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Gastrointestinal Biology > GB 20. Functions of the Colon > Flashcards

GB 20. Functions of the Colon Flashcards

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1
Q

What are the 3 major movements of the colon? Explain them.

A

[1] Segmentation

  • contractions (called haustrations)
  • they are more powerful in the colon than the small intestine
  • almost occlude the lumen
  • between the haustrations/contractions, you get periods of relaxation
  • occur at 2/min in the caecum + increase progressively to 6/min in the sigmoid colon

[2] Peristalsis
- weak and slow

[3] Mass Movement

  • powerful peristaltic waves
  • occurs 1-3x a day (typically after meals)
  • approx. 15 mins
  • triggered by:
    (a) gastrocolic reflexes
    (b) duodenocolic reflexes
  • propels feces into the rectum
  • colon transit is slow (approx. 18 to 24 hours) in comparison with small intestine
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2
Q

Explain a process of colon motility, Segmentation.

A
  • contractions (called haustrations)
  • they are more powerful in the colon than the small intestine
  • almost occlude the lumen
  • between the haustrations/contractions, you get periods of relaxation
  • occur at 2/min in the caecum + increase progressively to 6/min in the sigmoid colon
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3
Q

Explain a process of colon motility, Peristalsis.

A
  • weak and slow
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4
Q

Explain a process of colon motility, Mass Movement.

A
  • powerful peristaltic waves
  • occurs 1-3x a day (typically after meals)
  • approx. 15 mins
  • triggered by:
    (a) gastrocolic reflexes
    (b) duodenocolic reflexes
  • propels feces into the rectum
  • colon transit is slow (approx. 18 to 24 hours) in comparison with small intestine
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5
Q

What is the Mass Movement colon motility triggered by?

A

[1] Gastrocolic Reflex

[2] Duodenocolic Reflex

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6
Q

What are the 2 types of defecation reflexes?

A

[1] Intrinsic Reflex

[2] Extrinsic Reflex

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7
Q

What is the intrinsic defecation reflex?

A
  • “mass movement causes feces to enter the rectum, trigerring this reflex)
  • as food enters, rectal distensions causes:

(a) peristalsis in descending + sigmoid colon
(b) relaxation of internal anal sphincter

  • controlled by intramural plexuses (so even if lesion in spinal cord, defecation is still possible)
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8
Q

As food enters the rectum, rectal distension leads to 2 things: (parts of the intrinsic reflex of the defecation reflex)

A

[1] peristalsis in descending + sigmoid colon

[2] relaxation of internal anal sphincter

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9
Q

What is the innervation of the internal anal sphincter?

A

sacral parasympathetics

- under involuntary control

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10
Q

What is the innervation of the external anal sphincter?

A

pudendal nerve

- under voluntary control

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11
Q

What is the extrinsic defecation reflex?

A
  • distension of the intestine activates stretch receptors running in the sacral parasympathetics (involuntary)
  • the sacral parasympathetics send signals to the spinal cord
  • this reflexly activates the parasympathetic efferents causing REFLEX PERISTALSIS in the colon + rectum
  • it also causes relaxation of the internal anal sphincter
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12
Q

What are the effects of the sympathetic nervous system on the extrinsic defecation reflex?

A
  • does not cause reflex peristalsis in colon + rectum
  • does not lead to relaxation of the internal anal sphincter
  • opposite of parasympathetic nervous system
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13
Q

What is the type of muscle that makes up the external anal sphincter?

A

striated muscle

- voluntary + conscious control

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14
Q

What is the Valsalva Manoeuvre?

A

it is when you close your mouth and nose and try to expire

- during defecation, there is voluntary relaxation + the valsalva manoeuvre

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15
Q

What do the rectal stretch receptors convey to the brain?

A

convey the fullness of the rectum

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16
Q

What occurs to the conscious control of defecation in infants, people with cord and nerve injury and intellectual disability?

A

they do not have conscious control

17
Q

What is the total volume of fluid that is secreted by the intestine?

A

10L

18
Q

Describe the components of the fluids that are secreted into the intestine.

A 
Ingested - 2L
Saliva - 1L
Gastric Juice - 3L
Bile - 1L
Pancreatic Juice - 1L
Intestinal Juice - 2L
19
Q

How much fluid is absorbed by the small intestine?

