large bowel

Question 1

what does the large bowel consist of

Answer 1

colon, caecum, appendix, rectum and anal canal

Question 2

what is the caecum

Answer 2

blind pouch just distal to the ileocecal valve (communicates terminal ileum to colon) which is larger in herbivores

Question 3

what is the appendix

Answer 3

the appendix is a thin, finger like extension of the caecum - not physiologically relevant in humans

Question 4

what are the principal functions of the large bowel

Answer 4

reabsorption of electrolytes and water

the elimination of undigested food and waste

Question 5

what is the length and diameter of the large bowel

Answer 5

1.5m long - 6cm in diameter

Question 6

where is the ascending colon

Answer 6

is on the right side of the abdomen, runs from the caecum to the hepatic flexure (the turn of the colon by the liver)

Question 7

where is the transverse colon

Answer 7

runs from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen), hangs off the stomach, attached by a wide band of tissue called the greater omentum (posterior side, mesocolon)

Question 8

where is the descending colon

Answer 8

runs from the splenic flexure to the sigmoid colon

Question 9

where is the sigmoid colon

Answer 9

(s shaped) colon runs from the descending colon to the rectum

Question 10

what supplies the proximal transverse colon with blood

Answer 10

the proximal transverse colon is supplied with blood by the middle colic artery (branch of the superior mesenteric artery)

Question 11

what is the distal 3rd of the transverse colon perfused by

Answer 11

distal third of transverse colon is perfused by the inferior mesenteric artery

Question 12

what is the region between the distal and proximal third of the transverse colon sensitive to

Answer 12

sensitive to ischemia because blood supply is not as extensive

Question 13

what does the peritoneum carry

Answer 13

the peritoneum carries fatty tags (appendices epiploicae)

Question 14

how many bands does the muscle coat have

Answer 14

3 thick longitudinal bands (taeniae coli)

Question 15

what is the gut wall like in appearance

Answer 15

pouched in appearance (haustra)

Question 16

what is the structural or functional purpose of the appendices epiploicae

Answer 16

unknown - suggested to have a protective function against intra-abdominal infections

Question 17

what does the taenia coli do

Answer 17

necessary for large intestine motility

Question 18

what is present in the walls of the distal small intestine and large intestine

Answer 18

nodules of lymphoid tissue are common in the walls of the distal small intestine (Peyer’s patches) and large intestine (solitary nodules)

Question 19

what is the function of the colon

Answer 19

colon absorbs electrolytes and water
happens more in proximal colon
Na+ and Cl- absorbed by exchange mechanisms and ion channels
water follows by osmosis
K+ moves passively into the lumen
large intestine can reabsorb approx 4.5 litres water (usually 1.5) - above this threshold = diarrhoea

Question 20

what is the rectum

Answer 20

dilated distal portion of the alimentary canal of the large bowel

Question 21

how can you tell the colon and rectum apart

Answer 21

histology is similar to the colon but is distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa

Question 22

what is the anal canal and what is it surrounded by

Answer 22

terminal portion is anal canal

surrounded by internal (circular muscle) and external (striated muscle) and sphincters

Question 23

what is the mucosal structure of the large bowel

Answer 23

4 main layers
mucosa
submucosa
muscularis
serosa

Question 24

what is the mucosa

Answer 24

most inner layer and contains many glands covered by submucosa which also has many glands

Question 25

what does the muscularis have

Answer 25

harbours myenteric plexuses

Question 26

what does the serosa have

Answer 26

harbours vessels and nerve endings for large bowel

Question 27

what cells are abundant in the large bowel

Answer 27

enterocytes and goblet cells | abundant crypts

Question 28

what types of cells are found in crypts

Answer 28

stem cells

Question 29

at the gross level, how does the mucosa appear to look

Answer 29

smooth - no villi (smaller SA than small bowel)

Question 30

why do enterocytes have short irregular microvilli

Answer 30

concerned with reabsorption of salts

Question 31

how is water absorbed

Answer 31

passively as it follows electrolytes - resulting in more solid gut contents

Question 32

what cells are the crypts dominated by

Answer 32

goblet cells and stem cells | more in crypts than on surface - no of goblet cells increases distally towards the rectum

Question 33

where are goblet cells most abundant - small or large bowel

Answer 33

large bowel

Question 34

what are apical ends packed with

Answer 34

with mucus-filled secretion granules awaiting release

Question 35

what does mucus do

Answer 35

facilitates the passage of the increasingly solid colonic contents and covers bacteria and particulate matter

Question 36

what stimulates goblet cell secretion

Answer 36

acetylcholine (parasympathetic and enteric nervous system) stimulates goblet cell secretion

Question 37

what are the differences between the small and large bowel

Answer 37

no paneth cells and enteroendocrine cells are rarer than in the small bowel glycocalyx does not contain digestive enzymes crypts are dominated by goblet and stem cells > enterocytes - movement of large bowel is more complicated than the small intestine

Question 38

describe the muscle layers in the large bowel

Answer 38

- muscularis externa consists of an inner circular and outer longitudinal layer - circular muscles segmentally thickened - longitudinal layer concentrated in 3 bands - taenia coli - between the taenia, longitudinal layer is thin - bundles of muscle from the taenia coli penetrate the circular layer at irregular intervals + contribute to haustra formation - shorter than circular muscle layers are ovoid segments called haustracan contract individually - apart from rectum and anal canal - substantial and continuous

Question 39

what are colonic contractions

Answer 39

kneading process - minimally propulsive ~5-10 cm/hr at most | promotes absorption of electrolytes and water

Question 40

what patterns dominate in the proximal colon to retain chyme

Answer 40

anti propulsive patterns dominate to retain chyme

Question 41

what contractions occur in the transverse and descending colon

Answer 41

in transverse and descending colon, localised segmental contractions of circular muscle called haustral contractions cause back and forth mixing

Question 42

how often are short propulsive movements

Answer 42

short propulsive movements every 30 mins | increase in freq in response to a meal