28 - Water & electrolyte absorption and secretion in the GI tract Flashcards
Enteric nervous system
Myenteric and submucosal plexi
SI surface is amplified at 3 levels by
plicae circularis
villi + crypts of lieberkuhn
microvilli
LI surface is amplified at 3 levels by
Semi lunar folds (and haustra)
Crypts of Lieberkuhn (NO villi)
Microvilli
LI vs SI pathologies?
SI - voluminous/large volume
LI - less fluid but more frequent fluid loss
Does active Na absorprtion occur in the LI and SI?
Yes in both
Does active K+ secretion occur in SI and LI?
NOT in SI only in LI
Does nutrient absorption occur in SI and LI?
SI yes LI no - the absorption of non-electrolyte nutrients occurs in the SI while BOTH the SI and LI absorb water and electrolytes (na/Cl)
What is the net absorption/secretion in the SI/LI
- SI absorbs net amounts of water, Na, Cl and K and secretes HCO3-
- the LI absorbs net amounts of water, Na, CL and secretes K and HCO3-
Net amounts of secreted and absorbed fluid?
Saliva - 1.5 L Gastric secretion - 2 L Bile - 0.6 L Abs SI - 6.5 L Abs LI - 1.9 L Faecal fluid - 0.1 L
Na/K ATPases
- basis of membrane transport and membrane potential
- ACTIVELY remove 3 Na+ and bring in 2 K+
- use ATP
- means there is always a deficiency in cell
- conc and electrical gradient
- gradient for action and membrane potentials, transport of AA and glucose
Describe the transport of glucose in the cell?
- Na/K atpase (apical)
- SGLT
- GLUT (basolateral)
Block Na/K ATPases?
- diminish conc gradiet
- no glucose move into cells
- cells starve
- membrane potential affected as normally driven by sodium (intracellular is negative - electrical gradient)
- electrical gradient (membrane potential) lost if ATPases lost
Transepthelia movement of water is either
Paracellular or transcellular
Transcellular
Across 2 membranes in series
At least 1 membrane is active
The other is usually facillitated by solutes like glucose or AAs i.e. the active membrane creates an osmotic gradient for water to follow to osmotic movement
paracellular
Moves passively between cells via tight junctions
Paracellular movement of water predominates
Absorption of water
- Is entirely by osmosis
- Often coupled with solute movement
Majority of water absorption occurs in the
Jejunum of the SI
How does water help sodium and urea absorption in the jejunum
Solvent drag allows for considerable uptake of urea and sodium at the jejunum
Where does sodium absorption occur
The SI villus epithelial cells and surface epithelial cells of the LI
How is sodium absorbed
The transcellular absorption of sodium is mediated by the Na/K atpase at the BL surface which maintains a low intracellular conc
This provides the gradient/energy for Na movement by diffusion from the lumen into the cell across the apical surface and then into the ECF (by ATPase)
The apical movement of Na is mediated by Na coupled transporters OR Na channels
How does sodium move across the apical surface
either by cotransporters or sodium channels including..
- Na/glucose co transporters
- Na/H EXCHANGERS (H+ in duo to counteract bicarbonate)
- Na/H AND Cl-/HCO3- exchangers (Na and Cl in cell, HCO3- and H+ out of the cell)
- Epithelial Na channels
Where are you going to find Na epithelial channels
They are very specific and found in the distal colon
Sparse and only contribute to 5% of sodium absorption
How is most sodium absorbed across the apical membrane
Glucose co transporter
What CONTROLS sodium absorption etc
Sodium needs to be controlled as it is important in maintaining osmolarity, structure of the cell and enable solutes to move in/out of the cell
Controlled by aldosterone
