Large bowel

Question 1

What are the components of the large bowel

Answer 1

Colon, caecum, appendix, rectum and anal canal

Question 2

What is the caecum and appendix? Discuss any relevance

Answer 2

Caecum- blind pouch just distal to ileocecal valve; the caecum is larger in herbivores

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

Question 3

What are the principal functions of the large bowel

Answer 3

• Reabsorption of electrolytes and water
• Elimination of undigested food and waste

Question 4

What is the dimension of the large bowel

Answer 4

1.5m long

6cm in diameter

Question 5

What are the different parts of the colon and where do they start and end

Answer 5

Ascending colon- run from caecum to hepatic flexure

Transverse colon- from hepatic flexure to splenic flexue. It also hangs off stomach, attached by lesser omentum posteirorly (mesocolon)

Descending colon- from splenic flexure to sigmoid colon

Sigmoid colon - from descending colon to the rectum

Question 6

Explain the arterial supply of the colon

Discuss any clinical relevance

Answer 6

Proximal 2/3 of transverse colon (from caecum) is perfumed by middle colic artery; which is a branch of the superior mesenteric artery

Distal 1/3 is perfumed by inferior mesenteric artery.

this perfusion pattern demarcates the midgut from the hindgut.

The region between the 2 artery’s is sensitive to ischaemia

Question 7

What are the fatty tags carried by the peritoneum on the colon? What is their fucntion

Answer 7

Appendices epiploicae

fucntion: suggested to have a protective function against intra-abdominal infections

Question 8

What are the muscle coat which have 3 thick longitudinal bands on the colon ?

what is their fucntion

Answer 8

Taenia coli

necessary for large intestine motility.

it also cause the formation of pouched ovoid segments called haustra

Question 9

Compare the taenie coli in large and small bowel

Answer 9

Taenie coli in large bowel is shorter

Question 10

How much water can the large intestine absorb? How much does it usually absorb?

Answer 10

Can absorb 4.5 litres

USUALLY ABSORB 1.5 LITRES.

Anything less than 1.5 litres is diarrhoea

Question 11

What are the basic principles of how the colon absorb water and electrolytes?

Answer 11

This process occurs more in proximal colon

Na+ and Cl- absorbed by exchange mechanism and ion channels

Water follows by osmosis

K+ moves passively into lumen

Question 12

How is the rectum historically distinct from colin

Answer 12

Has transverse rectal folds in submucosa

Absence of taenia coli in muscularis external

Hence its the dilated distal portion of the alimentary canal

Question 13

What surrounds the anal canal?

Answer 13

Internal sphincter- circular muscle

External sphincter - striated muscle - hence under voluntary control

Question 14

Label this diagram

Answer 14

Question 15

What are the similarities and differences in the mucosal structure between small and large bowels?

Answer 15

Similarities :

• Abunndant enterocytes and goblet cells;
• Abundant crypts and stem cells are found in the crypts

Differences:

• large bowel DOES NOT HAVE VILLI ; unlike the small bowel
• large bowel has more goblet cells

Question 16

Describe the lymphoid tissue in small and large intestine. Give names

Answer 16

Small- Peyer’s patches

Large- solitary nodules

Question 17

Describe the mucosal organisation of the Large bowel

Answer 17

• Mucosa appears smooth at gross level- NO VILLI; (smaller SA)
• Enterocytes have short irregular Microvilli; primarily concerned with resorption of salts
• Crypts dominated by goblet cells
• No Paneth Cells
• Glycocalyx DOES NOT contain digestive enzymes
• enteroendocrine cells are rarer here than in small bowels

N.B Water is absorbed passively as it follows electrolytes

Question 18

Descirbe the distribution and features of the goblet cells in the large bowel

Answer 18

Higher number than in small bowels

More prevalent in crypts than in surface

Number increases as you approach the rectum

Apical ends are packed with mucus filled secretion granules awaiting release

Question 19

What stimulate goblet cell secretion in large bowels

Answer 19

Ach by PNS and enteric nervous system

Question 20

What is the fucntion of mucus in the large bowels

Answer 20

Facilitates the passage of the increasingly solid colonic contents and covers bacteria and particulate matter

Question 21

What covers the surface of Microvilli

what is the dimension of micorvilli in large Bowel

Answer 21

Glycocalyx

1.0uM high and make up brush border. About several thousands Microvilli per cell

Question 22

What are the features and fucntion of the glycocalx in the large bowel

Answer 22

• It’s a rich carb layer on apical membrane
• Serves as protection from digesitonal lumen yet allows for absorption
• traps a a layer of water and mucous known as “unstirred layer”
• Regulates rate of absorption from intesitnal lumen

Question 23

Describe the organisation of the muscularis layer of the large bowel

Answer 23

Contains circular and outer longitudinal muscle layers

• circular muscles are segmentally thickened
• longitudinal layers are arranged in 3 bands- Taenia coli

Between the taenia, the longitudinal layer is thin

bundles of Taenia coli penetrate the circular layer at irregular intervals

Question 24

Descirbe the action of the ovoid segment - haustra ?

