GI Physiology - The Large Intestine Flashcards
What is absorped from the large intestine?
- water
- sodium
- chloide (absorption) and bicarbonate (secretion)
- potassium
- vitamins and fats
How is sodium absorped from the large intestine?
- 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 is chloide absorped from the large intestine?
The movement of sodium into the plasma produces an electrochemical gradient to allow absorption of chloride. Chloride ions are exchanged for bicarbonate ions (causing net bicarbonate secretion).
How is water absorped from the large intestine?
The absorption of sodium and chloride creates an osmotic gradient to allow further absorption of water.
Is potasium absorbed or secreted in the large intestine?
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.
Which vitamns are absorbed from the large intestine?
Short-chain fatty acids, crucial B vitamins (such as B6 and B12) and vitamin K (required for blood clotting) are produced by the digestion of chyme by the commensal microbial flora of the colon.
How is absorption from the large intestine regulated by the enteric nervous system?
- Parasympathetic innervation promotes net secretion from the intestines .
- Sympathetic innervation promotes net absorption from the intestines.
Which neuroendocrine mechanisms regulate absorption from the gastrointestinal tract?
- Aldoesterone
- Glucocorticoids and somatostatin
How does Aldosterone regulate absorption from the gastrointestinal tract?
Aldosterone increases the net absorption of water and electrolytes by stimulating the basolateral sodium-potassium ATP-ase. This increases the electrochemical gradient and driving force for sodium absorption. It also increases transcription of epithelial sodium channels.
How do glucocorticoids and somaotstatin regulate absorption from the gastrointestinal tract?
Glucocorticoids and somatostatin act to increase water and electrolyte absorption by increasing the action of the basolateral sodium-potassium ATP-ase.
What is haustral shuttling?
Pacemaker cells send signals to the smooth muscle cells on the walls of the large intestine causing them to contract at regular intervals. The contraction causes the food to be churned in the intestine exposing the gut contents to a larger surface area of epithelium maximising absorption.
What is mass movement?
Mass movement only occurs once or twice per day. This involves a sudden, uniform peristaltic contraction of smooth muscle of the gut which originates at the transverse colon and rapidly moves formed faeces into the rectum, which is normally empty. The result of this is feeling the urge to defecate.
If mass movement is stimulated by eating what is this called?
The gastro-colic reflex.
What is the function of the rectum?
Temporary storage of faeces before defecation. As it becomes filled the rectal walls expand and stretch receptors stimulate the desire to defecate.
How does the urge to deficate arise?
The urge to defecate arises from contraction of rectal muscles, relaxation of the internal anal sphincter and an initial contraction of the external anal sphincter.