A

8.5 L

20
Q

How much fluid is absorbed by the large intestine?

A

1.4 L

21
Q

How much fluid from the intestines are excreted in the feces?

A
  1. 1 L (100mL)

exception: diarrhea

22
Q

Where are the Crypts of Lieberkuhn?

A

they are located in the small intestine

23
Q

Explain the mechanism about behind the Crypts of Lieberkuhn.

A
  • Na+ and Cl- (NaCl) enters the gut lumen [this brings water into the gut lumen as well]
  • the Na+/K+ channel pumps K+ into the cell and Na+ into the intracellular space
    (the Na+ goes from the intracellular space into the gut lumen)
  • a channel protein drives 1Na+, 1K+ and 2Cl- into the cell
  • K+ easily diffuses out of the blood down its concentration gradient (back into blood)
  • the CFTR (cystic fibrosis transmembrane regulator) pumps Cl- into the gut lumen
24
Q

What happens when the Cholera toxin (from bacterium Vibrio Cholerae) activates the CFTR?

A

when it activates the CFTR, it causes excessive NaCl and water secretion

  • producing as much as 20L of watery stool
  • as more Cl- exits the cell, more Cl- enters the cell (this leads to an increased secretion)
25
Q

What is the absorption processes that occur in the small intestine?

A
  • Na+/K+ pump pumps K+ into the cell and Na+ into the tight junction
  • Na+ enters the cell alongside (a) sugar and (b) amino acids [through secondary active transport]
  • CO2 and H2O combine in the cell to form H2CO3
  • H2CO3 breaks down to form HCO3- and H+
  • HCO3- enters the tight junction
  • the H+ exits the cell through the Na+/H+ pump (Na+ enters the cell)
  • once H+ exits the cell, it combines with HCO3- to form H2CO3
  • Cl- is absorbed through tight junctions due to the positive charge of intracellular space due to Na+
  • H2O is absorbed through the junctions due to accumulation of HCO3-, Na+ and Cl-
26
Q

What is a major route for salt absorption? Why is it special?

A

absorption from the small intestine

  • it is special because it is unaffected by bacterial toxins
  • oral rehydration therapy (salt solution with glucose in it)
    • this helps to bring nutrients into patient
27
Q

Explain the absorption processes that occur in the large intestine (colon)?

A

colon is excreting:

  • K+
  • HCO3-
  • H+

colon is getting in:
- NaCl

  • Na+/K+ pump pumps Na+ into interstitial space and K+ into the cell
  • K+ from the interstitial space leaves the cell
  • Na+ enters the cell
  • Na+/H+ pump pumps Na+ into the cell and H+ out of the cell
  • HCO3-/Cl- pumps Cl- into the cell and HCO3- out of the cell
28
Q

What inhibits the exchangers/membrane proteins present in the large intestine?

A

bacterial toxins inhibit:
[1] Na+/H+ exchanger
[2] HCO3-/Cl- exchanger

29
Q

What does the distal colon not have that the rest of the large colon have?

A

it doesn’t have the Na+/H+ exchanger so there is no net HCO3- secretion

30
Q

What are a list of effects that bacterial toxins have?

A
  • stimulates salt and water secretion
  • inhibits salt and water absorption
  • can cause diarrhea
  • have NO effect on absorption caused by sodium/sugar and sodium/amino acid co-transport
    [occurs in small intestine - and it is not affected]
  • diarrhea can be treated with oral solutions containing Na + glucose
31
Q

What is the normal level of HCO3- in the blood? What is the value it can be secreted up to in the blood?

A

normal level: 25 mmol/L of HCO3- in the blood

  • HCO3- is secreted up to a luminal concentration of 45mmol/L due to the HCO3-/Cl- exchanger
32
Q

What is the normal K+ secretion into the lumen?

A

normal level: 5mmol/L of K+ in blood

  • can be secreted up to a luminal concentration of 25 mmol/L
33
Q

Why does diarrhea cause hypokalaemia and metabolic acidosis?

A
  • if you are losing a lot of water from the large intestine, you are losing a lot of K+ [hypokalaemia]
  • excessive loss of bicarbonate (alkaline) it gets rid of H+ (acidic)
  • – this leads to an increase in acidity (decrease in pH)

Gastrointestinal Biology (15 decks)

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