case 4 - PBL Flashcards
(50 cards)
what is the chyme like in the colon
already very concentrated because most of the water has already been absorbed.
what is the large intestine lined with
mucosa with crypts of Lieberkuhn
what do these crypts contain
glands and mucus producing goblet cells
what do these cells protect from
protect the intestinal wall from the plethora of anaerobic bacteria in the colon and from the pressure exerted on the walls by the concentrated chyme
what else do the walls of the large intestine also contain
gastrointestinal lymphoid tissue (GALT)
what does this GALT do
contributes to the body’s immune defences
how much water does the colon help absorb from the lumen per day
400ml of wayer
what is the consequence of chyme being concentrated
by the tie it reaches the colon, the colon must work against a larger osmotic pressure gradient than in the rest of the GI tract
the water must move against the gradient for osmosis
what is there a net absorption of in the large intestine
sodium and chloride ions
what are the three methods that sodium is absorbed via
sodium-hydrogen antiporter on the luminal membrane
epithelial sodium channels
enhanced by absorption of short chain fatty acids in the colon via specialised symporters
how are chloride and bicarbonate absorbed
the movement of sodium into the plasma produces an electrochemical gradient to allow absorption of chloride
chloride ions are exchanged for bicarbonate ions
how is water further absorbed
the absorption of these electrolytes creates an osmotic gradient to allow further absorption of water
how is potassium absorbed
absorption of water along the length of the bowel concentrates potassium in the lumen
This provides an electrochemical gradient for the movement of potassium into the plasma
In the colon potassium may be absorbed or secreted depending on the remaining concentration in the lumen and the electrochemical gradient created by the active absorption of sodium
Secretion usually occurs when the luminal concentration of potassium ions is below 25mM
how are vitamins and fats absorbed
short chain fatty acids, crucial B vitamins, such as B6, and B12, and vitamin K are produced by the digestion of chyme by the commensal microbial flora of the colon
what are the 2 endocrine mechanisms used to regulate absorption in the GI tract
aldosterone
glucocorticoids
explain the aldosterone endocrine mechanism
increases the net absorption of water and electrolytes by stimulating the basolateral sodium potassium ATPase.
this increases the electrochemical gradient and driving force for sodium absorption
it also increases transcription of epithelial sodium channels
explain the glucocorticoid mechanism of absorption
(and somatostatin)
act to increase water and electrolyte absorption by increasing the action of the basolateral sodium-potassium ATPase
what does the parasympathetic innervation promote in the intestines
promotes net secretion from the testes
what does the sympathetic innervation promote in the intestines
promotes net absorption in the intestines
what is the large intestine naturally separated into
segments known as haustrau
along the course of the walls are groups of cells called what?
pacemaker cells
what do these pacemaker cells do
send signals to the smooth muscle cells on the walls of the large intestine causing them to contract at regular intervals
what did the contraction cause
the food to be churned in the intestine exposing the gut contents to a larger surface area of epithelium maximising absorption
what does each group of cells control
a certain number of haustra
