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Alimentary system > The large intestine > Flashcards

The large intestine Flashcards

1
Q

What is the large intestine made of?

A

Colon (ascending, transverse, descending, sigmoid), caecum, appendix, rectum and anal canal

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2
Q

What is the caecum?

A

A blind pouch not usually used in humans - first part of large intestine. Distal to the iloececal (sphincter seperating small and large intestine) valve and larger in herbivores

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3
Q

What is the appendix?

A

A thin, finger like extension of the caecum, not useful in humans but may be important for microbiome

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4
Q

How long and wide is the colon?

A

1.5m long, 6cm diameter

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5
Q

What is the main function of the colon?

A

To reabsorb electrolytes and water, and eliminate undigested food and waste

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6
Q

Describe the location and pathway of the ascending colon

A

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

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7
Q

Describe the pathway of the transverse colon and its connection to the stomach

A

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.

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8
Q

Describe the pathway of the descending colon

A

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

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9
Q

Describe the pathway and appearance of the sigmoid colon

A

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

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10
Q

Blood supply to the colon (2)

A
  • Proximal transverse colon supplied by middle colic artery (branch of superior mesenteric artery)
  • Distal third of transverse colon supplied by inferior mesenteric artery.

(Difference is because of embryological division between mid and hindgut - the area between the two is sensitive to ischaemia)

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11
Q

Features of the large intestine

A
  • The peritoneum carries fatty tags (appendices epiploicae)
    Suggestive to have protective features against intra abdominal infections
  • The muscle coat has 3 thick longitudinal bands (taeniea coli) which creates pouchinges (haustra). This is needed for motility.
  • Has nodule of lymphoid tissue: solitary nodule
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12
Q

Where is most of the water absorbed, SI or LI?

A

Small

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13
Q

How is water reabsorbed in the LI?

How much?

A

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).

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14
Q

The structure of the rectun

A
  • Dilated distal portion of the alimentary canal
  • Histology similar to the colon, but distinguished by transverse rectal folds in its submucosa (needed ti hold stool) 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.
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15
Q

Mucosal structure - compared to SI

A

Has lots of enterocytes and goblet cells, crypts and stem cells in crypts like SI
However, LI has no villi so smooth surface, the enterocytes are shorter and have irregular microvilil and re mainly involved with salt uptake

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16
Q

Goblet cells in the LI - distribution compared to in SI

What stimulates goblet cell secretion?

A

More goblet cells than the small intestine with number higher in the crypts (they dominate crypts) and more distally in the colon towards rectum - contents of LI become drier so more mucus needed to ease movement

ACh released by enteric nervous system and PNS

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17
Q

Paneth cells - their roles and distribution in LI

A

Contain large acidophilic granules containing: lysozymes (protect stem cells), glycoproteins and zinc (for enzymes). They also engulf bacteria and protozoa. May have a role in regulating intestinal flora. They are not found in the LI because we want bacteria!

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18
Q

Enteroendocrine cells in the LI

A

They are rarer than in the SI

19
Q

Microvilli in the LI

A
  • Microvilli form the brush border
  • There are several thousand microvilli per cell
  • The surface of the microvilli are covered with glycocalyx
20
Q

What is glycocalyx?

A
  • It is a rich carbohydrate layer on apical membrane
  • It is involved in protection from the digestional lumen, yet allows for absorption.
  • It traps a layer of water & mucous known as the “unstirred layer” which regulates rate of absorption from intestinal lumen
  • Unlike glycocalyx in SI, it doesn’t contain enzymes
21
Q

Muscle layers of the LI

A
  • Like small intestine, muscularis externa consists of an inner circular and outer longitudinal layer
  • The circular muscles are segmentally thickened
  • The longitudinal layers are concentrated in 3 bands - taenia coli
  • Between the taenia the longitudinal layer is thin
  • Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals
  • 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
22
Q

The motility of the LI

A
  • Colonic contractions lead to a kneading process
  • Slow propulsion promotes absorption of electrolytes and water
  • In the proximal colon, anti-propulsive patterns dominate to retain the chyme
  • In transverse and descending colon, haustral contractions cause back and fourth mixing
  • There are then short propulsions every 30 minutes that increase in frequency following a meal
  • Mass movement occur 1-3 times daily and can propel contents 1/3-3/4 length of large intestine in a few seconds – these movements are promoted by presence of indigestible fibre or food in the stomach
23
Q

What are mass movements in the LI?

