Large intestine

Question 1

What does the large intestine consist of

Answer 1

§ Consists of the colon (ascending, transverse, descending, sigmoid), caecum, appendix, rectum and anal canal.
§ The caecum is a blind pouch (distal to the ileocecal valve – not used in humans much – usually for fermenting and is larger in herbivores
§ The appendix is a thin, finger-like extension of the caecum – again not physiologically useful in humans, possibly a lifeboat for microbiome.

Question 2

What are the principal functions of the colon

Answer 2

The principal functions of the colon are the reabsorption of electrolytes and water and the elimination of undigested food and waste.
1.5m long, 6cm diameter.

Question 3

Describe the ascending colon

Answer 3

The ascending colon is on the right side of the abdomen, runs from the cecum to the hepatic flexure (the turn of the colon by the liver).

Question 4

Describe the transverse colon

Answer 4

The 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)- to the posterior abdominal wall

Question 5

Describe the descending colon

Answer 5

The descending colon runs from the splenic flexure to the sigmoid colon.

Question 6

Describe the sigmoid colon

Answer 6

Sigmoid (s-shaped) colon runs from descending colon to the rectum.

Question 7

Describe the ileocecal valve

Answer 7

The ileocaecal valve is a muscular sphincter that separates the distal ileum from the caecum, the first part of the large intestine. It is tonically active and constricted, and only relaxes to allow passage of the fluid chyme into the large intestine. Being tonically active, it also prevents the microbiota (gut bacteria) from migrating into the ileum.

Question 8

Describe appendicitis

Answer 8

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

Question 9

Describe the blood supply to the large intestine

Answer 9

It receives blood from both the middle colic artery (which perfuses the ascending and first two-thirds of the transverse portions) and the inferior mesenteric artery (which perfuses the final third of the transverse colon, descending colon, sigmoid colon and rectum).

Question 10

Why do different regions of the large intestine have different blood supplies

Answer 10

Reflects embryological division between the midgut and hindgut.
Region between the two is sensitive to ischemia- capillaries come from different sources- haemorrhage can effect this part of the gut.

Question 11

Describe the Appendices epiploicae

Answer 11

The peritoneum carries fatty tags (appendices epiploicae)

Can protect against intra-abdominal infections

Question 12

Describe the taeniae coli

Answer 12

muscle coat has 3 thick longitudinal bands (taeniae coli);
These bands are shorter than the length of the colon
which causes the colon to form regular ‘pouches’ caulled hastra.
Large intestine motility is different from small intestine, so need the taeniae coli

Question 13

Are the haustra always in the same place

Answer 13

No

Question 14

Describe the lymphatic nodules in the large intestine

Answer 14

Fairly common to have lymphoid tissue, usually occurring in solitary nodules.
Different to the Peyer’s patches in the small intestine
role in the large intestine is not well established

Question 15

Describe the formation of haustra

Answer 15

Instead of a continuous muscle layer like the rest of the GI tract, the colon has thee bands of longitudinal muscle around that are roughly equally spaced around the circumference. These bands are relatively thicker than typical longitudinal muscle layers. These are actually shorter than the length of the colon, which causes the colon to form regular ‘pouches’ caulled haustra.

Ovoid segments

Question 16

Summarise reabsorption in the large intestine

Answer 16

The large intestine reabsorbs ions and water. This is achieved predominantly in the proximal colon, where the chyme is more fluid-like. As the contents move along and have water reabsorbed, the contents become dehydrated.
Sodium and chloride are absorbed by exchange mechanisms. Water follows by osmosis. Potassium moves passively into the lumen via gap junctions (paracellularly)
The large intestine has the capacity to absorb 4500 mL per day, but usually only reabsorbs 1500 mL. The small intestine absorbs much more water! If the water volume entering the colon exceeds 4500 mL, then diarrhoea results.

Question 17

Describe the rectum

Answer 17

The rectum is 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 (see later) in the mucularis externa (muscle layer of gut wall). The transverse rectal folds form convenient ‘shelves’ for faeces to occupy until a convenient time to defaecate.

Question 18

Describe the anal canal

Answer 18

Terminal portion is anal canal.
ontrols the movement of things out of the GI tract, and is surrounded by two anal sphincters. The internal muscle is smooth muscle and is under central control. The external sphincter is striated muscle and is under voluntary control (which fortunately gives us control over defaecation). External anal sphincter is controlled by pudendal nerves.

Question 19

Why does more reabsorption occur at the proximal large intestine

Answer 19

This is achieved predominantly in the proximal colon, where the chyme is more fluid-like. As the contents move along and have water reabsorbed, the contents become dehydrated.

Question 20

How is the colon similar to the small intestine

Answer 20

Enterocytes and goblet cells are abundant.
Abundant crypts
Stem cells are found in the crypts.

