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
Cl- absorption
Tends to move with Na+ to neutralise
- Passive absorption/electrical gradient with Na+ (secondary active sodium movement sodium across apical membrane causes intracellular to be positive so CL follows to maintain membrane potential)
- Cl/HCO3- exchanger
- Na/H and HCO3-/Cl parallel exchangers
Where is chloride absorbed
jejunum, ileum and proximal colon
Is chloride secreted and absorbed passively or actively
secreted ACTIVELY
absorbed passively with sodium
How is chloride secreted
BL : na/k ATPase powers
Na/K/2Cl- CO transporter to bring Na, K, Cl into cell
Apical :CFTR facilitates passive diffusion of cl out of the cell into the lumen
The movement of Cl- into the lumen, osmotically drags Na and H20 through paracellular movement FROM the ECF
(movement of Cl causes an osmotic drag - can see how an increase in Cl secretion from CTFR would cause increased Na H20 paracellular secretion from the ECF causing secretory diarrhoea)
Is sodium absorbed or secreted
ABSORBED in both si and li
What can cause excessive cl secretion
- cholera toxin causes excessive cAMP to bind to and stimulate CTFR (like secretion normally does)
- cAMP
- Ca
How does potassium move in the gut
- absorbed in SI
- Secretion in LI
Passive movement
What 3 systems control absorption and secretion in the gut
- Enteric nervous system
- Endocrine system
- Paracrine system
How does the Enteric nervous system, Endocrine system and Paracrine system control abs/secretion
- enteric releases ACh, VIP and other secratogogues
- endocrine - aldosterone
- paracrine - 5HT
What does the enteric system release
Ach
VIP
Other secretagogues
Solvent Drag?
Where the solute may be moved by coupled to fluid movement as it is swept away by the bulk movement of the solute
Anything that causes an increase in secretion or decrease in absorption can cause
diarrhoea
Def of diarrhoea
more than 200g/0.2L or more than 2 liquid bowel movements a day
Diarrhoea is the .. most common cause of infant death worldwide
2nd
Difference between osmotic and secretory
osmotic - Is a disturbance of absorption
- often due to enzymes malfunction so increase in osmotic particles/load
secretory - is a disturbance of secretion
- increased channel activity and fluid loss from the body
- often due to infection or tumour
Osmotic Diarrhoea
Macronutrient malabsorption causes an increase in osmotic load in lumen cause water to be retained
- Can occur is pancreatic disease/enzyme insufficiency, large intake of sugar or alcohol, intolerances, celiac
- lactose intolerance (more than 5% of the world more common in asia and africa)
Secretory Diarrhoea
- an increase in active secretion
How does cholera/E.Coli cause secretory diarrhoea
- cholera/e coli most common causes
- release an enterotoxin that causes an increase in cAMP/cGMP/Ca++
- stimulation of CFTR
- Cl secretion and paracellular H20 and Na movement to maintain electrical and osmolarity balance
Besides cholera what else can cause secretory diarrhoea
Can get an absence of the CL/hco3 exchanger
How does secretory diarrhoea impact Na absorption
No impact on nutrient absorption or Na coupled co exchangers
Basis for ORT
Treat with gluc/na/h20 to utilise these co transporters to get water back into the ECF
What kind of fluid loss does secretory diarrhoea cause
ISO-OSMOTIC fluid loss
- essentially tap from ECF as losing both electrolytes and water some osmolarity
- this fluid is coming from the blood resulting in patients quickly becoming dehydrated
What does secretory diarrhoea result in
- loss of ECF/blood plasma
- loss of blood volume (and venous return so can lead to shock)
- decrease in blood pressure
- increase in HR as compensated for by baroreceptors and increase in sympathetic stimulation
What does ORT consist of any why
- water (rehydrate fluid loss)
- na (enable uptake via co transport gradients to restore Na and glucose loss)
- glucose
- HCO3 - losing fluid/ECF so losing the HCO3 that is usually secreted. This means the pH drops so need to restore HCO3- to prevent acidosis
