8 Large Intestine

Question 1

Q: What does the large intestine consist of? (6) What’s the main part? How long is it? wide? Compared to small intestine?

Answer 1

A: (end of ileum)

• caecum
• ileocecal valve/ junction/ sphincter
• colon *** (a, d, t, s)
• appendix
• rectum
• anal canal

150 cm long and 6 cm wide
(shorter and wider)

Question 2

Q: What is the caecum? size? where in conjunction to other GI tract parts?

Answer 2

A: blind pouch (larger in herbivores as more important for them- grass heavy diet)

where the small intestine meets the large

immediately distal to the ileocaecal valve

Question 3

Q: What is the appendix? role? Current opinion? Problems with it result in?

Answer 3

A: thin, finger like extension of the caecum

limited physiological role in humans

Current opinion is that it provides a safe refuge for gut bacteria

Appendicitis is a common problem treated with surgical removal of the appendix; patients undergoing this procedure go on to live perfectly normal lives.

Question 4

Q: What is the ileocecal valve? role? Normal state? (2) positive?

When does its state change?

Answer 4

A: muscular sphincter

separates the distal ileum (end of SI) from the caecum (the first part of the large intestine)

It is tonically active and constricted
-prevents the microbiota (gut bacteria) from migrating into the ileum

Only relaxes to allow passage of the fluid chyme into the large intestine

Question 5

Q: What are the main functions of the colon? (4)

Answer 5

A: -re/absorption of water

• re/absorption of electrolytes/ions
• elimination of waste
• microenvironment for gut bacteria

Question 6

Q: Describe the blood flow to the large intestine. (2) What does this reflect? Problem?

Answer 6

A: cecum, a colon and first 2/3 of t colon
=> receives blood from middle colic artery

final third of the transverse colon, descending colon, sigmoid colon and rectum
=> receives blood from inferior mesenteric artery

embryological division between the midgut and hindgut

region between the 2 is sensitive to ischaemia

Question 7

Q: How in the innervation of the large intestine distributed? (4) Reflects?

Answer 7

A: superior mesenteric plexus provides sympathetic innervation to the cecum, appendix, ascending and transverse colon (near to the left colic flexure)

inferior mesenteric plexus innervates the colon from the left colic flexure to the rectum

inferior hypogastric plexus also innervates the rectum

possibly- embryological division between the midgut and hindgut

Question 8

Q: Describe the different parts of the colon in terms of location. (4)

Answer 8

A: ascending colon starts at the ileocaecal valve -> runs up the right-hand side of the body to the hepatic flexure (a flexure is a ‘bend’, and this one is near the liver)

transverse colon starts at the hepatic flexure and runs across the abdomen to the splenic flexure (‘bend’ near the spleen)

descending colon starts at the splenic flexure and runs inferiorly to the first bend of the sigmoid colon (this is a less defined junction than the aforementioned flexures)

sigmoid colon is an S-shaped part of bowel that starts at the descending colon and runs until the rectum

Question 9

Q: What is the rectum? role? Histology compared to colon? (2)

Answer 9

A: dilated portion of the colon that can act as a storage site for faeces

It has a similar histological structure to the colon, however it has

• transverse rectal folds in the submucosa
• and no taeniae coli in the mucularis externa (muscle layer of gut wall)

Question 10

Q: What do transverse rectal folds form? function?

Answer 10

A: transverse rectal folds form convenient ‘shelves’ for faeces to occupy until a convenient time to defaecate (so that faeces = not constantly pushing against anal sphincter)

Question 11

Q: What makes up the anal canal? (2) Control? Function? Nerves?

Answer 11

A: surrounded by two anal sphincters

• internal (circular smooth muscle NOT under conscious control// under central control)
• external (circular muscle under conscious control- gives us control over defaecation)

controls the movement of things out of the GI tract

External anal sphincter is controlled by pudendal nerves

Question 12

Q: What are 4 unique features of the large intestines?

Answer 12

A: Appendices epiploicae

Longitudinal muscle

Circular muscle

Nodules of lymphoid tissue

Question 13

Q: What are appendices epiploicae? function?

