lare intestine

Question 1

what does the large intestine consist of?

Answer 1

colon, caecum, appendix, rectum and anal canal

Question 2

define caecum

Answer 2

is a blind pouch just distal to the ileocecal valve-larger in herbivores

Question 3

define appendix

Answer 3

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

Question 4

what is the positioning of each section of the large intestine:

• ascending colon
• transverse colon
• descending colon
• sigmoid colon

Answer 4

• ascending colon: right side of the abdomen, runs from the caecum to the hepatic flexure (the turn of the colon by the liver)
• transverse colon: 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)
• descending colon: runs from the splenic flexure to the sigmoid colon
• sigmoid colon: (S shaped) colon runs from descending colon to the rectum

Question 5

the proximal transverse colon is supplied with blood from

Answer 5

blood by the middle colic artery (branch of the superior mesenteric artery)

Question 6

distal third of transverse colon is perfused by

Answer 6

the inferior mesenteric artery

(Difference is due to embryological division between midgut and hindgut. Region between the two is sensitive to ischaemia.)

Question 7

the peritoneum carries
and the muscle coat has
the gut wall is ‘ ‘ in appearance

Answer 7

the peritoneum carries fatty tags (appendices epiploicae) and the muscle coat has 3 thick longitudinal bands (taeniae coli), the gut wall is pouched in appearance (haustra).

Question 8

nodules of lymphoid tissue are common in the walls of the distal small intestine called peyer’s patch- what are they called in the large intestine

Answer 8

solitary nodules

Question 9

structural or functional purpose of appendices epiplocia

Answer 9

protective function against intra-abdominal infections

Question 10

taenia coli necessary for

Answer 10

large intestine motility

Question 11

Taenia coli cause the formation of

Answer 11

pouched ovoid segments called haustra (singlular haustrum).

Muscular tone element

Question 12

Functions of the large intestine

Answer 12

Functions
§ Absorption of electrolytes and water and to eliminate unwanted waste. Na+ and CL- absorbed by exchange mechanisms and ion channels. Water follows by osmosis. K+ moves passively into lumen.
o More absorption in proximal colon.
§ Small intestine absorbs the most water, large just absorbs some more [4.5L].

Question 13

Rectum
How is it different to the colon?

Transverse rectal-folds act as?

Answer 13

Rectum

• absent taenia coli in muscularis externa
• has transverse rectal folds in submucosa.

Transverse rectal-folds act as shelves for stool.

Question 14

Which sphincter is under unconscious/reflex control?

Answer 14

The internal anal sphincter is under unconscious/reflex control while the external sphincter is under voluntary control.

Question 15

Similarities between small and large intestine

Answer 15

The large intestine features similarities with the small intestine:
o Enterocytes and goblet cells abundant.
o Abundant crypts with stem cells found in crypts.

Question 16

Difference between small and large intestine

Answer 16

However, the large intestine is distinguishable from the small intestine by:
o No villi (just an enterocyte surface) so smooth surface.
o Enterocytes have short and irregular microvilli – for resorption of salts (and thus water by co-transport).

Question 17

Mucosal Structure
Goblet cells:
More goblet cells in the small or large intestine?
What nerves stimulate goblet cell secretion?
Which cells are absent?
Enteroendocrine cells are rarer than in
Glycocalyx present on microvilli BUT does not ?

Answer 17

Mucosal Structure
Goblet cells:
More goblet cells in large intestine with number higher in the crypts and more distally in the colon. No. increases distally towards rectum.
Mucus facilitates the passage of the increasingly solid colonic contents, and covers bacteria and particulate matter.
Acetylcholine (PNS and enteric NS) stimulate goblet cell secretion.
NO Paneth cells at all.
Enteroendocrine cells are rarer than small intestine.
Glycocalyx present on microvilli BUT does not hold digestive enzyme in ‘unstirred layer’.

Question 18

• Paneth cells found in
• they contain
• also engulf
• may have a role

Answer 18

• the bases of the crypts
• contain large, acidophilic granules containing
  - antibacterial enzyme lysozyme (protects stem cells)
  - glycoproteins and zinc (essential trace metal for a no of enzymes)
• also engulf some bacteria and protozoa
• may have a role in regulating intestinal flora

Question 19

large intestine mucosal organisation

• crypts dominated by
• what cells are absent?
• what cells are rarer?
• what does glycocalyx not contain?

Answer 19

• crypts dominated by goblet cells
• No Paneth cells
• enteroendocrine cells are rarer in large intestine
• glycocalyx does not contain digestive enzymes

Question 20

Microvilli

• size
• how many per cell?
• surface of the microvilli are covered with?

