The large intestine

Question 1

what does the large intestine consist of?

Answer 1

consists of the colon, caecum, appendix, rectum and the anal canal

Question 2

what is the caecum?

Answer 2

the caecum is a blind path

it is not often used in humans

it is usually fermenting

Question 3

what is the appendix?

Answer 3

the appendix is a thin fingerlike projection extension of the caecum

• not very useful in humans
• potentially a lifeboat for mirobiome

Question 4

what are the dimensions of the colon?

Answer 4

1.5 m long , 6cm diameter

Question 5

what is the function of the colon?

Answer 5

to reabsorb electrolytes and water and also to flush the waste

Question 6

where are the ascending colon, transverse colon, descending colon and sigmoid colon positioned?

Answer 6

the ascending colon = caecum to the hepatic flexure

transverse colon = hepatic flexure to splenic flexure

descending colon = splenic flexure to sigmoid colon

sigmoid colon = descending colon to rectum

Question 7

what is the blood supply of the colon?

what is the proximal transverse colon supplied by?

what is the distal third colon supplied by?

Answer 7

Proximal transverse colon supplied by middle colic artery

(branch of the superior mesentric artery)

Distal third of transverse colon supplied by inferior mesenteric artery.

Question 8

what are some of the structural features of the colon?

and what are the functions of these features?

Answer 8

• fatty tags (appendices epiploicae)

protective function against infection

• longitudinal muscle bands ( taeniae coli)

Creates haustra, necessary for large intestine motility.

• nodules of lymphoid tissue (Solitary nodules)

Question 9

what are the histological differences of the rectum?

Answer 9

Histologically similar but absent taeniae coli ( longitudinal muscle bands) and transverse rectal folds in submucosa.

Question 10

what control is the internal anal sphincter and external sphincter under?

Answer 10

internal anal sphincter - unconcious control

external spinchter - voluntary control

Question 11

Diagram of the Rectum ?

Answer 11

Question 12

what are the similarities between the large intestine and the small intestine?

Answer 12

• abundant enterocytes
• abundant goblet cells
• abundant crypts with stem cells present

Question 13

how is the large intestine distinguishable from the small intestine?

Answer 13

• there are no villi in the large intestine
• Enterocytes have short and irregular microvilli

Question 14

what is the mucosal structure?

Answer 14

• goblet cells present
• More goblet cells than the small intestine with number higher in the crypts and more distally in the colon
• no paneth cells
• Enteroendocrine cells are rarer than small intestine
• Glycocalyx present on microvilli BUT does not hold digestive enzyme in ‘unstirred layer’

Question 15

what are the muscle layers of the large intestine ?

Answer 15

• like small intestine
• inner circular and outer longitudinal is present
• 3 clear bands – taeniae coli
• there are oval segments called haustra that can contract induvidually

Question 16

what are the movements of the rectum and anal canal?

Answer 16

these movements are substantial and continuous – used for kneading contents.

Question 17

how fast are the colonic contractions?

Answer 17

5- 10 cm / AN HOUR

Question 18

What are the movements of the colon and proximal colon like?

Answer 18

the colon is slow propulsion promotes absorption of electrolytes and water.

the proximal colon is ‘anti-propulsive’ patterns

Question 19

what are the movements like in the transverse and descending colon?

Answer 19

haustral contractions cause back and fourth mixing

There are then short propulsions ever 30 minutes

these increase in frequency following a meal

Question 20

when do mass movements happen and what can they propel?

Answer 20

• occur 1- 3 times a day
• • propel contents 1/3-3/4 length of large intestine in a few seconds

these movements depend upon fibre

Question 21

what is the nervous control of the large intestine?

parasympathetic ?

sympathetic ?

enteric?

Answer 21

parasympathetic ? ascending colon, most of transverse colon by VAGAL nerve. More distal by PELVIC nerves

sympathetic ? – lower thoracic and upper lumbar spinal cord.

enteric

Myenteric plexus ganglia concentrate below the taeniae coli.

Question 22

where do the faeces travel and where are they stored?

Answer 22

the rectum becomes filled with faeces by mass movements in sigmoid column which then stores the stool

Question 23

what is the defecation reflex controlled by?

Answer 23

controlled by the sacral spinal-cord

Question 24

how does defecation work?

Answer 24

• reflex inhibition of internal sphincter to balloon the walls of the rectum
• pressure receptors send signals to Myenteric plexus to initiate peristaltic waves
• external sphincter under voluntary control which means one can choose whether to defecate or resist the urge

Question 25

what can the last few centimetres of the rectum determine?

Answer 25

determines solid, liquid or gas

Question 26

where are most of the the bacteria present?

Answer 26

the stomach and small intestine has a few bacteria

the large intestine has many bacteria - essential to function

Question 27

what are the functions of the intestinal flora to humans?

Answer 27

• they synthesis and excrete vitamins like vitamin K
• they prevent colonisation by pathogens by outcompeting them
• they kill pathogens by producing antibacterials

Question 28

what are the beneficial roles of the intestinal flora to humans ?

Answer 28

• immunity - production of cross-reactive antibodies
• growth - stimulates the development of certain tissues like the caecum
• fibre breakdown - flora can breakdown the fibre
• fatty acids - produces short-chain fatty acids which might be used as hormone release, energy, or influence food intake

Question 29

what are the most prevalent bacteria?

Answer 29

• gram negative bacteria

-