Water And Electrolyte Movement In The GI Tract Flashcards
Why is only a small amount of water absorbed by the stomach?
Only small amounts of water move across the gastric mucosa, because of the high osmotic load of secreted hydrochloric acid.
How is water absorbed in the SI?
Water moves freely across the small intestine in accordance with osmotic gradients (solute driven) or possibly via channels called aquaporins (solvent driven) although the precise mechanisms or their relative importance are not known.
Na+ is actively transported from the lumen by pumps located in the cell membranes in the ileum and jejunum.
This can decrease the water potential and hence aid osmosis into the enterocytes
How is water absorbed by the colon?
The contents are iso-osmotic in the colon, Na+ is actively pumped into the lumen and water follows.
Also note:
Chloride & Bicarbonate reabsorption in the ileum and colon Cl- is actively reabsorbed in exchange for bicarbonate, this results in the intestinal contents becoming more alkaline.
Simple cellular shape
Vs multicellular
Simple = large SA:volume for absorption
Multicellular = smaller SA:volume may need a transport system
Function of the GI:
Take large solids and digest them into smaller molecules that can be absorbed, whilst acting as a barrier to toxins, bacteria and parasites.
Daily gut fluid balance:
Saliva, gastric secretions, pancreatic juices, bile, intestinal secretions all produced throughout the day and the water is reabsorbed by the SI, absorbs 7.5L per day
Colon absorbs 1.5L
How is water moved?
Water moves by osmosis (down an osmotic gradient)
Electrolytes move dow an electrochemical gradient.
Moving against an conc grad requires energy energy is supplied by the sodium gradients generated by the pumps.
They decrease the water potential inside cells and hence cause osmosis into the cell.
Trans cellular pathway =
Paracellular pathway =
Apical membrane =
Base lateral membrane =
Trans cellular pathway = through the cells
Paracellular pathway = route between cells
Apical membrane = membrane facing towards lumen of gut, kidney etc
Base lateral membrane = membrane facing away from the lumen
The small intestine is arranged into villi and crypts, what are the jobs of these?
Villi with microvilli increase the SA for absorption
The crypts are important in secretions
How does NA+ couple nutrient absorption?
Na+ are AT out of the cell by NA+/K+ ATPase
Na+ move into the cell from the lumen
As they move in they carry a glucose molecule, this glucose molecule or a.a is carried against their concentration gradient (cotransport)
This also decreases the water potential of the cell and hence water also moves in by osmosis
What factors affect absorption and secretions of enterocytes?
X 4
Number of enterocytes
Structure of villi and crypts
Nutrient intake
Bile
Overview of intestinal secretions:
Cl- into enterocyte via NA+/K+/2Cl- cotransporter
This leads to increased levels of cAMP
cAMP influence Cl- channels causing more Cl- into the lumen of the GI tract
This causes an electro gradient so NA+ follow and due to a decreased osmotic pressure so will water.
Cholera will increase this process leading to diarrhoea
What is coeliac disease?
This is when you are gluten intolerant
Gluten is found in wheat, barley etc
When it is ingested it leads to damage to the villi and hence patients can present with weight loss, diarrhoea and deficiencies due to malabsorption
What is oral rehydration what does it contain?
It is a solution of glucose and salt, as this is absorbed into cells it rings water in due to its osmotic effect and therefore it is a better rehydrated.
What is the definition of digestion?
It is the breakdown of large complex molecules into smaller ones that can be absorbed
2 x types = mechanical and chemical
Examples =
