Physiology: Large Intestine

Question 1

What is the approximate length and diameter of the large intestine?

Answer 1

1.7m long and 6cm in diameter

Question 2

What are the different parts of the large intestine?

Answer 2

• caecum
• appendix
• colon (ascending, transverse, descending, sigmoid)
• rectum
• anal canal and anus

Question 3

What is the name of the 3 longitudinal strands of muscle that run along the colon and caecum?

Answer 3

Taeniae coli

Question 4

How is the smooth muscle arranged around the rectum and anal canal?

Answer 4

encircles the rectum and anal canal

Question 5

Where is smooth muscle thickened?

Answer 5

At the internal anal sphincter which is surrounded by the skeletal muscle of the external anal sphincter

Question 6

What is the name of the sac-like bulges of the colon?

Answer 6

haustra

Question 7

What causes the haustra to form sac-like bulges?

Answer 7

activity of the taeniae coli and circular smooth muscle layers, haustra change location

Question 8

How much material does the caecum normally receive and what is this material?

Answer 8

Approx. 1-2 L - indigestible residues, unabsorbed fluids and unabsorbed biliary products

Question 9

What is the gastroileal reflex?

Answer 9

entry of substance from terminal ileum into caecum through the ileocaecal valve

Question 10

What triggers the gastroileal reflex?

Answer 10

gastrin and CCK

Question 11

How does the ileocaecal valve work? (3 ways)

Answer 11

1. maintains a positive resting pressure
2. relaxes in response to distension of the duodenum
3. contracts in response to distension of the ascending colon

Question 12

What controls the ileocaecal valve?

Answer 12

• vagus nerve
• sympathetic nerve
• enteric neurones
• hormonal signals

Question 13

What connects the appendix to the caecum?

Answer 13

appendiceal orifice

Question 14

What tissue is the appendix made of?

Answer 14

lymphoid tissue

Question 15

What can cause appendicitis?

Answer 15

obstruction of the appendiceal orifice by a faecalith

Question 16

What are the absorption functions of the colon?

Answer 16

• Colon absorbs Na+, Cl- and H20 to condense stool

- Colon absorbs short chain fatty acids

Question 17

What happens to carbohydrates in the digestive tract and where are they absorbed?

Answer 17

Carbohydrates are fermented by colonic flora to short chain fatty acids in the colon

Question 18

What does the colon secrete?

Answer 18

• K+
• HCO3-
• mucous

Question 19

What does faeces consist of?

Answer 19

• mostly H20
• cellulose
• bacteria
• bilirubin
• small amount of salt

Question 20

What does the surface of the colon have in order to increase surface area?

Answer 20

• microvilli
• crypts
• colonic folds
• (no villi)

Question 21

What cells in the colon mediate ion secretion?

Answer 21

crypt cells

Question 22

What cells in the colon mediate electrolyte absorption and by what mechanism does this lead to H2O reabsorption?

Answer 22

• colonocytes

- osmosis

Question 23

What do goblet cells secrete?

Answer 23

• lots of mucous containing glycosaminoglycans (this forms slippery surface gel)
• trefoil proteins involved in host defence

Question 24

What hormone enhances K+ and Na+ absorption?

Answer 24

aldosterone

Question 25

What can occur in secretory diarrhoea?

Answer 25

significant K+ loss

Question 26

How much material approx. does the colon absorb each day?

Answer 26

1L

Question 27

What are the 3 patterns of motility in the large intestine?

Answer 27

1. haustration (non-propulsive segmentation)
2. peristaltic propulsive movements (mass movement)
3. Defaecation (periodic egestion)

Question 28

What is haustration?

Answer 28

the formation of haustra (succules) made by alternating contraction that has function similar to segmentation but slower

Question 29

How is haustration generated?

Answer 29

slow wave activity probably

Question 30

What is the function of haustration?

Answer 30

mixes content to allow time for fluid and electrolyte reabsorption

Question 31

What happens to haustra before and after a mass movement?

Answer 31

disappear before and reappear after

Question 32

What is mass movement (peristaltic propulsive movements)?

Answer 32

simultaneous contraction of circular muscle of the ascending and transverse colon that drives faeces distally

Question 33

How often do mass movements occur in a day?

Answer 33

1-3 times

Question 34

What triggers mass movement?

Answer 34

triggered by a meal (often breakfast) via the gastrocolic response

Question 35

What is the gastrocolic response and what does it involve?

Answer 35

• the urge to poop after a meal

- involving gastrin and extrinsic nerve plexuses

Question 36

What triggers the defaecation reflex?

Answer 36

rectal stretch caused by faeces entering the rectum