haustra (single is haustrum)

Question 10

What are functions of the colon?

Answer 10

Reabsorption:

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 1.5L). Above this threshold causes diarrhoea

Question 11

What is the rectum?

Answer 11

Distal part of the alimentary canal
Its histology is similar to the colon but its distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa

Question 12

What is the anal canal?

Answer 12

Terminal portion is the anal canal

Its surrounded by internal (circular muscle) and external (striated muscle) anal sphincters

Question 13

What is the mucosal structure of the large bowel?

Answer 13

Innermost first:
Mucosa:
- epithelium 
- lamina propria 
- muscular mucosae 

Submucosa:
- connective tissue (containing nerve plexus)

Muscularis:
- smooth muscle (containing nerve plexus)

Serosa/ adventitia:
- connective tissue +/- epithelium

Question 14

What kind of cells are found in the mucosal structure of the large bowel?

Answer 14

Like the small intestines:
Enterocytes and goblet cells are abundant
Abundant crypts
Stem cells are found in the crypts

Question 15

How is the mucosa organised?

Answer 15

Mucosa appears smooth at the gross level because it has no villi (smaller SA than small bowel)
Enterocytes have short, irregular microvilli- primary concerned with the reabsorption of salts
Crypts dominated by goblet cells
Glycocalyx are also found on apical membrane

Question 16

How are goblet cells structured?

Answer 16

Higher number of goblet cells than small bowel
More prevalent in the crypts than along the surface, number increases distally towards rectum
Apical ends are packed with mucus filled secretion granules
Mucus- facilitates the passage of the increasingly solid colonic contents and covers bacteria and particulate matter
Acetylcholine (parasympathetic and enteric NS) stimulates goblet cell secretion

Question 17

What cells are not found in the large bowel?

Answer 17

No paneth cells

Enteroendocrine cells are rarer than in the small bowel

Question 18

How does the dominant cell of small bowel compare with large bowel?

Answer 18

In large bowel enterocytes are still the dominant cell facing the lumen but mucus-secreting goblet cells dominate crypts

Question 19

What are microvilli?

Answer 19

Make up the brush border
Several thousand microvilli per cell
Surface of microvilli covered in glycocalyx

Question 20

What is glycocalyx?

Answer 20

This is rich in carbohydrates
Serves as a protection from digestional lumen yet allows absorption
Traps a layer of water and mucous known as “unstirred layer”
Regulates rate of absorption from intestinal lumen

Question 21

What muscles are found in the large bowel?

Answer 21

Like the small bowel muscularis externa consists of inner circular and outer longitudinal layer
Circular muscles segmentally thickened
Longitudinal layer concentrates in 3 bands- taenia coli
Between taenia, longitudinal layer is thin
Bundles of muscle from the taenia coli penetrate the circular layer at irregular intervals
Taenia coli form haustra which can contract individually

Question 22

How does the large bowel produce motility?

Answer 22

Colonic contraction (usually segmental) are minimally propulsive: 5-10cm/hr
This promotes the absorption of electrolytes and water
In the proximal colon, ‘anti-propulsive’ patterns dominate to retain chyme
In the transverse and descending colon, localised segmental contractions of circular muscle called Haustral contraction cause back and fourth mixing
Short propulsive movements occur every 30 mins. These increase in frequency following a meal

Question 23

What is mass movement?

Answer 23

1-3 times daily there is mass movement (high amplitude propagated contractions)- it resembles peristaltic wave
It can propel contents 1/3- 3/4 of length in large intestine in a few seconds
Food that contains fibre promotes rapid transport through the colon

Question 24

How is the large bowel controlled by nerves?

Answer 24

Parasympathetic: ascending colon and most of transverse colon is innervated by vagus nerve. More distal innervated by pelvic nerves
Sympathetic: lower thoracic and upper lumbar spinal cord
External sphincter controlled by somatic motor fibres in the pudendal nerves
Afferent sensory neurones detect pressure

Question 25

How else is the large bowel controlled?

Answer 25

Enteric nervous system also important- in Hirschprung’s disease there no enteric intramural ganglia
Myenteric plexus of ganglia is concentrated below taenia coli
Presence of food in stomach can stimulate mass movement
Hormonal/paracrine control e.g. aldosteone promotes sodium and water absorption (synthesis of Na+ ion channel)

Question 26

How does defecation occur?

Answer 26

Rectum is filled with faeces by mass movement in the sigmoid colon
It stores stool until its convenient to void
Defecation reflex is controlled primarily by 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 colon, sigmoid colon and rectum. Internal anal sphincter inhibited
Weak intrinsic signal augmented by autonomic reflex
The external anal sphincter is under voluntary control
If the urge is resisted then the sensation subsides

Question 27

What is the role of the rectum?

Answer 27

Last few cm of the rectum is known as the ‘social part’ of the rectum
It can distinguish between solid, liquid and gas
That perceptual ability is important in knowing what can be passed appropriately in what circumstances

Question 28

What is faeces?

Answer 28

150g/day in adult
2/3 water
Solid: cellulose, bacteria, cell debris, bile pigment, salts
Bile pigment gives colour
Bacterial fermentation gives odour

Question 29

What is the large bowel flora?

Answer 29

All mammals have symbiotic relationship with their gut microbial community
Stomach and small bowel have few bacteria- protected
Large bowel has many bacteria
The large bowel flora is essential to normal function
Its a diverse, highly metabolically active community
The microbiome on an average human has 1.5kg of live bacteria

Question 30

What are roles of the intestinal flora regarding pathogens and vitamins?

Answer 30

Synthesises and excretes vitamins e.g. vit K germ-free animals can have clotting problems
Prevents colonisation by pathogens by competing for attachment sites for essential nutrients
Antagonise other bacteria through the production of substances which inhibit or kill non-indigenous species

Question 31

What are roles of the intestinal flora regarding antibodies and tissue development?

Answer 31

Stimulates the production of cross-reactive antibodies. Antibodies produced against components of normal flora can crossreact with certain related pathogens and so prevent infection or invasion

Stimulates the development of certain tissues including caecum and lymphatic tissues

Question 32

What are the roles of the flora regarding fibre and fatty acids?

Answer 32

Fibre can be broken down by colonic bacteria

Produces short chain fatty acids which regulate gut hormone release or be absorbed and used as an energy source or to influence functions such as food intake or insulin sensitivity directly

Question 33

What are types of normal flora?

Answer 33

Most prevalent bacteria are bacteroids- Gram- negative, anaerobic, non-spore forming bacteria. They’re implicated in the initiation colitis and colon cancer

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

Question 34

How can the microbiome be replenished?

Answer 34

Faecal matter can be transplanted from a healthy individual into another individual to replenish their microbiome