Gastro - Large bowel

Question 1

What is the Caecum? (Location)

Answer 1

The caecum is a blind pouch, just distal to the ileocaecal valve which communicates the terminal ileum to the colon, and it tends to be larger in herbivores.

Question 2

What is the appendix?

Location, physiological relevance

Answer 2

The appendix is a thin, finger-like extension of the caecum. It’s generally thought not to be physiologically relevant in humans, although in the last few years there’s been more data potentially linking it with the microbiome of the colon, and that potential link with certain diseases.

Question 3

What is the function of the large bowel?

Answer 3

The principal functions of the large bowel are the reabsorption of electrolytes and water, and the elimination of undigested food and waste. Overall the colon is about 1.5 metres long and it can extend to a maximum of six centimetre diameter.

Question 4

Provide an anatomical course in the context of the colon

Answer 4

the ascending colon is on the right side of the abdomen. And it runs from the caecum to the hepatic flexure, which is the turn around the liver. It’s then linked to the transverse colon, which runs from the hepatic flexure to the splenic flexure, which is the turn around the spleen. The transverse colon hangs on the stomach attached by a wide band of tissue called the greater omentum. It then goes into the descending colon which runs from the splenic flexure into the sigmoid colon. The sigmoid colon is an S shaped part of the colon, and it runs from the descending colon to the rectum

Question 5

Blood supply of the PTC,DTC notes for ischemia

Answer 5

The proximal transverse colon is supplied with blood by the middle colic artery, which is a branch of the superior mesenteric artery. The distal third of the transverse colon is perfused by the inferior mesenteric artery. This reflects the embryological division between the midgut and the hind gut. The region between these two areas can be sensitive to ischaemia, because the blood supply is not as extensive as to the neighbouring regions.

Question 6

Provide an overview of the peritoneum (Fatty tags, Tenaie, hustra, lymph)

Answer 6

To look into further anatomy, the peritoneum carries fatty tags called appendices epiploicae, and the muscle coat has three thick longitudinal bands called the taeniae coli. The gut wall is pouched in appearance and is named haustra. The structural or functional purpose of the appendices epiploicae is unknown. It is suggested to have a protective function against intraabdominal infections. The taeniae coli, however, are necessary for large intestine motility. There are also nodules of lymphoid tissue which are common in the walls of the distal small intestines, Peyer’s patches, and the large intestine, which can be seen as solitary nodules

Question 7

How are hustra formed? What shape is produced

Answer 7

The haustra are made as the taeniae coli are shorter in the large bowl compared to the small intestine. This causes the formation of pouched ovoid segments.

Question 8

Main function of the LB, solute flow, threshold for diarrhoea

Answer 8

With regards to function, the main function of the large bowel is reabsorption. It absorbs electrolytes and water, which happens more in the proximal colon. Sodium and chloride are absorbed by exchange mechanisms and ion channels. Water follows by osmosis and potassium moves passively into the lumen. The large intestine can reabsorb approximately 4.5 litres of water. Anything above this threshold results in diarrhoea.

Question 9

Rectal histology compared to colon

Answer 9

From the sigmoid colon, we get into the rectum, and this is the dilated distal portion of the alimentary canal of the large bowel. The histology is similar to the colon, but it is distinguished by the transverse rectal folds in its submucosa and the absence of taenia coli and its muscularis externa.

Question 10

What is the terminal region and what surrounds the terminal region?

Answer 10

the terminal portion, and the anal canal is surrounded by the internal and external anal sphincters.

Question 11

Provide a list of the 4 histological layers of the large bowel as well as some key structures running within them

Answer 11

The mucosa, the submucosa, the muscularis and the serosa. The mucosa is the most inner layer, and it usually contains multiple glands. It’s then covered by the submucosa, which also contains many glands, followed by the most muscularis, which can harbour the myenteric plexuses, and then the serosa which can also harbour the vessels and the nerve endings to the large bowel.

Question 12

Give a similarity and difference between the small intestines and large bowel

Answer 12

Like the small intestines, the large bowel contains enterocytes and goblet cells which are seen in abundance. There are multiple crypts which usually harbours the stem cells in them. The mucosa appears smooth in the large bowel, as it has no villi. Compare that to the small bowel, for example. The enterocytes have short, irregular microvilli, however, but they are primarily concerned with the resorption of salts.

Question 13

Describe the path of water and an overview of crypts

Answer 13

Water is absorbed as it passively follows electrolytes, and it results in more solid gut contents. The crypts are dominated by goblet cells. Goblet cells are seen in a higher number compared to the small bowel. They are more prevalent in the crypts in the large bowel than along the surface. The number of these goblet cells increase distally towards the rectum. These characteristically contain apical ends which are packed with mucus filled secretion granules awaiting release. The mucus is important because it facilitates the passage of increasingly solid colonic contents, and it covers bacteria and particulate matter. And it’s usually controlled by acetylcholine, which stimulates goblet cell secretion, usually under the parasympathetic and enteric nervous system control.

Question 14

what are absent in the large bowel? what dominates in crypts

Answer 14

the villi are absent from large bowel, enterocytes are still the dominant cell facing the gut lumen. But the mucus secreting goblet cells dominate the crypts. As in the small bowel, new cells arise from crypts stem cells, these crypts are dominated by goblet cells. No Paneth cells are seen and enteroendocrine cells are rarer than in small bowel.

