Lecture 32 Flashcards
Where are the sites of absorption in the GI tract?
small intestine and large intestine
What is absorbed in the small intestine?
There is bulk absorption of isosmotic fluid and nutrients
What is absorbed in each part of the large intestine?
- proximal colon: absorption of products of fermentation such as fibre as well as fluid and electrolytes
- distal colon: selective absorption of fluid and electrolytes
What drives water absorption?
Na+
Explain the nutrient dependent Na+ absorption in the small intestine
Na+ is coupled to solutes such as monosaccharides and amino acids. There is also specialised absorption for bile salts and vitamins
Explain the nutrient absorption in the proximal colon
Na+ dependent absorption of short chain fatty acids
In which parts of the body is there nutrient independent Na+ absorption?
small intestine
large intestine
In which parts of the body is there nutrient independent Na+ absorption?
small intestine
large intestine
Describe the nutrient dependent Na+ absorption in the small intestine
This is an Na+-coupled solute absorption through the SGLT1 (this cotransports glucose or galactose with Na+ and water follows
Describe the nutrient independent absorption of Na+ in the small intestine
This uses the Na+/H+ exchanger (NHE3) of the Cl-/HCO3- exchanger in the apical membrane
What happens in the proximal colon?
There is fermentation of indigestible sugars and proteins which generated short chain fatty acids such as acetate, propionate and butyrate
There is also Na+ absorption
Describe the Na+ absorption in the proximal large intestine
This is an electroneutral NaCl absorption (there is paired Na+/H+ and Cl-/HCO3- exchangers in the small intestine). The Na+ absorption is associated with absorption of short chain fatty acids
What is an examples of Na+ transporter in the proximal colon? How does it work?
In the apical membrane there is a sodium monocarboxylate transporter such as SMCT1. This brings in one SCFA and Na+ into the lumen. The SCFAis used by colonocyte for intracellular metabolism and the Na+ leaves the basolateral membrane via the Na+/K+ ATPase and then Cl- and water follow paracellularly
Where does the bulk reabsorption of H2O occur?
In the small intestine
What is the purpose of the distal colon?
Regulation of composition of faeces
Explain the Na+ absorption in the distal colon
This varies depending on fluid and electrolyte status.
Explain the Na+ absorption in the distal colon
This varies depending on fluid and electrolyte status.
How does aldosterone affect the absorption in the distal colon? Why is this?
The distal colon is tight epithelia which means that Na+ must be absorbed transcellularly. If we are in a salt depleted condition, there is high levels of plasma aldosterone which means that ENaC is inserted into the apical membrane to increase the Na+ absorption. If we are in a salt replete condition, we have low levels of plasma aldosterone and we don’t reabsorb as much Na+
Is there secretion in the small and large intestine?
yes
Why do we have secretions into the small and large intestine?
To dilute the contents of the lumen in order to reabsorb the last bit of nutrients
How much do we normally secrete into the small and large intestine?
1-2L