GB 20. Functions of the Colon Flashcards
What are the 3 major movements of the colon? Explain them.
[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
Explain a process of colon motility, 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
Explain a process of colon motility, Peristalsis.
- weak and slow
Explain a process of colon motility, 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
What is the Mass Movement colon motility triggered by?
[1] Gastrocolic Reflex
[2] Duodenocolic Reflex
What are the 2 types of defecation reflexes?
[1] Intrinsic Reflex
[2] Extrinsic Reflex
What is the intrinsic defecation reflex?
- “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)
As food enters the rectum, rectal distension leads to 2 things: (parts of the intrinsic reflex of the defecation reflex)
[1] peristalsis in descending + sigmoid colon
[2] relaxation of internal anal sphincter
What is the innervation of the internal anal sphincter?
sacral parasympathetics
- under involuntary control
What is the innervation of the external anal sphincter?
pudendal nerve
- under voluntary control
What is the extrinsic defecation reflex?
- 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
What are the effects of the sympathetic nervous system on the extrinsic defecation reflex?
- does not cause reflex peristalsis in colon + rectum
- does not lead to relaxation of the internal anal sphincter
- opposite of parasympathetic nervous system
What is the type of muscle that makes up the external anal sphincter?
striated muscle
- voluntary + conscious control
What is the Valsalva Manoeuvre?
it is when you close your mouth and nose and try to expire
- during defecation, there is voluntary relaxation + the valsalva manoeuvre
What do the rectal stretch receptors convey to the brain?
convey the fullness of the rectum
What occurs to the conscious control of defecation in infants, people with cord and nerve injury and intellectual disability?
they do not have conscious control
What is the total volume of fluid that is secreted by the intestine?
10L
Describe the components of the fluids that are secreted into the intestine.
Ingested - 2L Saliva - 1L Gastric Juice - 3L Bile - 1L Pancreatic Juice - 1L Intestinal Juice - 2L
How much fluid is absorbed by the small intestine?
8.5 L
How much fluid is absorbed by the large intestine?
1.4 L
How much fluid from the intestines are excreted in the feces?
- 1 L (100mL)
exception: diarrhea
Where are the Crypts of Lieberkuhn?
they are located in the small intestine
Explain the mechanism about behind the Crypts of Lieberkuhn.
- 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
What happens when the Cholera toxin (from bacterium Vibrio Cholerae) activates the CFTR?
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)
What is the absorption processes that occur in the small intestine?
- 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-
What is a major route for salt absorption? Why is it special?
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
Explain the absorption processes that occur in the large intestine (colon)?
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
What inhibits the exchangers/membrane proteins present in the large intestine?
bacterial toxins inhibit:
[1] Na+/H+ exchanger
[2] HCO3-/Cl- exchanger
What does the distal colon not have that the rest of the large colon have?
it doesn’t have the Na+/H+ exchanger so there is no net HCO3- secretion
What are a list of effects that bacterial toxins have?
- 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
What is the normal level of HCO3- in the blood? What is the value it can be secreted up to in the blood?
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
What is the normal K+ secretion into the lumen?
normal level: 5mmol/L of K+ in blood
- can be secreted up to a luminal concentration of 25 mmol/L
Why does diarrhea cause hypokalaemia and metabolic acidosis?
- 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)
