The Large Intestine

Question 1

Answer 1

Question 2

What does the large intestine consist of?

• The cecum is a blind pouch just distal to the ……………… valve- larger in herbivores.
• The appendix is a thin, finger-like extension of the ………….- not physiologically relevant in humans.

Answer 2

• Consists of the colon, cecum, appendix, rectum and anal canal.
• The cecum is a blind pouch just distal to the ileocecal valve- larger in herbivores.
• The appendix is a thin, finger-like extension of the cecum- not physiologically relevant in humans.

Question 3

What is the function of the large intestine?

Answer 3

• 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 4

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

The innervation is similarly distributed, which the parasympathetic nervous system (via the vagus nerve) innervates …………….. and most of ……………. colon. The distal colon is innervated by …………….. nerves. Sympathetic innervation arises from the lower thoracic and upper lumbar spinal cord.

Answer 4

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).

The innervation is similarly distributed, which the parasympathetic nervous system (via the vagus nerve) innervates ascending and most of transverse colon. The distal colon is innervated by pelvic nerves. Sympathetic innervation arises from the lower thoracic and upper lumbar spinal cord.

Question 5

Colon

• The ascending colon is on the …………. side of the abdomen, runs from the …………. to the ………….…………. (the turn of the colon by the liver).
• The transverse colon runs from the ………….………….to the………….…………. (the turn of the colon by the spleen). Hangs off the stomach, attached by a wide band of tissue called the ………….…………. (posterior side, mesocolon).
• The descending colon runs from the ………….………….to the………….…………. .
• Sigmoid (s-shaped) colon runs from descending colon to the …………..

Answer 5

Colon

• 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).
• 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).
• The descending colon runs from the splenic flexure to the sigmoid colon.
• Sigmoid (s-shaped) colon runs from descending colon to the rectum.

Question 6

The proximal transverse colon is supplied with blood by the ………… ……….. artery (branch of the superior mesenteric artery).

Distal third of transverse colon is perfused by the …………… ……………. artery

Answer 6

Question 7

The anal canal controls the movement of things out of the GI tract, and is surrounded by …… anal sphincters.

The internal muscle is ……………. muscle and is under …………… control.

The …………… sphincter is …………… muscle and is under ……………control (which fortunately gives us control over defaecation).

External anal sphincter is controlled by …………… nerves.

Answer 7

The anal canal controls 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 8

State four unique features of the colon?

Answer 8

Appendices epiploicae

These are fatty tags that arise from the serosa, and do not seem to have a physiologically meaningful function.

Longitudinal muscle

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 hastra.

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

Circular muscle

Circular muscles are segmentally thickened and bundles of muscle from the taeniae coli penetrate the circular muscle at irregular intervals to keep them together.

Nodules of lymphoid tissue

Fairly common to have lymphoid tissue, usually occurring in solitary nodules.

Question 9

What are the appendices epiploicae?

Answer 9

Appendices epiploicae

These are fatty tags that arise from the serosa, and do not seem to have a physiologically meaningful function.

Question 10

Describe the longitudinal muscle of the colon

Answer 10

Longitudinal muscle

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 hastra.

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

Question 11

What are the nodules of lymphoid tissue in the colon called

Answer 11

Nodules of lymphoid tissue

Fairly common to have lymphoid tissue, usually occurring in solitary nodules.

Question 12

Answer 12

Reabsorption

• Colon absorbs electrolytes and water.
• More in proximal colon.
• Na+ and Cl- absorbed by exchange mechanisms and ion channels.
• Water follows by osmosis.
• K+ moves passively into lumen.
• Large intestine can reabsorb approx 4.5 litres water (usually 1.5 litres). Above this threshold diarrhoea.

Question 13

What is the difference between the colon and the rectum in terms of histology?

Answer 13

Rectum

• Dilated distal portion of the alimentary canal.
• Histology similar to the colon, but distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa.
• Terminal portion is anal canal. Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters.

•

Question 14

LIke the small intestines

Answer 14

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

Question 15

Cells of the colon 1

Abundant ……………….. and…………….. cells. Lots of invaginations called ‘…………….. ……………..’, which have …………….. 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.