Question 21

How can the large intestine be distinguished from the small intestine

Answer 21

Mucosa appears smooth at the gross level because it has no villi (smaller SA than small intestine).
Enterocytes have short, irregular microvilli and primarily concerned with resorption of salts.
(Water is absorbed as it passively follows the electrolytes, resulting in more solid gut contents)

Question 22

Describe the colic crypts

Answer 22

Abundant enterocytes and goblet cells. Lots of invaginations called ‘colonic crypts’, which have stem cells at the bottom, similar to the small intestine. Cells migrate up the crypts and into the lumen, and are sloughed off after a few days.
Crypts dominated by goblet cells.

Question 23

Describe the goblet cells

Answer 23

Higher no. of goblet cells than small intestine.
More prevalent in the crypts than along the surface, no. increases distally towards rectum.
The mucus facilitates the passage of the increasingly solid colonic contents, and covers bacteria and particulate matter.
Acetylcholine (parasympathetic and enteric nervous system) stimulates Goblet Cell secretion.

Question 24

Why is it important that the number of goblet cells increases

Answer 24

As more water is reabsorbed- the contents of the colon becomes more solid- needs more lubricant to keep it moving
Mucus also covers faces with particulate matter to prevent infection and abrasion

Question 25

What is found at the apical ends of the goblet cells

Answer 25

apical ends are packed with mucus-filled secretion granules awaiting release.

Question 26

What are the colic crypts dominated by

Answer 26

Enterocytes are still the dominant cells facing the gut lumen, but the mucus-secreting goblet cells (stained red) dominate the crypts.

Question 27

Why are the enterocytes of the colon different to that of the small intestine

Answer 27

Enterocytes have short irregular microvilli, and the intracellular machinery reflects their role in reabsorption of salt (i.e. electrolytes instead of nutrients). This movement of ions creates an osmotic gradient to absorb water passively.
There should be no reabsorption of macronutrients in the colon

Question 28

Why may Paneth cells be absent from the colic crypts

Answer 28

Need to maintain commensal gut flora in the colon
Food will also contain more bacteria in the small intestine - but has since been covered in acid and digested and so the colon does not require the same degree of protection.

Question 29

Why may there be fewer enteroendocrine cells in the colon than in the small intestine

Answer 29

This is probably because the regulation of digestion and absorption in small intestine is very complicated (gut hormones need to regulate secretions and appetite) requiring lots of enteroendocrine the large bowel is simpler.)

Question 30

Describe the glycocalyx in the colon

Answer 30

The glycocalyx is still present, but it does not contain the digestive brush border enzymes that the small intestinal glycocalyx does.

Question 31

Summarise the key differences between the mucosal organisation of the colon and the small intestine

Answer 31

Crypts dominated by goblet cells.
No Paneth cells and enteroendocrine cells are rarer than in small intestine.
Glycocalyx does not contain digestive enzymes.

Question 32

Summarise the muscular layer of the colon

Answer 32

Like the small intestine, muscularis externa consists of an inner circular and outer longitudinal layer.
Circular muscles segmentally thickened.
Longitudinal layer concentrated in three bands- taenia coli
Between the taenia, longitudinal layer is thin
Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals.

Question 33

how does the longitudinal layer compare to the circular layer

Answer 33

Shorter than circular muscle layer, ovoid segments called haustra- can contract individually.
Apart from rectum and anal canal- substantial and continuous.
Movements of large intestine more complicated than small intestine- contents are more solid and therefore harder to move

Question 34

Summarise large intestine motility

Answer 34

Colonic contractions- kneading process- minimally propulsive- 5-10cm/hr at most.
Promotes absorption of electrolytes and water.
In the proximal colon, ‘antipropulsive’ patterns dominate to retain chyme.
In transverse and descending colon, localised segmental contractions of circular muscle called Haustral contractions cause back and forth mixing.
Short propulsive movements every 30 mins.
Increase in frequency following a meal

Question 35

Describe mass movement in the large intestine

Answer 35

1-3 times daily- mass movement- resembles peristaltic wave.
Can propel contents 1/3-3/4 of length of large intestine in few seconds.
Food that contains fibre (indigestible material) promotes rapid transport through colon)- by promoting mass movements

Question 36

What is the importance of the anti-propulsive contractions in the proximal colon

Answer 36

Basic colonic contractions are a kneading process that provide minimal propulsion (about 5-10 cm/h), which allow chyme to stay in colon for long time to promote absorption.
Proximal colon also elicits ‘antipropulsive’ contractions to impede propulsion and keep food in that region for longer.

Question 37

Describe the contractions in the transverse and descending colon

Answer 37

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. Also, there are short propulsive movements every 30 mins.

