Large Intestine Structure and Function

Question 1

What valve is between the ileum and the caecum?

Answer 1

Ileocaecal valve

Question 2

What is the length of the colon?

Answer 2

Ranges from 1.5 to 1.8m

Question 3

What are the 4 parts of the large intestine?

Answer 3

Ascending colon

Transverse colon

Descending colon

Sigmoid colon

Question 4

Explain how the circular and longitudinal muscles of the colon are different from the small intestine?

Answer 4

Circular muscle is complete but the longitudinal is not

Question 5

What are the three bands present along the entire length of the colon?

Answer 5

Teniae coli

Question 6

What are tenaei coli?

Answer 6

Three seperate longitudinal ribbons of smooth muscle running through the entire colon

Question 7

What are the pouches present throughout the colon called?

Answer 7

Haustra

Question 8

What produces haustra?

Answer 8

Contractions of the teniae coli

Question 9

What is the classification of the mucosa in the colon?

Answer 9

Simple columnar epithelium that is flat

Question 10

What is contained in the crypts within the colon?

Answer 10

Goblet cells

Question 11

Why are goblet cells present in the colon?

Answer 11

Secrete mucous that is used as lubrication for faeces to move

Question 12

What is the rectum?

Answer 12

Straight, muscular tube between the end of sigmoid colon and anal canal

Question 13

What is the classification of the mucosa of the rectum?

Answer 13

Simple columnar epithelium

Question 14

How does the muscularis externa of the rectum compare to other regions of the alimentary canal?

Answer 14

It is thicker

Question 15

What is the anal canal?

Answer 15

2-3cm between distal rectum and anus

Question 16

How does the muscularis externa of the anal canal compare to the rectum?

Answer 16

Is thicker forming the internal anal sphincter

Question 17

What is the internal anal sphincter formed from?

Answer 17

Muscularis externa (smooth muscle)

Question 18

What is the external anal sphincter formed from?

Answer 18

Skeletal muscle

Question 19

How does the epithelium change in the anal canal?

Answer 19

Changes from simple columnar to stratified squamous

Question 20

Does the colon have an important nutrient absorpion role?

Answer 20

Not in humans

Question 21

What is the function of the colon?

Answer 21

Actively transport sodium from lumen into the blood, causing osmotic absorption of water

This causes dehydration of chyme causing solid faecal pellets

Question 22

What cause faeces to be solid?

Answer 22

Water is absorbed, causing dehydration of faeces, due to the colon transporting sodium from the lumen to the blood creating an osmotic difference

Question 23

What causes bacterial colonisation of the colon?

Answer 23

Large residence time

Question 24

How much bacteria is present in the colonc microflora?

Answer 24

10¹⁴ (about 1kg)

Question 25

What happens to undigested carbohydrates in colon?

Answer 25

Bacterial fermentation

Question 26

What are consequences of bacterial colonisation of the colon?

Answer 26

Bacterial fermentation of undigested carbohydrates, short chain fatty acids and vitamin K Production of gas

Question 27

Why does defaecation not occur passively?

Answer 27

Anus is closed by internal anal sphincter (smooth muscle under autonomic control) and external anal sphincter (skeletal muscle under voluntary control)

Question 28

Explain the mechanism that occurs following a meal that leads to defaecation?

Answer 28

1) Wave of intense contraction (mass movement contraction) from the colon into the rectum 2) Distension of rectal wall produced by mass movement of faecal material into rectum 3) Activated mechanoreceptors activate the defaecation reflex 4) Causes urge to defaecate

Question 29

What controls the defaecation reflex?

Answer 29

Parasympathetic nervous system (via pelvic splanchnic nerves) with no sympathetic influence

Question 30

How can defaecation be voluntary delayed?

Answer 30

By descending neural pathways

Question 31

Explain the mechanism of the defaecation reflex under parasympathetic control?

Answer 31

1) Contraction of rectum 2) Relaxation of internal and contraction of external anal sphincters 3) Increased peristaltic activity in colon (increased pressure on external anal sphincter causing expulsion of faeces)

Question 32

What is constipation?

Answer 32

Condition where there is difficulty emptying the bowels

Question 33

Does constipation lead to absorption of toxins?

Answer 33

No, there is no absorption of toxins from faecal material following long periods of retention

Question 34

How does frequency of bowel movements vary from person to person?

Answer 34

Vary considerably

Question 35

What are some symptoms associated with constipation?

Answer 35

Headaches Nausea Loss of appetite Abdominal distension

Question 36

Why does constipation cause symptoms?

Answer 36

It causes distension of the rectum

Question 37

What is diarrhoea?

Answer 37

Too frequent passage of faeces which are too liquid

Question 38

What are some causes of diarrhoea?

Answer 38

Pathogenic bacteria Protozoans Viruses Toxins Food

Question 39

How much deaths occur per year due to diarrhoea?

Answer 39

0.5 million deaths/year, mostly children under 5

Question 40

What is E-coli an example of?

Answer 40

Enterotoigenic bacteria that normally lives in the intestines

Question 41

What are examples of enterotoxigenic bacteria?

Answer 41

Vibrio cholerae Escherichia coli

Question 42

What does enterotoxigenic bacteria produce that causes diarrhoea?

Answer 42

Enterotoxins that maximally turn on intestinal chloride secretion from crypt cells, increasing H₂O secretion

Question 43

How does enterotoxigenic bacteria maximally turn on intestinal chloride secretion from crypt cells?

Answer 43

Enterotoxins that they produce elevate the levels of intracellular second messengers like: cAMP cGMP Calcium

Question 44

Why does excess H₂O secretion in the colon lead to diarrhoea?

Answer 44

It swamps absorptive capacity of villus cells leading to profuse watery diarrhoea

Question 45

How much water per day is lost in diarrhoea due to cholerae?

Answer 45

25L

Question 46

Do enterotoxins damage villus cells?

Answer 46

No

Question 47

What is the treatment of secretory diarrhoea?

Answer 47

Sodium/glucose solution which drives H₂O absorption leading to rehydration

Question 48

What is the administration of sodium/glucose solutions to people suffering from secretory diarrhoea called?

Answer 48

Oral rehydration therapy (ORT)

Question 49

What does ORT stand for?

Answer 49

Oral rehydration therapy