Mucosa is …………….. and does not have any …………….., 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 have short irregular …………….., 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.

Answer 15

Cells of the colon 1

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.

Mucosa is smooth and does not have any 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 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.

Question 16

Cells of the colon 2

Colonic crypts are dominated by ……………… cells, which 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. The mucus also ‘covers’ bacteria and particulate matter, to protect the luminal surface from infect and/or abrasion. …. from local nerves stimulates goblet cells.

…………….. cells are absent in the large intestine, because of the commensal (commensal roughly means one organism living inside another organism) bacteria. Enteroendocrine cells are also much fewer in number. 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.)

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

Answer 16

Cells of the colon 2

Colonic crypts are dominated by goblet cells, which 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. The mucus also ‘covers’ bacteria and particulate matter, to protect the luminal surface from infect and/or abrasion. ACh from local nerves stimulates goblet cells.

Paneth cells are absent in the large intestine, because of the commensal (commensal roughly means one organism living inside another organism) bacteria. Enteroendocrine cells are also much fewer in number. 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.)

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

Question 17

Answer 17

Question 18

Motility

Basic contractions

Large intestine motility is more complicated than that of the small intestine.

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 ‘………………..’ contractions to impede propulsion and keep food in that region for longer. In the transverse and descending portions there are localised ……………… contractions of circular muscle called ………………contractions, which helps to shuffle contents forwards and backwards. Also, there are short propulsive movements every 30 mins.

Mass movement

Between 1 and 3 times a day there is a coordinated mass movement event in the colon. These contractions can propel contents up to three quarters of the length of the large intestine in a few seconds. High-fibre food (which is indigestible) promotes mass movements.

Answer 18

Motility

Basic contractions

Large intestine motility is more complicated than that of the small intestine.

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. 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.

Mass movement

Between 1 and 3 times a day there is a coordinated mass movement event in the colon. These contractions can propel contents up to three quarters of the length of the large intestine in a few seconds. High-fibre food (which is indigestible) promotes mass movements.

Question 19

Defaecataion

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.

Usually ‘suddenly’ following a mass movement, the afferent sensation of needing to defaecate is perceived, however most people are capable of storing the faeces until it is convenient to void - there is no need to go immediately. …………… ……… sphincter is relaxed in the defecation reflex, which is a spinal reflex via the ………… spinal cord. The same nerves that are involved in voluntary activity.

Distension of the rectal wall is detected by pressure receptors, which send signals via the …………………. plexus to initiate ……….. waves in descending, sigmoid colon and rectum. The internal anal sphincter relaxes, but if someone resists the urge, the sensation usually subsides until your next mass movement .

The last few centimetres of rectum 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.

Healthy adults produce 150g of faeces per day, of which two thirds of it is water. Solid components include cellulose, bacteria, cell debris and some bile pigments/salts. Bile pigments give faeces its healthy brown colour, and the (variable) smell arises from bacterial

Answer 19

Defaecataion

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.

Usually ‘suddenly’ following a mass movement, the afferent sensation of needing to defaecate is perceived, however most people are capable of storing the faeces until it is convenient to void - there is no need to go immediately. Internal anal sphincter is relaxed in the defecation reflex, which is a spinal reflex via the sacral spinal cord. The same nerves that are involved in voluntary activity.

Distension of the rectal wall is detected by pressure receptors, which send signals via the myenteric plexus to initiate peristaltic waves in descending, sigmoid colon and rectum. The internal anal sphincter relaxes, but if someone resists the urge, the sensation usually subsides until your next mass movement .

The last few centimetres of rectum 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.

Healthy adults produce 150g of faeces per day, of which two thirds of it is water. Solid components include cellulose, bacteria, cell debris and some bile pigments/salts. Bile pigments give faeces its healthy brown colour, and the (variable) smell arises from bacterial

Question 20

Answer 20

Question 21

State some physiological and pathophysiologic roles of gut flora

Answer 21

Physiological roles

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.

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

Pathophysiology

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!)