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BS - Gastrointestinal System pt.1 > Large Intestine > Flashcards

Large Intestine Flashcards

1
Q

When can ingestion be considered a function of the large intestine?

A

When suppositories are used as a way of administering drugs

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2
Q

What are the main functions of the large intestine?

A
  1. mechanical processing - especially through reabsorption of fluid
  2. digestion - bacteria perform this
  3. secretion
  4. absorption - mainly of water
  5. excretion
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3
Q

Why does the large bowel act as more of a ‘storage space’?

A

The peristaltic waves in the large bowel are much slower

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4
Q

What part of the small intestine will open into the large intestine?

A

The terminal ileum is the last part of the small bowel

This opens into the cecum

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5
Q

What controls the flow of chyme from the small to large intestine?

A

Ileocecal valve

This is the opening between the ileum and the cecum (first part of large bowel)

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6
Q

Between which structures does the ascending colon run?

A

The ascending colon runs from the cecum to the hepatic flexure

This is a sharp bend between the ascending and transverse colons

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7
Q

Between which structures does the transverse colon run?

A

Between the hepatic flexure and the splenic flexure

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8
Q

What happens to the blood supply of the transverse colon as it moves towards the splenic flexure?

why is this important?

A

2/3 of the way along, the blood supply changes from the superior mesenteric artery to the inferior mesenteric artery

The distal 1/3 of the transverse colon is a hindgut structure

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9
Q

Where does the descending colon run to?

A

It runs from the splenic flexure into the sigmoid colon

The large bowel becomes the sigmoid colon when it enters the pelvis

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10
Q

what does the sigmoid colon lead to?

A

The rectum, and then eventually to the anus

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11
Q

What is the difference in the muscle of the small and large bowels?

A

In the SI there is a continuous sheet of smooth muscle

In the LI there are taenia coli - strips of longitudinal muscle

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12
Q

Where are the three taenia coli of the large bowel found?

What are they called?

A
  1. Tenia libera (free coli) - found on the interior
  2. Tenia mesocolica (mesocolic coli) - found on the posterior aspect of the large bowel
  3. Tenia omentalis (omental coli) - found on the superior aspect of the transverse colon
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13
Q

What results from tonic contractions of the teniae coli?

A

It bunches up the colon into a succession of pouches called haustra

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14
Q

What are the haustra?

A

Small pouches caused by sacculation, which give the colon a segmented appearance

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15
Q

What are epiploic appendages?

A

Small, fat-filled sacs of visceral peritoneum that are attached to the teniae coli

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16
Q

What is involved in a barium enema?

A

radioopaque barium is passed into the colon through the anus

this will show the very prominent haustra

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17
Q

What is shown in a barium enema about the nature of the haustra?

A

The dilated part of the haustrum is where the smooth muscle is relaxed

The narrowed part of the haustrum is where the smooth muscle is contracted

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18
Q

Why is the surface epithelium of the LI much flatter than the SI?

A

There are no circular folds and there are no villi

This is because no amplification of surface area is needed

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19
Q

In what other way does the nature of the LI differ to the SI?

A

There are very few enzyme-secreting cells located in the large intestine

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20
Q

Other than in the anal canal, what is the nature of the mucosa of the colon?

A

It is simple columnar epithelium

It is made mostly of enterocytes (absorptive cells) and goblet cells

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21
Q

What is the role of deep pit glands and enterocytes in the large bowel?

A

Deep pit glands contain goblet cells and enterocytes

Enterocytes absorb water, salts and vitamins produced by intestinal bacteria

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22
Q

what is the role of goblet cells in the large intestine?

A

They secrete mucus which eases movement of the faeces

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23
Q

what are the key roles of mucus in the large intestine?

A
  1. it protects the intestine from the effects of the acids and gases produced by enteric bacteria
  2. layers of mucus allow faecal material to be dried and compacted as it passes through the large bowel
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24
Q

Why does the large intestine contain lymphatic tissue?

A

As the colon contains a lot of bacteria

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25
Q

How may the ileocecal valve be described?

A

It is a one-way flutter valve that connects the terminal ileum to the cecum

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26
Q

What happens to the ileocecal valve when pressure is high in the colon?

A

The leaflets of the ileocecal valve are pushed together

This prevents chyme from returning to the terminal ileum after it has entered the cecum

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27
Q

What happens when the ileocecal valve relaxes?

A

During periodic relaxation of the valve, substances can flow into the caecum

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28
Q

When will the ileocecal valve be open and when will it be closed?

A

It is open during ileal distention

It is closed during caecal distention

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29
Q

What is significant about the relationship between the caecum and the appendix in most people?