Answer 24

They can contract individually

this occurs almost everywhere except rectum and anal canal where contraction is substantial and continuous

Question 25

Which movement is more complicated ? Large or small bowel

Answer 25

Large

Question 26

Describe the pattern of segmental motility throughout the large bowel

Answer 26

In Proximal colon, there's antipropulsive pattern dominating to retain chyme in transverse and descending colon, localised segmental contractions of circular msucle called Haustral contraction; causes back and forth mixing theres short minimal propulsive movements every 30 mins (5-10cm/hr) This increase in freqency following a meal

Question 27

Describe when mass movement occurs in large bowels and how?

Answer 27

1-3times a day; this resembles peristaltic wave This can propel contents form 1/3 to 75% of length of large intestine within a few seconds Food with fibres speed up this process and promote rapid transport

Question 28

Descirbe the PNS control of the large bowel. What part of the bowel does it control

Answer 28

Ascending colon and most of transverse colon innervates by Vagus nerve More distal part of large bowel innervated by pelvic nerves

Question 29

Describe the SNS control of Colon

Answer 29

Sympathetic by T8 to L2 this stop motility and peristalsis

Question 30

What controls the external anal sphincter muscle

Answer 30

Somatic motor fibres in **pudendal nerves** Affrent sensory neurones detect pressure in rectum

Question 31

What disease can occur as a result of no enteric intramural ganglia

Answer 31

Hirschsprung disease

Question 32

Describe the defecation reflex and how it can be overridden

Answer 32

Rectum filled with faecesby mass movement in sigmoid colon; causes distension in walls of rectum Pressure receptors send signals via myenteric plexus to initiate peritalstic waves in descending, sigmoid colon and rectum. Internal anal sphincter inhibited this weak intrinsic signal is augmented by autonomic reflex when we decide not to devoid bowels; as external anal sphciter is under voluntary control urge resisted and sensation subsides

Question 33

What controls the defecation reflex

Answer 33

Sacral spinal cords- both reflex and voluntary actions

Question 34

Descibe the "social part" of the rectum

Answer 34

Last few centimetres of rectum Can distinguish between solid, liquid and gas hence know what can be passed appropriately in what circumstance

Question 35

Descibe the composition of faeces. Explain what gives it's colour and odour

Answer 35

* 150g/day made by adult * 2/3 of it is water * solids are: cellulose , bacteria , cell debris , bile pigments, salts (K+) * bile pigment (**stercobilinogen**) gives it colour * **Bacterial fermentation** gives odour

Question 36

Compare the amount of gut bacteria in large bowel and (stomach and small bowel)

Answer 36

A LOT in large bowel small amounts in stomach and small bowel- protected by acidic conditions

Question 37

Describe the features and relationship of microbiome with large bowel

Answer 37

Symbiotic - hence essential for fucntion Diverse and highly metabolically active weight of all micorbiome together is 1.5kg

Question 38

Explain the different roles of intesitnal flora

Answer 38

* Make and excrete vitmains ;e.g Vit k * Prevent colonisation of pathogens by competing for attachment site/nutrients * Make substances that inhibit/kill non-endogenous bacteria * stimulate development of some tissues; like caecum and lymphatic tissue * stimulate production of cross-reactive antibodies * Breaks down fibre * Make short fatty acid chains which can regulate gut hormone release

Question 39

Describe the action of cross-reactive antibodies made by flora

Answer 39

Antibodies made against compent of flora can cross react with some related pathogens and hence prevent invasion and infection

Question 40

What are the actions of the short chain fatty acids made by flora

Answer 40

Regulate gut hormone release Fatty acid absorbed and used as energy source Influence functions such as food intake or insulin sensitivity directly Can also reduce appetite (fatty acid acetate)

Question 41

What are 2 different bacteria in flora. Contrast them

Answer 41

**Bacteriodes**: * Most prevalent * Gram negative, anaerobic * non-spore forming * Implicated in the initiation colitis and colon cancer **Bifidobacteria** * Gram postive , lactic acid bacteria * non spore forming * been descirbed as friendly bacteria * Thought to prevent colonisation by potential pathogens

Question 42

In recent research, what the different links to flora

Answer 42

Drug metabolism Insulin resistance Bile acid metabolism Lipid metabolism Obesity

Question 43

Where is the myenteric plexus ganglia concentrated?

Answer 43

Below taenia coli

Question 44

Descirbe how the presence of food in stomach can stimulate mass movement (peristalsis)

Answer 44

Via hormonal/paracrine control i.e. aldosterone promotes sodium and water absorption via synthesis of Na+ ion channel and Na/K+ pump