Answer 20

-0.5-1.5 micrometers high and make up the brush border
several thousand microvilli per cell
surface of the microvilli is covered with glycocalyx

Question 21

Glycocalyx

• rich in?
• provides protection from?
• traps a layer of what and what is this called?

Answer 21

rich in carbohydrate layer on apical membrane
protection from the digestional lumen, yet allows for absorption
traps a layer of water and mucous known as the unstirred layer which regulates rate of absorption from intestinal lumen

Question 22

Muscles Layers of the Large Intestine

Muscularis externa consists of

Answer 22

Muscles Layers of the Large Intestine
§ Like small intestine, Muscularis externa consists of an inner circular and outer longitudinal layer (which are concentrated in 3 clear bands – taeniae coli). Between the taenia, longitudinal layer is thin.

Question 23

what do the haustra do?

Answer 23

Oval segments called haustra can contract individually – apart from the rectum and anal canal, these movements are substantial and continuous – used for kneading contents.

Question 24

Colonic contractions how regularly?
What causes back and forth mixing?
Short propulsions occur when?
MASS MOVEMENTS occur when?

Answer 24

Colonic contractions lead to a kneading process – 5-10cm/hour at most.
o Slow propulsion promotes absorption of electrolytes and water.
§ In the proximal colon, ‘anti-propulsive’ patterns dominate to retain the chyme.
§ In transverse and descending colon, haustral contractions cause back and fourth mixing.
§ There are then short propulsions ever 30 minutes that increase in frequency following a meal.
§ MASS MOVEMENTS occur 1-3 times daily and can propel contents 1/3-3/4 length of large intestine in a few seconds – these movements are promoted by presence of indigestible fibre or food in the stomach.

Question 25

Nervous Control of the Large Intestine 
§ PARASYMPATHETIC –
§ SYMPATHETIC – 
§ External anal sphincter controlled by 
§ Afferent sensory pressure neurons detect

Answer 25

Nervous Control of the Large Intestine
§ PARASYMPATHETIC – ascending colon, most of transverse colon by VAGUS nerve. More distal by PELVIC nerves.
§ SYMPATHETIC – lower thoracic and upper lumbar spinal cord.
§ External anal sphincter controlled by somatic motor fibres in the PUDENDAL nerves.
pressure

Question 26

ENTERIC – complicated but important- no enteric ganglia.

Myenteric plexus ganglia concentrate

Answer 26

ENTERIC – complicated but important – Hirschsprung’s disease = no enteric ganglia.
§ Myenteric plexus ganglia concentrate below the taeniae coli.

Question 27

Defecation 
Rectum becomes filled with faeces by 
The defecation reflex is then controlled primarily by 
Last few centimetres of rectum = 
How much faeces made? 
What is it made of?

Answer 27

Defecation
§ Rectum becomes filled with faeces by mass movements in the sigmoid colon which stores the stool.
§ The defecation reflex is then controlled primarily by the sacral spinal-cord (reflex and voluntary).
§ Defecation urges come in waves:
o § Last few centimetres of rectum = “social part” of rectum and can differentiate between solid, liquid and gas.
§ 150g/day of faeces made = 2/3rds water, cellulose, bacteria (smell), cell debris, bile pigments (colour), salts.

Question 28

Describe the reflex that takes place during defecation

Answer 28

pressure receptors send signals via Myenteric plexus to initiate peristaltic waves in descending sigmoid colon and rectum
Internal anal sphincter inhibited
weak intrinsic signal augmented by autonomic reflex
external anal sphincter under voluntary control
Urge resisted, sensation subsides

Question 29

Flora -

- role of intestinal flora

Answer 29

Flora -
§ Roles of the intestinal flora include:
o VITAMINS - Synthesise and excrete vitamins such as Vitamin K.
o COMPETITION – Prevent colonisation by pathogens by outcompeting them.
o KILL PATHOGENS – Antagonize other bacteria by producing anti-bacterials etc.
§ Beneficial roles of the intestinal flora to humans:
o IMMUNITY – Stimulate production of cross-reactive antibodies that can protect against some pathogens.
o GROWTH – Stimulate development of certain tissues such as caecum and lymphatic tissues.
o FIBRE BREAKDOWN – Flora can breakdown fibre.
o FATTY ACIDS – Produce short chain fatty acids = regulate hormone release, used as energy, influence food intake or insulin sensitivity directly.

Question 30

• Most prevalent bacteria are the?

- Another example?

Answer 30

§ Most prevalent bacteria = bacteroides = gram –ve bacteria.
o Another bacterium is gram +ve bifidobacteria = prevent colonisation by potential pathogens.
o Thought to play a role in: drug metabolism, insulin resistance, bile acid metabolism, lipid metabolism and obesity.