A

The lumen of the cecum continues into the appendix

This means that material can be trapped in the blind-ended tube

This leads to infection and appendicitis

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30
Q

Why is much of the motility of the colon NOT designed to move the contents along?

A

material needs to transit the colon slowly to allow water to be reabsorbed

The content is also compacted slowly

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31
Q

How do peristaltic waves in the large bowel compare to the small bowel?

A

The peristaltic waves are much less frequent and move a much larger distance

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32
Q

What is meant by the process of haustration?

A

There are slow contractions of the circular muscle to squeeze the content to and fro, mix material and release water

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33
Q

What stimulates slow-moving haustral contraction?

A

presence of food residues in the colon

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34
Q

What is movement like during haustration?

A

it involves sluggish segmentation, primarily in the transverse and descending colons

When a haustrum is distended with chyme, its muscle will contract to push the residue into the next haustrum

These movements happen every 30 minutes

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35
Q

What is meant by ‘mass movement’ in the colon?

A

Strong peristaltic waves which occur three to four times per day

36
Q

Where does a mass movement commence?

A

The wave starts midway along the transverse colon

This forces the contents towards the rectum

37
Q

what happens to the haustra during a mass movement?

A

the haustra briefly disappear as the mass movement sweeps by

they will reform immediately afterwards

38
Q

When do mass movements usually occur?

A

During eating or immediately afterwards

39
Q

How is motility of the large intestine mostly controlled?

A
  1. most control is intrinsic via the enteric nervous system
  2. there is some parasympathetic control
  3. there are enteroendocrine and neurocrine influences
40
Q

What are the enteroendocrine and neurocrine influences on the large bowel?

A

They come from cells releasing 5-HT and Peptide YY

41
Q

How is the action of Peptide YY an example of feedback activity?

A

Local cells produce Peptide YY

This acts locally and enters the circulation to influence regions elsewhere

42
Q

What reflexes can trigger mass movement?

A

The gastrocolic and the orthocolic reflexes

43
Q

When does the gastrocolic reflex occur and what is its effect?

A

It occurs when the stomach becomes full

It triggers a mass movement in the colon to move the faeces along

44
Q

When does the orthocolic reflex occur and what is its effect?

A

It occurs when someone moves from lying in a horizontal position to standing in an upright position

It triggers a mass movement in the colon

45
Q

What triggers the “ileal brake”?

why is it necessary?

A

The presence of undigested lipid in the distal ileum and proximal colon

If there is fat in the colon, this means insufficient processing has occurred earlier on

46
Q

What is the action of the ileal brake?

A

Undigested lipid triggers the release of Peptide YY

Peptide YY slows gastric emptying and small bowel peristalsis

47
Q

What is the difference in digestion in the colon and the small intestine?

A

Digestion in the colon is performed by bacteria

Digestion in the SI is performed by human enzymes

48
Q

What will the bacteria in the colon digest?

A
  1. fibre
  2. urea and amino acids
  3. bilirubin from haemoglobin
  4. cysteine and methionine
  5. conjugated bile acids
49
Q

What does the digestion of fibre produce?

A

Short chain fatty acids such as butyric acid, hydrogen and methane

Butyric acid is used as an energy source by colon cells

50
Q

what does the digestion of urea and amino acids produce?

A

ammonia

51
Q

What does the digestion of bilirubin produce?

A

urobilinogen and stercobilinogen

52
Q

What happens to the urobilinogen produced from the digestion of bilirubin?

A

Half of the urobilinogen is reabsorbed

It is taken to the liver by the portal vein

It then enters circulation to be secreted by the kidneys in the urine

53
Q

What happens to the stercobilinogen produced from the digestion of bilirubin?

A

Stercobilinogen is known as faecal urobilinogen

It is oxidised to stercobilin which is responsible for pigmentation of faeces

54
Q

What does digestion of cysteine and methionine produce?

A

hydrogen sulphide

55
Q

What does the digestion of conjugated bile acids produce?

Why is this process important?

A

Hydroxyl groups are removed to produce unconjugated bile acids

It is important for bile salt recycling as making the bile salts uncharged means that they can pass into circulation

56
Q

How much gas is released by fermentation each year?

A

1L of nitrogen, hydrogen and CO2 is released a year

57
Q

What transporter is needed for short-chain fatty acid absorption in the colon?

What type of transport is this?

A

It is secondary active transport via a Na+-linked transporter

This is the sodium monocarboxylate transporter

58
Q

How are short-chain fatty acids absorbed in the colon?

