The Large Intestine

Question 1

How long is the large intestine and what is its diameter?

Answer 1

1.5 m

6 cm

Question 2

What are the principal functions of the large intestine?

Answer 2

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

Question 3

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

Answer 3

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

Question 4

State three features that are unique to the colon.

Answer 4

Appendices epiploicae
Taenia coli
Pocketed/segmented appearance - haustra

Question 5

Describe how the distribution of lymphoid tissue is different in the large intestine compared to the small intestine.

Answer 5

Lymphoid tissue in the terminal ileum is arranged into peyer’s patches
Large intestine - individual nodules

Question 6

What is the reason for the haustra?

Answer 6

The taenia coli are shorter than the intestines so the large intestine is contracted into a pocketed appearance

Question 7

Describe the movement of ions into and out of the large intestine lumen.

Answer 7

Na+ and Cl- move out of the lumen
Water follows
K+ moves into the lumen passively

Question 8

How much water can the large intestine absorb?

Answer 8

4.5 L/day (usually around 1.5 L/day)

Question 9

How is the histology of the rectum different to the large intestine?

Answer 9

No taenia coli

Transverse rectal folds present

Question 10

What are the sphincters in the anal canal and what controls them?

Answer 10

Internal anal sphincter - ANS control

External anal sphincter - somatic control (via pudendal nerve)

Question 11

What are the main cells of the large intestine?

Answer 11

Enterocytes and goblet cells

Crypts are abundant (goblet cells dominate the crypts)

Question 12

Describe some differences between the epithelium of the large intestine and the small intestine.

Answer 12

Large intestine does not have villi
There are no paneth cells
Goblet cells dominate the crypts 
There are stem cells in the crypts (this is a similarity) 
Microvilli are small and irregular

Question 13

What is the role of the goblet cells in the large intestine?

Answer 13

Produce mucus which provides lubrication for the movement of the stools down the colon.

Question 14

Describe the distribution of goblet cells in the large intestine.

Answer 14

Goblet cells increase in number distally.

Question 15

What stimulates goblet cell secretion?

Answer 15

Acetylcholine - they are under parasympathetic control

Question 16

Why are there NO PANETH CELLS in the large intestine?

Answer 16

Because the bacteria in the large intestine are integral to its function so you don’t want to be secreting antimicrobial products from paneth cells that will kill the flora.

Question 17

Describe the abundance of enteroendocrine cells in the large intestine.

Answer 17

There are FEWER enteroendocrine cells in the large intestine compared to the small intestine.

Question 18

How is the glycocalyx of the large intestine different to the small intestine?

Answer 18

There are no digestive enzymes in the large intestine glycocalyx whereas in the small intestine there are carbohydrate digesting enzymes in the glycocalyx.

Question 19

What are taenia coli?

Answer 19

The longitudinal muscle layer is concentrated into three bands which are the taenia coli. The longitudinal muscle between these layers is thin.

Question 20

Describe the relationship between the taenia coli and the circular muscle layer? Which is longer?

Answer 20

Taenia coli penetrate the circular muscle at irregular intervals. The circular muscle layer is longer which is why you get haustra.

Question 21

What are the different types of contractions in the large intestine?

Answer 21

Colonic contractions - not really propulsive, it is a kneading process that promotes absorption of electrolytes and water
In the proximal colon you get ‘antipropulsive’ patterns which try and retain the chyme in that part of the colon to promote more water and electrolyte reabsorption.
You can get localised, haustral contractions that cause back and forth mixing. There are short propulsive movements every 30 minutes.
MASS MOVEMENT - occurs 1 to 3 times a day. Resembles a peristaltic wave and moves the bolus 1/3 to 3/4 of the way along the colon in seconds.

Question 22

Where does the parasympathetic innervation of the colon come from?

Answer 22

Vagus nerve and pelvic nerves

Question 23

Where does the sympathetic innervation of the colon come from?

Answer 23

Lower thoracic and upper lumbar spinal cord

Question 24

What nerve allows control of the external anal sphincter?

Answer 24

External anal sphincter - pudendal nerve - somatic motor

Question 25

What causes Hirschsprung’s disease?

Answer 25

Absence of enteric intramural ganglia so you don’t really get proper colonic function.

Question 26

Where are the myenteric plexus ganglia found?

Answer 26

Concentrated below the taenia coli.

Question 27

What can stimulate the mass movement?

Answer 27

Food in the stomach - by neural and hormonal controls

Question 28

Describe the hormonal control of the colon.

Answer 28

Aldosterone stimulates reabsorption of Na+ and water (removes K+ and H+)

Question 29

What causes the defecation reflex?

Answer 29

It is controlled by the SACRAL spinal cord

Question 30

Describe the process of defecation.

Answer 30

Sudden distension of the walls of the rectum. Pressure receptors send signals via the myenteric plexus to initiate a peristaltic wave from the descending colon, sigmoid colon and rectum.
Internal anal sphincter is inhibited.
External anal sphincter is under voluntary control.

Question 31

What is the significance of the last few centimetres of the rectum?

Answer 31

The social part of the rectum can distinguish between solid, liquid and gas. Helps you determine what can be passed appropriately.

Question 32

How much faeces does an adult produce per day? What does it mostly consist of?

Answer 32

150 g/day
2/3 water
Bilirubin gives it the brown colour (stercobilin)
Bacterial fermentation gives the smell

Question 33

Describe some of the roles of intestinal flora.

Answer 33

Synthesise and excrete vitamin K
Prevent colonisation by pathogens
Antagonise other bacteria through production of substances that inhibit or kill
Stimulate production of cross-reactive antibodies
Stimulate development of certain tissues
Fibre can be broken down by colonic bacteria
Produces short chain fatty acids that can regulate gut hormone release

Question 34

What are the most prevalent bacteria in the large intestine?

Answer 34

Bacteroides - Gram-negative, anaerobic, non-sporeforming

Implicated in initiation of colonic cancer and colitis

Question 35

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

Answer 35

Bifidobacteria - Gram-postive, non-sporeforming, lactic acid bacteria.
Thought to prevent colonisation by host cells.