Answer 13

A: fatty tags that arise from the serosa, and do not seem to have a physiologically meaningful function

-suggested to have protective function against intra-abdominal infections

Question 14

Q: Describe the longitudinal muscle of the large intestine. (4) Why is it structured this way? What structure do they create?

Answer 14

A: colon has three bands of longitudinal muscle = taeniae coli (doesn’t have continuous layer like rest of GI tract)

roughly equally spaced around the circumference

bands are relatively thicker than typical longitudinal muscle layers

are actually shorter than the length of the colon, which causes the colon to form regular ‘pouches’ called hastra

Large intestine motility is different from small intestine, so need the taeniae coli

Question 15

Q: Describe the circular muscle of the large intestine. In tandem with longitudinal?

Answer 15

A: segmentally thickened

bundles of muscle from the taeniae coli penetrate the circular muscle at irregular intervals to keep them together

Question 16

Q: What are haustra? What causes them?

Answer 16

A: regularly occurring ‘pouched’ segments

taenia coli (longitudinal muscle) = shorter than the length of the colon, which causes the colon to gather together and form haustra

Question 17

Q: Compare lymphoid tissue found in the small intestine and large.

Answer 17

A: nodules of lymphoid tissue are common in the walls of the distal small intestine (peyers patches)

large intestine as solitary nodules

Question 18

Q: What is reabsorbed in the large intestine? Which part specifically does this? why?

How is the resorption function of the large intestine achieved? (4)

Answer 18

A: electrolytes/ions and water- more in proximal colon where the chyme is more fluid-like (As the contents move along and have water reabsorbed, the contents become dehydrated)

1. Sodium and chloride are absorbed by exchange mechanisms and ion channels
2. -> drive osmotic gradient
3. Water follows by osmosis
4. Potassium moves passively into the lumen via gap junctions -> lost in faeces

Question 19

Q: How much can the large intestine absorb a day? usually? why? When do you get diarrhoea?

Answer 19

A: has the capacity to absorb 4500 mL per day, but usually only reabsorbs 1500 mL

vast majority is absorbed by the small intestine

If the water volume entering the colon exceeds 4500 mL, then diarrhoea results (too much to reabsorb)

Question 20

Q: Describe the general gut tube plan.

Answer 20

A: middle->

epithelium= 1 cell thick and contains diff specialised cells ->

lamina propia (with epithelium and basement membrane makes mucosa) ->

2 thin layers of muscle= muscularis mucosa= can throw internal gut surface into folds ->

submucosa= has nerve plexus and blood vessels travelling through ->

inner circular muscle layer ->

outer longitudinal muscle layer= gathered into 3 bands that run along length of gut ->

serosa= allows gut to move over itself

Question 21

Q: Describe the mucosal of the large intestine.

Answer 21

A: (abundant crypts) smooth and does not have many villi, which means it has a considerably small surface area than the small intestine (because the small intestine is where nutrient absorption really takes place) as no glucose or amino acid absorption in the colon

Enterocytes (most abundant cell)

• short irregular microvilli
• reabsorption of salt (i.e. electrolytes instead of nutrients)
• movement of ions creates osmotic gradient to absorb water passively

abundant goblet cells:
-dominate crypts

stem cells:

• found at bottom of invaginations called ‘colonic crypts’
• similar to the small intestine. Cells migrate up the crypts and into the lumen, and are sloughed off after a few days
• no paneth cells: SI needs them to prevent pathogens that have been ingested causing problems-> LI have a bacterial population (commensal) you need for normal gut function
• enteroendocrine cells are rarer than in SI (LI has simpler role)
• glycocalyx does not contain digestive brush border enzymes that the small intestinal glycocalyx does as digestion has been completed by this point

Question 22

Q: Describe the goblet cells of the large intestine. Where are they? Role? (2) abundance? Stimulation?

Answer 22

A: Colonic crypts are dominated by goblet cells (less along the surface)

• The mucus ‘covers’ bacteria and particulate matter, to protect the luminal surface from infect and/or abrasion -> less likely to escape and cause infections
• secrete mucous to facilitate the passage of increasingly dry luminal contents (as more and more water is reabsorbed)

Because of this, their abundance increases markedly along the length // more in LI than SI per sq cm as food there is liquid and easier to transport

ACh from local nerves (enteric NS and PNS) stimulates goblet cells to secrete // sympathetic activity tends to inhibit function and it causes the f/l/ight response

Question 23

Q: What is glycocalyx? role? how? name?