A

The SMCT1 transports fatty acids into the cell alongside Na+ ions

This is driven by the export of Na+ from the cell by the Na+/K+ ATPase

59
Q

why is it important that short-chain fatty acids are absorbed in the colon?

A

they are an important source of energy for colonocytes

60
Q

What channels allow for transcellular sodium transport into the colon cells?

A

Epithelial sodium channels - ENaC

61
Q

During salt absorption in the colon, what does the entry of Na+ ions into the colonocyte cause?

A

A paracellular Cl- flux as the Cl- ions leave the cell

This then leads to water following the osmotic gradient

62
Q

What is salt absorption in the colon stimulated by?

How can the colon respond?

A

the process is stimulated by aldosterone

the colon can respond by expressing more channels (ENaC) and absorbing more sodium

63
Q

What creates the sodium gradient needed for salt absorption in the colon?

A

The Na+/K+ ATPase pumps Na+ out of the cell

The K+ that is pumped into the cell leaves via a K+ channel

64
Q

What are the major secretagogues of chloride in the colon?

what is a secretagogue?

A

cAMP and Ca2+

These are substances which promote secretion of Cl-

65
Q

How do toxins increase the amounts of the chloride secretagogues in the colon?

A

cholera and E. coli toxins increase the amount of cAMP by triggering the insertion of CFTR channels

C. difficile toxin increases the amount of Ca2+

66
Q

What is the result of inserting more CFTR channels into epithelial cells in the colon?

A

There is increased chloride excretion into the lumen

This leads to salty water being secreted back into the faeces

This leads to severe dehydration

67
Q

What is the effect of increasing cAMP or Ca2+?

A

Secretory diarrhoea

68
Q

what is the movement of potassium like in the colon?

A

It can undergo secretion or absorption

There is usually a net secretion, and passive secretion exceeds active secretion

69
Q

How is passive secretion of K+ achieved in the colon?

A

This involves K+ moving paracellularly

As Na+ is reabsorbed, this creates an electrochemical gradient

70
Q

How is active secretion of K+ achieved in the colon?

A

This involves opening the BK channel

This allows K+ to go from a high intracellular concentration, into the lumen

71
Q

What determines the balance of passive and active secretion of K+ in the colon?

A

It is determined by plasma potassium concentration, aldosterone and cAMP

72
Q

What is the role of the H+/K+ ATPase in movement of K+ across the colon?

A

It is usually involved in the absorption of K+ and the secretion of H+ into the lumen

73
Q

How is the transfer from sigmoid colon to rectum described?

What does the rectum act as?

A

The sigmoid colon enters the rectum at a very sharp angle

The rectum is a storage space

74
Q

what are rectal valves and what is their function?

A

They are folds in the wall of the rectum

They help to support the faecal material and prevent it from pushing down too quickly

75
Q

What are the 2 different anal sphincters?

A
  1. internal anal sphincter

2. external anal sphincter

76
Q

What is the role of the internal anal sphincter?

What is it made of and what control is it under?

A

It is made from smooth muscle

It contracts to constrict the faecal material moving through the anus

It is under involuntary control

77
Q

What is the external anal sphincter made from and what type of control is it under?

A

It is made from skeletal muscle

It is under voluntary control

78
Q

what is the process of anal sampling?

A

As tension rises, the internal anal sphincter relaxes

This allows some faecal material to pass into the space between the sphincters

79
Q

What happens after anal sampling?

A

The external anal sphincter contracts

Voluntary control allows it to relax to empty the bowels at the appropriate time

80
Q

What is the Dentate (pectinate) line?

What does it signify?

A

It is the boundary between the normal gut lining and the anal canal

  1. the epithelium becomes stratified squamous
  2. it reflects the different innervations of the different areas
81
Q

What happens to the internal anal sphincter when faeces enter the rectum?

A

When faeces enter the rectum, the pressure rises

The rise in pressure triggers reflex relaxation of the internal anal sphincter

This comes with the urge to defecate

82
Q

What happens after the reflex relaxation of the internal anal sphincter?

A

A small amount of material enters the anus, allowing for anal sampling

The external anal sphincter contracts if defecation is not desired

83
Q

Why is sitting or squatting recommended during defecation?

A

It increases the rectosigmoid angle

This facilitates the passage of faeces

84
Q

What other action will increase the rectosigmoid angle during defecation?

A

Relaxation of the puborectalis muscle

85
Q

How is the passage for faeces opened during defecation?

A

Relaxation of the external anal sphincter and pelvic floor muscles

86
Q

What is the Valsalva manoeuvre?

A

This describes that the motive force behind defecation is triggered by rectal peristalsis plus raised intraabdominal pressure

BS - Gastrointestinal System pt.1 (22 decks)

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