Lecture 12 Large Intestine Structure and Function

Question 1

What connects the ileum with the cecum

Answer 1

Ileocaecal valve

Question 2

What are the four parts of the colon

Answer 2

Ascending colon, Transverse colon, Descending Colon and Sigmoid colon

Question 3

Describe the muscular layer of the colon

Answer 3

Circular muscle layer complete but longitudinal muscle layer incomplete

Question 4

What are the 3 bands that run the full course of the large intestine

Answer 4

Teniae coli

Question 5

Contractions of the Teniae coli form

Answer 5

Haustra

Question 6

What cells are the mucosa of the large intestine made of

Answer 6

Simple columnar flat epithelial cells

Question 7

What and where is the rectum

Answer 7

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

Question 8

What type of cells make up the lining of the rectum

Answer 8

Simple columnar epithelium

Question 9

Describe the muscular externa of the rectum

Answer 9

Thick compared to other regions of the alimentary canal

Question 10

Where is the anal canal located

Answer 10

2-3cm between distal rectum and anus

Question 11

Describe the Internal and external anal sphincter

Answer 11

Muscularis thicker than rectum  internal anal sphincter

External anal sphincter = skeletal muscle

Question 12

What type of cells line the anal canal

Answer 12

Transitions from simple columnar to stratified squamous

Question 13

Does nutrient absorption occur in the colon

Answer 13

No

Question 14

How does colon absorb water from chyme to produce faeces

Answer 14

• Actively transports sodium from lumen into blood this causes osmotic absorption of water and dehydration of chyme into solid faecal pellets

Question 15

What does the long residence time in the con cause

Answer 15

Bacterial colonisation

Question 16

What is Gas (flatus) made up of

Answer 16

nitrogen, CO2, hydrogen, methane, hydrogen sulphide

Question 17

Describe the normal physiology of the anal sphincter muscles

Answer 17

• Normally, anus closed by internal anal sphincter (smooth muscle under automatic control) and external anal sphincter (skeletal muscle under voluntary control)

Question 18

What happens at the rectum following a meal

Answer 18

• Wave of intense contraction (Mass Movement Contraction) travels from the colon to rectum
Mechanoreceptors detect this causing a deflection reflex and an urge to defaecate

Question 19

Describe the defection reflex

Answer 19

Under parasympathetic control – via pelvic splanchnic nerves (no sympathetic influence)
A Contraction of rectum
B Relaxation of internal and contraction of external anal sphincters
C Increased peristaltic activity in colon
 Pressure on external anal sphincter - relaxes under voluntary control  expulsion of faeces

Question 20

When does constipation occur

Answer 20

• No absorption of toxins from faecal material following long periods of retention

Question 21

Symptoms of constipation

Answer 21

Headaches
Nausea
Loss of appetite
Abdominal distension

Question 22

Causes of diarrhoea

Answer 22

pathogenic bacteria protozoans
viruses
toxins
food

Question 23

Give examples of Enterotoxigenic Bacteria

Answer 23

Vibrio cholerae,

Escherichia coli

Question 24

How do enterotoxigenic bacteria increase water secretion

Answer 24

• Produce protein enterotoxins which maximally turn on intestinal chloride secretion from crypt cells
increase H2O secretion which swamps the capacity of the villus leading to watery diarrhoea
• Act by elevating intracellular second messengers:
cAMP
cGMP
calcium

Question 25

Treatment of Secretory Diarrhoea

Answer 25

•	Enterotoxins don’t damage villus cells
•	Give sodium/glucose solution 
	Drive H2O absorption  rehydration
•	Secretion still going  wash away infection
•	Oral rehydration therapy (ORS)