Answer 23

A: rich carbohydrate layer on apical membrane

protects digestive lumen but allows absorption

traps layer of water and mucous known as unstirred layer = regulates rate of absorption from intestinal lumen

Question 24

Q: Describe the muscle layers of the large intestine. (4) How do movements compare to the SI?

Answer 24

A: (Like the small intestine, muscularis externa consists of an inner circular and outer longitudinal layer)

Circular muscles segmentally thickened= typically found between haustra

Longitudinal layer concentrated in three bands- taenia coli

Between the taenia, longinitudinal layer is thin

Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals and because it’s shorter than circular muscle layer, ovoid segments called haustra form which can contract individually

Movements of large intestine more complicated than small intestine

Question 25

Q: What are the basic contractions of the large intestine? What do they provide? allows? Describe 3 other contractions/ movements. Following a meal?

Answer 25

A: Basic colonic contractions= kneading process that provide minimal propulsion (about 5-10 cm/h), which allow chyme to stay in colon for long time to promote absorption of electrolytes and water

Proximal colon also elicits ‘antipropulsive’ contractions to impede propulsion and keep food in that region for longer.

In the transverse and descending portions there are localised segmental contractions of circular muscle called haustral contractions, which helps to shuffle contents forwards and backwards between ovoid segments

Also, there are short propulsive movements every 30 mins= push food down

increase in frequency

Question 26

Q: Describe the mass movement of the large intestine. What does it result it? How can diet effect? Compared to SI?

Answer 26

A: Between 1 and 3 times a day there is a coordinated mass movement event in the colon

These contractions can propel contents a third to up to three quarters of the length of the large intestine in a few seconds-> sweeps colon clean

High-fibre food (which is indigestible) promotes mass movements-> promotes rapid transport through colon

resembles peristaltic waves that occur in SI but in SI they don’t cause much movement

Question 27

Q: Parasympathetic influence on the large intestine causes? Different nerves? (2)

Answer 27

A: Parasympathetic:

• mainly stimulates gut activity
• ascending colon and most of transverse colon innervated by vagus nerve (cranial nerve 10)
• More distal innervated by pelvic nerves.

Question 28

Q: Sympathetic influence on the large intestine causes? Originate?

Somatic nervous system?

Answer 28

A: Sympathetic:

• tends to reduce blood supply to it and reduce motility
• lower thoracic and upper lumbar spinal cord.

-External anal sphincter (striated muscle) controlled by somatic motor fibres in the pudendal nerves = conscious control

Question 29

Q: What does the enteric nervous system refer to? in large intestine? (2) (compared to SI) Importance of enteric NS?

Answer 29

A: nerve plexuses in gut wall that communicate with eachother

• submucosa has network of neurons
• myenteric plexus= sandwiched between muscle layers (concentrated below taenia coli whereas in SI = evenly spread)

for colonic function

Question 30

Q: Apart from nerves, what can affect the large intestine? (2)

Answer 30

A: Presence of food in stomach can stimulate mass movement- hormonal? Neural?

Hormonal/paracrine control= shorter time scale
-e.g. aldosterone promotes sodium and water absorption (synthesis of Na+ ion channel, Na+/K+ pump)

Question 31

Q: What is faeces? What does it contain? (4) Colour? Smell?

Where does it go before exiting our bodies? until? How much do healthy adults make?

Answer 31

A: indigestible waste of the chyme that entered the large intestine

Solid components include cellulose (can’t digest, bacteria, cell debris and some bile pigments, salts (K+)

Bile pigments give faeces its healthy brown colour, and the (variable) smell arises from bacterial

rectum steadily fills with faeces by mass movement in the sigmoid colon which sit on the shelves creted by membranous folds
-convenient to void

150g of faeces per day, of which two thirds of it is water

Question 32

Q: What is defecation? control? (2)

Answer 32

A: reflex to sudden distension of rectum walls by spinal reflex via the sacral spinal cord

Defecation reflex controlled primarily by the sacral spinal cord- both reflex and voluntary actions

Question 33

Q: How is defaecation initiated? What follows this? (2) Control? End result usually?

