large bowel

Question 1

• What is the principle function of the large bowel?

- Dimensions of the large bowel?

Answer 1

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

    1.5m long and 6cm in diameter.

Question 2

• Describe the anatomical position of the transverse colon.

Answer 2

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 3

• What supplies blood to the proximal transverse colon?

- What is the distal third of the transverse colon perfused by and what does this reflect?

Answer 3

Middle colic artery

Inferior mesenteric artery Embryological division between the midgut and the hindgut - region between the two is sensitive to ischaemia

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 anatomical position of the ascending colon?

what is the anatomical position of the descending colon?

Answer 5

right side of the abdomen
from caecum to the hepatic flexure (the turn of the colon by the liver)

from splenic fixure to the sigmoidal colon

Question 6

• What is the functional purpose of the appendices epiploica?
• What are taenia coli?

Answer 6

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

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

Answer 7

Distal small intestine - Peyer’s Patches
Large intestine - Solitary nodules

Haustra

Question 8

• What does the colon (re)absorb?

- What happens if a person’s large intestine reabsorbs more than 4.5L of water?

Answer 8

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

Diarrhoea

Question 9

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

Answer 9

Distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa

Question 10

• What is the anal canal surrounded by?

Answer 10

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

Question 11

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

Answer 11

Mucosa

Submucosa

Muscularis 

Serosa

Question 12

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

Answer 12

Enterocytes and goblet cells are abundant

Abundant crypts 

Stem cells are found in the crypts

Question 13

• Why does mucosa of the large bowel appear smooth at the gross level?
• Do large bowel enterocytes have microvilli?

Answer 13

No villi (therefore smaller surface area than small bowel)

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

Question 14

• What type of cell are the crypts in the large bowel dominated by?
• What happens to the number of goblet cells present in the crypts of the large bowel as you move towards the rectum (distally)?

Answer 14

Goblet cells

Increase

Question 15

• What stimulates mucous secretion by goblet cells?

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

Answer 15

Acetylcholine (ParaNS and enteric nervous system)

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

Question 16

• Are villi present on the large bowel epithelia?

- What make up the brush border in the large bowel?

Answer 16

No
(enteroendocrine cells present, but rarer than in small bowel)

Microvilli (~0.5-1.5 micrometres high) 
Surface covered with glycocalyx (but no digestive enzymes)

Question 17

• Explain the muscular arrangement of the large bowel.

Answer 17

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 18

• Explain the movements of the large bowel.

Answer 18

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 19

• What causes an increase in frequency of bowel movements?
• What does mass movement mean?
• How often does mass movement occur daily?

Answer 19

having a meal

High-amplitude propagated contraction similar to a peristaltic wave

1-3 times

Question 20

• What proportion of the contents of the large bowel can mass movement propel in just a few seconds?
• What innervates the ascending colon and most of the transverse colon and what type of control is this?

Answer 20

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

Vagus nerve, more distal large bowel innervated by pelvic nerves 

Type of control: Parasympathetic

Question 21

• Where does the sympathetic control of the large bowel stem from?
• What controls the external anal sphincter?

Answer 21

Lower thoracic and upper lumbar spinal cord

Somatic fibres in the pudendal nerves

Question 22

• What process fills the rectum with faeces?

- What is the defecation reflex controlled primarily by?

Answer 22

Mass movement in the sigmoid colon
(store stool until convenient to void)

Sacral spinal cord - parasympathetic reflex

Question 23

• Outline the events leading to defection

Answer 23

Reflex to sudden distention 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 contraction inhibited.

Weak intrinsic signal augmented by autonomic reflex.

External anal sphincter under voluntary control. Urge resisted until sensation subsides.

Question 24

• What is different about the last few centimetres of the rectum?
• Roughly how much faeces does an adult defecate daily?

Answer 24

Can distinguish between solid, liquid and gas
This ability is important in knowing what can be passed appropriately and in what circumstance

150g

Question 25

- Outline the composition of faeces | - What gives the odour of faeces?

Answer 25

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

Question 26

- Compare the number of bacteria (flora) between the large bowel and the small bowel and stomach - What mass of live bacteria is present as a part of the gut microbial community (microbiome) in humans?

Answer 26

Large bowel contains many more than the small bowel and stomach 1.5 Kg

Question 27

- List all the roles of intestinal flora

Answer 27

Synthesise and excrete vitamins e.g. Vitamin K germ-free animals can have clotting problems 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

Question 28

what do the FAs that intestinal flora produce do?

Answer 28

Regulating hormone release Or absorbed and used as energy source or to influence functions such as food intake or insulin sensitivity

Question 29

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

Answer 29

Bacteroides (more prevalent) - Gram -ve, anaerobic non-spore forming bacteria (Implicated in the initiation of colitis and colon cancer) Bifidobacteria - Gram +ve, non-spore forming, lactic acid bacteria → Thought to prevent colonisation by potential pathogens

Question 30

- What links have been found to gut bacteria in recent research?

Answer 30

Drug metabolism Insulin resistance Bile acid metabolism Lipid metabolism Obesity

Question 31

- What is the purpose of a stool transplantation?

Answer 31

Source of introducing microbiome from healthy individuals to replenish microbiome