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Alimentary system > 5. Large intestine > Flashcards

5. Large intestine Flashcards

1
Q

How long is the large intestine and what is its diameter?

A

Length: 1.5 m
Diameter: 6 cm

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2
Q

What is the caecum?

A

a blind pouch just distal to the ileocecal valve

Larger in herbivores

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3
Q

What is the appendix?

A

a thin, finger-like extension of the cecum

Not physiologically relevant in humans

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4
Q

What are the principal functions of the large intestine?

A

Reabsorption of electrolytes and water

Elimination of undigested food and waste.

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5
Q

Location of ascending colon

A

Right side of the abdomen

Runs from the cecum to the hepatic flexure

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6
Q

Location of transverse colon

A

Runs from the hepatic flexure to the splenic flexure

Hangs off the stomach, attached to the greater omentum (posterior side, mesocolon).

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7
Q

Location of the descending colon

A

Runs from the splenic flexure to the sigmoid colon.

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8
Q

Location of Sigmoid colon

A

Runs from descending colon to the rectum

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9
Q

What supplies the proximal transverse colon with blood?

A

Middle colic artery

branch of the superior mesenteric artery

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10
Q

What supplies the distal 1/3 of transverse colon with blood?

A

Inferior mesenteric artery

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11
Q

State a region of the colon that is particularly vulnerable to ischaemia.

A

The area between the area that is perfused by the middle colic artery and the area perfused by the inferior mesenteric artery.

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12
Q

State 3 features that are unique to the colon.

A

Appendices epiploicae
Taenia coli
Haustra

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13
Q

What are appendices epiploicae?

A

Fatty tags

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14
Q

What is the taeniae coli?

A

Longitudinal muscle layer concentrated into 3 thick bands which are the taenia coli.
The longitudinal muscle between these layers is thin.
Necessary for large intestine motility

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15
Q

What are haustra?

A

Pouched ovoid segments formed by the short taenia coli

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16
Q

Describe how the distribution of lymphoid tissue is different in the large intestine compared to the small intestine.

A

Lymphoid tissue in the terminal ileum is arranged into peyer’s patches
Large intestine: individual nodules

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17
Q

What is the suggested function of appendices epiploicae?

A

protective function against intra-abdominal infections.

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18
Q

Describe the movement of ions into and out of the large intestine lumen.

A

Na+ and Cl- absorbed by exchange mechanisms and ion channels
Water follows by osmosis
K+ moves into the lumen passively

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19
Q

How much water can the large intestine absorb?

A

4.5 L/day

but usually absorbs 1.5 L/day

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20
Q

What results if there is >4.5L of water to be reabsorbed?

A

Exceeds capacity of absorption

Diarrhoea

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21
Q

How is the histology of the rectum different to the large intestine?

A

No taenia coli in muscular externa

Transverse rectal folds present in submucosa

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22
Q

Muscle types in sphincters of the rectum

A

Internal: Circular muscle
External: Striated muscle

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23
Q

What are the main cells of the large intestine?

A

Enterocytes and goblet cells
Crypts are abundant (goblet cells dominate the crypts)
Stem cells are found in the crypts.

24
Q

Describe some differences between the epithelium of the large intestine and the small intestine.

A

Large intestine mucosa appears smooth (has no villi, thus smaller SA)
No paneth cells
Enteroendocrine cells are rarer
Goblet cells dominate the crypts
Enterocytes have short, irregular Microvilli, primarily concerned with resorption of salts
Glycocalyx does not contain digestive enzymes.

25
Q

How does number of goblet cells change along the large intestine? Why?

A

Increases

Mucous facilitates the passage of the increasingly solid colonic contents, and covers bacteria and particulate matter

26
Q

What stimulates goblet cell secretion?

A

Acetylcholine

parasympathetic and enteric nervous system

27
Q

Describe the appearance of the apical ends of goblet cells

A

Packed with mucous filled secretion granules

28
Q

Muscle composition of the large intestine

A

Outer longitudinal layer

Inner circular layer

29
Q

Describe the circular and longitudinal muscles of the large intestine

A

Circular muscles segmentally thickened.
Longitudinal layer concentrated in 3 bands (taenia coli)
Between the taenia, longitudinal layer is thin
Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals.

30
Q

Why are there NO PANETH CELLS in the large intestine?

A

Because the bacteria in the large intestine are integral to its function.
Don’t want to secrete antimicrobial products from paneth cells that will kill the flora.

31
Q

Describe the relationship between the taenia coli and the circular muscle layer? Which is longer?