Answer 33

A: Pressure receptors/ mechanoreceptors send signals via myenteric plexus to initiate peristaltic waves in descending, sigmoid colon and rectum

• Internal anal sphincter inhibited mainly by enteric NS
• Weak intrinsic signal augmented by autonomic reflex

External anal sphincter under voluntary control= allows people to be capable of storing the faeces until it is convenient to void - there is no need to go immediately

Urge resisted, sensation subsides

Question 34

Q: Which part of the rectum is relevant to defecation? What can it do? Flaw?

Answer 34

A: last few centimetres of rectum = known as ‘social part’

can distinguish between solids, liquids and gases, which can help an individual perceive what and when it is appropriate to let things pass

That said, it can struggle to tell the difference between gas and oil, which can lead to ‘spotting’ underwear with small amounts of faecal waste

Question 35

Q: How does cell loss differ in the SI and LI?

Answer 35

A: cells lost in SI tend to be broken down and reabsorbed and replaced

in LI, tend to be lost in faeces-> doesn’t have digestive enzymes/ absorptive capacity

Question 36

Q: Describe gut flora. Large intestine? Compared to large intestine and stomach?

Answer 36

A: All mammals have symbiotic relationships with their gut microbial community (microbiome)

Large intestine contains many, essential to normal function= Diverse, highly metabolically active community

Stomach and small intestine have few bacteria as they are quite harsh environments

Question 37

Q: What’s the size of a microbiome in an average adult human? equivalent to? Development?

Most prevalent species?

Answer 37

A: comprises approximately 1.5 kg of live bacteria, with the active biomass equivalent to a major human organ

Humans are thought to be born with a bacteria-free gut, which develop during the first years of life, and eventually declines in later life

bacteriodetes (phylumm) bacteriodes (gram negative, anaerobic non-spore forming bacteria)

Question 38

Q: Physiological roles of intestinal flora. Synthesise? Protection? (2)

Answer 38

A: -Synthesise and excrete Vitamin K (important for coagulation; it is almost exclusively produced by gut bacteria)

• Prevent pathogen colonisation by competing for attachment sites and nutrition.
• Antagonise other bacteria which can inhibit or kill non-indigenous species.

Question 39

Q: What do intestinal flora stimulate in terms of development? What do they do that we can’t? result? (2)

Answer 39

A: Stimulate development of some tissues, including cecum and lymphatic tissues

Fibre (indigestible carbohydrate) can be broken down by colonic bacteria.

Produces short chain fatty acids which can regulate gut hormone release, or be absorbed to be used as an energy source or to influence functions such as food intake or insulin sensitivity directly.

Question 40

Q: What do intestinal flora stimulate the production of? describe. Use?

Answer 40

A: cross-recative antibodies =
Antibodies produced against components of the normal flora can to cross react with certain related pathogens, and thereby prevent infection or invasion

Question 41

Q: What has intestinal flora got links to according to recent research? (5)

Answer 41

A: Links with drug metabolism, insulin reistance, bile acid metabolism, lipid metabolism and obesity

Question 42

Q: What’s the pathophysiological role of gut bacteria? What can an inappropriate population lead to?

Answer 42

A: Gut bacteria populations help to maintain and prime the immunological system

Inappropriate population or loss of commensal bacteria can predispose to infection and illness throughout the body

Question 43

Q: What’s the potential pathophysiological of gut bacteria?

Answer 43

A: Potential value in ‘faecal transplant’ to re-instate bacteria (remember the potential role of the appendix!)

Question 44

Q: What’s the most prevalent bacteria in the gut? describe (3). Implicated in? (2)

Answer 44

A: Most prevalent bacteria are the Bacteroides- Gram-negative, anaerobic, non-sporeforming bacteria

Implicated in the initiation colitis and colon cancer

Question 45

Q: Apart from the most prevalent kind, give an example of a bacteria from normal gut flora? Describe (3). Known as? why?

Answer 45

A: Bifidobacteria

Gram-positive, non-sporeforming, lactic acid bacteria

Have been described as “friendly” bacteria. Thought to prevent colonisation by potential pathogens.