A

They occur 1-3 times daily and can propel contents 1/3-3/4 length of large intestine in a few seconds – these movements are promoted by presence of indigestible fibre or food in the stomach

24
Q

Nervous control of the LI

A
  1. PARASYMPATHETIC – ascending colon, most of transverse colon by vagal nerve. More distal by pelvic nerves.
  2. SYMPATHETIC – lower thoracic and upper lumbar spinal cord
  3. External anal sphincter controlled by somatic motor fibres in the pudenal nerves.
  4. ENTERIC – complicated but important
  5. Myenteric plexus ganglia concentrate below the taeniae coli
25
Q

What is Hirschsprung’s disease?

A

No enteric ganglia.

26
Q

Hormonal /paracine control of the LI

A

Aldosterone promotes sodium and water absorption (synthesis of Na+ ion channel, Na+/K+ pump

27
Q

What happens before defecation?

A

Rectum becomes filled with faeces by mass movements in the sigmoid colon which stores the stool. Stores stool until convenient.

28
Q

What controls the defecation reflex?

A

The sacral spinal cord - reflex and voluntary

29
Q

What is the process of defecation - signals involved?

A

Defecation urges come in waves:

  • Reflex inhibition of internal sphincter to distension of walls of rectum -> pressure receptors send signals to myenteric plexus to initiate peristaltic waves -> external sphincter under voluntary control -> defecate or resist urge and sensation subsides
  • weak intrinsic control from ANS
30
Q

What is the social part of the rectum and why is it important?

A
  • Last few centimetres of rectum
  • It can differentiate between solid, liquid and gas
  • That is important in knowing what can be passed appropriately in what circumstance
31
Q

How much faeces is produced daily?

A

1500 g - equal to an organ

32
Q

What is the composition of faeces and what gives it the colour and smell?

A

2/3rds water, cellulose, bacterial fermentation (smell), cell debris, bile pigments (colour), salts

33
Q

How much bacteria is there in the LI?

A

1.5 kg

Diverse, highly metabolically active community

34
Q

Compare the amount of bacteria in SI, stomach and LI

A
  • Stomach and small intestine = few bacteria (harsh environments)
  • Large intestine = many bacteria – essential to function
35
Q

What are the roles of the intestinal flora?

A
  1. VITAMINS - Synthesise and excrete vitamins such as Vitamin K
  2. COMPETITION – Prevent colonisation by pathogens by outcompeting them
  3. KILL PATHOGENS – Antagonize other bacteria by producing anti-bacterials etc
  4. IMMUNITY – Stimulate production of cross-reactive antibodies that can protect against some pathogens
  5. GROWTH – Stimulate development of certain tissues such as caecum and lymphatic tissues
  6. FIBRE BREAKDOWN – Flora can breakdown fibre
  7. FATTY ACIDS – Produce short chain fatty acids = regulate hormone release, used as energy, influence food intake or insulin sensitivity directly
36
Q

What is the relationship with gut microbes in mammals?

A

symbiotic

37
Q

What are the most prevalent bacteria in flora?

A
  1. Bacteroidetes - Gram negative
    They are anaerobic, non spore forming bacteria. Implicated in the initiation of colonitis and colon cancer
  2. Bifidobacteria - Gram positive
    They are non spore forming, lactic acid bacteria. They are friendly and prevent colonisation by pathogens
38
Q

What has recent research into gut bacteria shown links with?

A
  • drug metabolism
  • insulin resistance
  • bile acid metabolism
  • lipid metabolism
  • obesity
39
Q

Are humans born with bacteria in the gut?

A

No they are thought to develop in first few years and then it declines

40
Q

Pathophysiology relating to gut bacteria

A

Inappropriate population or loss of commensal bacteria can predispose to illness

41
Q

What does the proximal colon do to keep food in the region for longer?

A

It elicits antipropulsive contractions to impede propulsion

42
Q

SI vs LI

A

SI- longer but narrower in diameter

LI - shorter but wider

43
Q

What are the layers of the gut tube?

A 
epithelium
lamina propria
muscularis mucosae
submucosa
inner circular muscle layer
outer longitudinal muscle layer
serosa

Alimentary system (14 decks)

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