Large Bowel

Question 1

What is the principle function of the large bowel?

Answer 1

Reabsorption of electrolytes and water, and the elimination of undigested food and waste.

Question 2

Dimensions of large bowel?

Answer 2

1.5m long and 6cm in diameter

Question 3

Describe the anatomical position of the transverse colon.

Answer 3

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

Question 4

What part of the colon runs from the descending colon to the rectum?

Answer 4

Sigmoid (s-shaped) colon

Question 5

What is the functional purpose of the appendices epiploica?

Answer 5

Unknown, but suggested to have a protective function against intra-abdominal infections.

Question 6

What are taenia coli?

Answer 6

3 separate longitudinal bands of smooth muscle on the outside of the ascending, transverse, descending and sigmoid colons.

Question 7

What nodules of lymphoid tissue are common in the walls of the distal small intestine and large intestine respectively?

Answer 7

Distal small intestine - Peyer’s Patches

Large intestine - Solitary nodules

Question 8

What are the small ovoid segments that give the gut wall a pouched appearance?

Answer 8

Haustra

Question 9

What does the colon (re)absorb?

Answer 9

Water (by osmosis) and electrolytes including Na+ and Cl- by exchange mechanisms and ions channels.
(K+ moves passively into the lumen)

Question 10

What happens if a persons large intestine reabsorbs more than 4.5L of water?

Answer 10

Diarrhoea

Question 11

In terms of histology, how is the rectum different to the colon?

Answer 11

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 surrounded by?

Answer 12

Internal (circular muscle) and external (striated muscle) anal sphincters

Question 13

What are the 4 main layers of the mucosal structure of the large bowel from deep to superficial?

Answer 13

Submucosa
Mucosa
Muscularis
Serosa

Question 14

What similarities are there between the small and large intestine in terms of their mucosa?

Answer 14

Enterocytes, goblet cells and crypts are abundant

Stem cells are found in the crypts

Question 15

Why does mucosa of the large bowel appear smooth at the gross level?

Answer 15

No villi (therefore small surface area than small bowel)

Question 16

Do large bowel enterocytes have microvilli?

Answer 16

Short, irregular microvilli are present (primarily concerned with resorption of salts)

Question 17

What type of cell are the crypts in the large bowel dominated by?

Answer 17

Goblet Cells

Question 18

What happens to the number of goblet cells present in the crypts of the large bowel as you move towards the rectum (distally)?

Answer 18

Increase in number.

Question 19

What stimulates mucous secretion by goblet cells?

Answer 19

Acetylcholine (ParaNS and enteric nervous system)

Question 20

What is the purpose of mucous being secreted by goblet cells in the large bowel?

Answer 20

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

Question 21

Are villi present in the large bowel?

Answer 21

No

enteroendocrine cells present, but rarer than in small bowel

Question 22

What make up the brush border in the large bowel?

Answer 22

Microvilli (~0.5-1.5 micrometres high)

Surface covered with glycocalyx

Question 23

Explain the muscular arrangement of the large bowel.

Answer 23

Muscular externa consisting of inner circular and outer longitudinal layer. Circular muscles segmentally thickened. Longitudinal layer concentrated in 3 bands - taenia coli.

Bundles of muscle from the taenia coli penetrate the circular layer at irregular intervals.

Question 24

Explain the movements of the large bowel.

Answer 24

Colonic contractions promote absorption of electrolytes and water. In the proximal colon, “antipropulsive” patterns dominate to retain chyme.
In the transverse and descending colon, there are localised segmental contractions of circular muscle called Haustral contractions causing back and forth mixing.

Short propulsive movements every 30 minutes.

Question 25

What causes an increase in frequency of bowel movements?

Answer 25

Having a meal

Question 26

What does mass movement mean?

Answer 26

High-amplitude propagated contraction similar to a peristaltic wave.

Question 27

How often does mass movement occur daily?

Answer 27

1-3 times

Question 28

What proportion of the contents of the large bowel can mass movement propel?

Answer 28

1/2 - 3/4 of length of large intestine in a few seconds.

Question 29

What innervates the ascending colon and most of the transverse colon?

Answer 29

Vagus nerve
More distal large bowel innervated by pelvic nerves.
Type of control: Parasympathetic

Question 30

What controls the external anal sphincter?

Answer 30

Somatic fibres in the pudendal nerves

Question 31

What do afferent sensory neurones detect in the large bowel?

Answer 31

Pressure

Question 32

What condition would a person have if they had no enteric intramural ganglia?

Answer 32

Hirschsprung’s Disease

Question 33

Where are myenteric plexus ganglia more concentrated?

Answer 33

Below taenia coli

Question 34

What process fills the rectum with faeces?

Answer 34

Mass movement in the sigmoid colon

store stool until convenient to void

Question 35

What is the defecation reflex controlled primarily by?

Answer 35

Sacral spinal cord

Question 36

Outline the events leading to defection.

Answer 36

Reflex to sudden distention of rectum wall. 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 until sensation subsides.

Question 37

What is different about the last few centimetres of the rectum?

Answer 37

Can distinguish between solid, liquid and gas.

This ability is important in knowing what can be passed appropriately and in what circumstance.

Question 38

Roughly how much faeces does an adult defecate daily?

Answer 38

150g

Question 39

Outline the composition of faeces.

Answer 39

2/3 Water
Solids: Cellulose, bacteria, cell debris, bile pigments (give colour), salts e.g. K+

(Bacterial fermentation gives odour.)

Question 40

Compare the number of bacteria (flora) between the large bowel and the small bowel and stomach.

Answer 40

Large bowel contains many more than the small bowel and stomach.

Question 41

What mass of live bacteria is present as a part of the gut microbial community (microbiome) in humans?

Answer 41

1.5 Kg

Question 42

List all the roles of intestinal flora.

Answer 42

Synthesise and excrete vitamins
Prevent colonisation by pathogens via competitive inhibition.
Antagonise other bacteria by producing inhibitory substances that kill foreign species.
Stimulate production of cross-reactive Abs (Abs produced against normal flora can cross-react with related pathogens and prevent infection or invasion).
Stimulate development of certain tissues, including caecum and lymphatic tissues.
Fibre (indigestible carbohydrate) can be broken down by colonic bacteria
Produce short chain FAs regulating hormone release or be absorbed and used as energy source to influence functions such as food intake or insulin sensitivity directly.

Question 43

What are the main types of normal flora and which is more prevalent?

Answer 43

Bacteroides (more prevalent) - Gram -ive, anaerobic non-spore forming bacteria. (Implicated in the initiation of colitis and colon cancer)

Bifidobacteria - Gram +ive, non-spore forming, lactic acid bacteria → Thought to prevent colonisation by potential pathogens.

Question 44

What links have been found with gut bacteria in recent research?

Answer 44

Drug metabolism 
Insulin resistance 
Bile acid metabolism 
Lipid metabolism 
Obesity

Question 45

What is the purpose of a stool transplantation?

Answer 45

Source of introducing microbiome from healthy individuals to replenish microbiome.