A

Taenia coli penetrate the circular muscle at irregular intervals.
Circular muscle layer is longer which is why haustra are formed (which can contract individually)

32
Q

What are the different types of contractions in the large intestine?

A

Colonic contractions: kneading process, minimally propulsive (5-10 cm/hr) promotes absorption of electrolytes and water
In the proximal colon: ‘antipropulsive’ patterns dominate to retain chyme
In the transverse and descending colon: localised, haustral contractions, cause back and forth mixing.
Short propulsive movements every 30 minutes.
Increase in frequency following a meal

33
Q

What is mass movement and how often does it occur?

A

Occurs 1 to 3 times a day.
Resembles a peristaltic wave (but much larger)
Propels contents 1/3 to 3/4 of length of large intestine in seconds.

34
Q

What promotes rapid transport through the colon?

A

Food that contains fibre (indigestible material)

35
Q

Where does the parasympathetic innervation of the colon come from?

A 
Vagus nerve (ascending and most of transverse colon) 
Pelvic nerves (distal 1/3 of transverse colon onwards)
36
Q

Where does the sympathetic innervation of the colon come from?

A

Lower thoracic and upper lumbar spinal cord

37
Q

What nerve allows control of the external anal sphincter?

A

Somatic motor fibres in the pudendal nerves

38
Q

What causes Hirschsprung’s disease?

A

Absence of enteric intramural ganglia

Results in unsatisfactory colonic function.

39
Q

Where are the myenteric plexus ganglia found?

A

Concentrated below the taenia coli.

40
Q

Describe the hormonal control of the colon.

A

Aldosterone promotes reabsorption of Na+ and water (synthesis of Na+ ion channel, Na+/K+ pump)

41
Q

What can stimulate the mass movement?

A

Food in the stomach: by neural and hormonal controls

42
Q

What controls the defecation reflex?

A

SACRAL spinal cord: both reflex and voluntary actions

43
Q

What causes the rectum to fill with faeces?

A

Mass movement in the sigmoid colon

Stores stool until convenient to void

44
Q

Describe the process of defecation

A

Reflex to sudden distension of the walls of the rectum.
Pressure receptors send signals via myenteric plexus to initiate peristaltic waves in descending, sigmoid colon and rectum.
Internal anal sphincter is inhibited.
Weak intrinsic signal, augmented by autonomic reflex
External anal sphincter is under voluntary control.
Urge resisted, sensation subsides

45
Q

What is the significance of the last few centimetres of the rectum?

A

“Social part” of the rectum
Can distinguish between solid, liquid and gas.
Helps you determine what can be passed appropriately.

46
Q

How much faeces does an adult produce per day? What does it mostly consist of?

A 
150 g/day
2/3 water 
Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+)
Bile pigments gives brown colour
Bacterial fermentation gives odour
47
Q

Mass of microbiome in an average adult human

A

1.5 kg

48
Q

Describe some of the roles of intestinal flora.

A

Synthesise and excrete vitamin K
Prevent colonisation by pathogens by competing for attachment sites/ nutrients
Antagonise other bacteria through production of substances that inhibit or kill
Stimulate production of cross-reactive antibodies to prevent infection
Stimulate development of certain tissues e.g. cecum, lymphatic
Fibre can be broken down by colonic bacteria: ferment fibre to produce short chain fatty acids that can regulate gut hormone release, be used as an energy source or influence functions e.g. food intake

49
Q

What are the most prevalent bacteria in the large intestine?

A 
Bacteroides: 
Gram-negative
Anaerobic
Non-spore forming
Implicated in initiation of colonic cancer and colitis
50
Q

Name a bacterium that is thought to prevent colonisation by potential pathogens.

A

Bifidobacteria:
Gram-postive
Non-sporeforming
Lactic acid bacteria

51
Q

Recent research has found links between gut bacteria and…

A 
Drug metabolism
Insulin resistance
Bile acid metabolism
Lipid metabolism
Obesity
52
Q

Control of internal and external anal sphincters

A

Internal: Central control
External: Voluntary control

53
Q

Presence of taenia coli in the large intestine

A

Substantial and continuous apart from rectum and anal canal

54
Q

Name 2 stimulants other than food that cause mass movement

A

Caffeine

Tobacco smoke

55
Q

Necessity of microbiome proved by germ free mice

A

Couldn’t produce Vit. K so had clotting problems

Had shorter guts and undeveloped caecum

Alimentary system (15 decks)

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