Question 15

describe microvilli, its contents and function

Answer 15

Microvilli make up the brush border, they’re made of several thousand microvilli per cell and the surface of microvilli are covered by glycocalyx, which is a rich carbohydrate layer on the apical membrane. It serves as a protection from digestion of the lumen, yet allows for absorption. It traps a layer of water and mucus along it, which is usually referred to as the unstirred layer. This regulates the rate of absorption from the intestinal lumen.

Question 16

Describe the muscular layers of the large bowel, what constitutes haustra formation and where are they formed

Answer 16

Like the small bowel that muscularis externa consists of an inner circlular and an outer longitudinal layer. The circular muscles are segmentally thickened, whereas the longitudinal layers are concentrated in three bands, which is the taeniae coli that we mentioned earlier. Between the taeniae coli, the longitudinal layer is actually is actually thin. The bundles of muscle from the taeniae coli penetrate the circular layer at a irregular intervals and also contributes to the haustra formation. These longitudinal layers are shorter than circular muscular layers, and the ovoid segments are called the haustra And these are usually seen throughout the large colon apart from the rectum and the anal canal.

Question 17

Describe the motility of the large bowel and what this allows for

Answer 17

The motility of the large bowel is usually segmental contractions. The colonic contractions are minimally propulsive. They do about five to centimetres per hour at most. This promotes absorption of electrolytes and water. In the proximal colon antipropulsive patterns dominate the movement, and this helps with retaining chyme and giving it more time for the colon to absorb the water and electrolytes.

Question 18

Describe TDC segmental movements, when may this increase?

Answer 18

In the transverse and descending colon, localised segmental contractions of the circular muscle causes back and forth mixing. There is usually short propulsive movements every minutes. This increases in frequency following a meal. In addition to these short movements, we usually have mass contractions, which are high amplitude propagated contractions. It happens one to three times daily and resembles a peristaltic wave. It can propel up to three quarters of the contents of the large intestine in a few seconds.

Question 19

Describe the regulation of the Large bowel from a neurological perspective

Answer 19

Usually food that contains fibre can provoke rapid transport through the colon. The regulation of the large bowel is usually complex, multidimensional. It’s usually driven by the parasympathetic and the sympathetic, in addition to afferent sensory neurones. The parasympathetic controls the ascending colon and most of the transverse colon, and is innervated by the vagus nerve. More distal large bowel is usually innervated by the pelvic nerves.

Question 20

Describe the regulation of the Large bowel from a sympathetic perspective and by the ENS

Answer 20

From a sympathetic point of view, it’s usually driven from the lower thoracic and the upper lumbar spinal cord. The external anal sphincter is controlled by somatic motor fibres in the pudendal nerve, the afferent sensory neurones detect pressure. Additionally, the enteric nervous system is also important. In addition to the myenteric plexus and a combination of hormonal and paracrine control and therefore the control of the large bowel is rather complex from this point of view.

Question 21

Describe the control of defecation

Answer 21

Finally, with regards to control, when it comes to defecation, the rectum is usually filled with faeces by a mass movement from the sigmoid colon. The rectum store’s stool until convenient to void, and the defaecation process is a reflex controlled primarily by the sacral spinal cord, which is both reflex and voluntary action. It’s a reflex to sudden distension of the walls of the rectum. The pressure receptors send signals via the myenteric plexus to initiate peristaltic waves in the descending colon, sigmoid collarbone and the rectum. The internal anal sphincter is then inhibited. There is weak intrinsic signal usually, which is augmented by an autonomic reflex.

Question 22

Under what control is the external sphincter, what can the rectum distinguish between?

Answer 22

In addition, the external, anal sphincter is usually under voluntary control, and when we have the urge to pass a motion, if it is resisted, that sensation usually subsides because of the voluntary control, the rectum it is the last few centimetres of the large bowel, and it can distinguish between solid liquid and gas content. And that is usually important in knowing when to pass appropriately whatever content it is.

Question 23

How much feaces do we produce? What pigments it and what is the flora

Answer 23

in general we produce 150 grams per day as adults, and it contains water and solids. The bile usually pigments it and gives it the colour, whereas bacterial fermentation gives it the odour. The flora is present in all mammals and has a symbiotic relationship with their gut microbial community or the microbiome.

Question 24

Summarise the bacteria of the large bowel compared to other structures, what is its quantity?

Answer 24

The stomach and small bowel have few bacteria, it’s usually protected and it’s usually protected by the high acid content of the stomach. The large bowel, on the other hand, contains billions of colonies of microbiome, and this is seen to be essential for normal function. The microbiome of the large bowel is usually diverse, highly metabolically active, and the microbiome in an average adult human comprises of approximately 1.5 kilograms of live bacteria, with the active biomass equivalent to a major human organ.

Question 25

What is the role of the intestinal flora

Answer 25

The role of the intestinal flora is to synthesise and excrete vitamins. It can prevent colonisation by pathogens by competing for attachment sites or for essential nutrients. And it can antagonise other bacteria through the production of substances which inhibit or kill non-indigenous species. In addition, it can stimulate the production of cross-reactive antibodies and stimulate the development of certain tissue, including caecum and lymphatic tissues.

Question 26

What can fibre be broken down by, what does this produce and regulate therfore

Answer 26

Fibre can be broken down by colonic bacteria, and it 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 27

What is the most prevelant microbiome, what are bifido bacteria?

Answer 27

There are many types of microbiome, but the most prevalent bacteria are the bacteroides, which are gram-negative anaerobic, non-spore forming bacteria. It’s implicated in the initiation of colitis and colon cancer. There’s also bifido bacteria, which are gram-positive, non-spore forming lactic acid bacteria. They have been described as friendly bacteria and are thought to prevent colonisation by potential pathogens