Question 38

Summarise the innervation of the colon

Answer 38

Parasympathetic: ascending colon and most of transverse colon innervated by vagus nerve. More distal innervated by pelvic nerves.
Sympathetic- lower thoracic and upper lumbar spinal cord.
External anal sphincter controlled by somatic motor fibres in the pudendal nerves.

Question 39

What is the role of the SNS in the colon

Answer 39

Inhibits movements and secretions

Question 40

Describe the enteric nervous system

Answer 40

Enteric nervous system also important- Hirschsprung’s disease (no enteric intramural ganglia).
Myenteric plexus ganglia concentrated below taenia coli
In coeliac disease- the plexi inflamed

Question 41

What triggers mass movements of the colon

Answer 41

Presence of food in stomach can stimulate mass movement- hormonal? Neural?
Hormonal/paracrine control. e.g. aldosterone promotes sodium and water absorption (synthesis of Na+ ion channel, Na+/K+ pump)
Even caffeine and nicotine can trigger these mass movements

Question 42

What is meant by faeces

Answer 42

Faeces is the indigestible waste of the chyme that entered the large intestine. The rectum steadily fills with faeces, which sit on the shelves creted by membranous folds.

Question 43

Summarise defecation

Answer 43

Rectum filled with faeces by mass movement in the sigmoid colon.
Stores stool until convenient to void.
Defecation reflex controlled primarily by the sacral spinal cord- both reflex and voluntary actions.

Question 44

Describe the defecation reflex

Answer 44

Reflex to sudden distension of walls of rectum.
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 45

What is the importance of the last few cm of the rectum

Answer 45

Last few centimeters of the rectum known as the “social part” of the rectum
Can distinguish between solid, liquid and gas. That perceptual ability is important in knowing what can be passed appropriately in what circumstance.
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- STEATORRHOEA

Question 46

Describe the characteristics of faeces

Answer 46

150g/day adult.
Two thirds water.
Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+).
Bile pigments give colour.
Bacterial fermentation gives odour.

Question 47

Why may cell debris be found in the faeces

Answer 47

Colon does not have capacity to digest cell components within the cells

Question 48

Summarise the large intestine flora

Answer 48

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

Stomach and small intestine have few bacteria- protected.

Large intestine contains many, essential to normal function.

Question 49

Describe the microbiome in the large intestine

Answer 49

Diverse, highly metabolically active community.
The microbiome in an average adult human comprises approximately 1.5 kg of live bacteria, with the active biomass equivalent to a major human organ.

Question 50

How does our microbiome develop

Answer 50

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.

Question 51

List the physiological roles of the colon flora

Answer 51

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.
Stimulate the production of cross-recative antibodies.
Stimulate development of some tissues.
Fibre can be broken down.

Question 52

What is the purpose of these cross-reactive antibodies

Answer 52

Antibodies produced against components of the normal flora can to cross react with certain related pathogens, and thereby prevent infection or invasion.

Question 53

What is the purpose of fermentation

Answer 53

Humans don’t have the enzymes to digest fibre (which is a carbohydrate)- but bacteria do and can ferment it- make short chain fatty acids which can bind to receptors on gut, vagus nerve, pancreas and liver- energy source and signalling molecules- can regulate homeostasis in the hypothalamus (reduce appetite)

Question 54

What are the roles of these fatty acids

Answer 54

regulate hormone release, used as energy, influence food intake or insulin sensitivity directly

Question 55

Describe the importance of the colon flora in development

Answer 55

Stimulate the development of certain tissues, including cecum and lymphatic tissues- anatomical structures may not develop normally without the flora

Question 56

What is the most prevalent type of flora found in the large intestine

Answer 56

Most prevalent bacteria are the Bacteroides- Gram-negative, anaerobic, non-spore forming bacteria. Implicated in the initiation colitis and colon cancer (proportion of Bacteroides)

Question 57

Describe the other types of flora found in the colon

Answer 57

Bifidobacteria are Gram-positive, non-sporeforming, lactic acid bacteria. Have been described as “friendly” bacteria. Thought to prevent colonization by potential pathogens.

Question 58

What has recent research shown regarding the colon flora

Answer 58

Links between gut bacteria and:

1) Drug metabolism
2) Insulin resistance
3) Bile acid metabolism
4) Lipid metabolism
5) Obesity

Question 59

Describe a key study that illustrated the importance of gut bacteria

Answer 59

One of the key studies demonstrating the importance of commensal bacteria was by Hooper et al. (2001). The authors reported that a single bacterium introduced into a germ-free mouse (a mouse that has been kept in bacteria-free sterile conditions) switched on genes affecting mucosal barrier function, nutrient absorption/dietary energy extraction, enteric nervous system, intestinal maturation and immune system development.

Question 60

Describe the roles of the gut bacteria in pathophysiology

Answer 60

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

Question 61

Can you change your microbiome

Answer 61

Yes- by changing your diet.