Lecture 20: Salt and water Flashcards
Describe the water compartments:
ICF (2/3)
ECF (1/3) = Interstitial fluid (80%) and plasma (20%)
What is osmolality?
Osmolality is used clinically and is the number of osmotically active particles per a unit weight of solvent…. osmoles per kg of water
Osmolarity is number of osmotically active particles per litre of total solution (Osmol/L)
Molal and molar are very similar between the two i.e theyre much of the same.
What is tonicity?
NOT OSMOLALITY
Describes the osmotic pressure a solute exerts across a cell membrane (thereby causing movement of water)
What does tonicity account for?.
Accounts ONLY for osmotically active IMPERMEABLE solutes i.e proteins rather than all osmotically active solutes.
- In reference to a particular membrane (osmolality is indp. of a membrane)
- Not measurable
Describe the permeability of the plasma membrane and how this relates to tonicity:
Plasma membrane of cells is semi permeable and thus is permeable to water but NOT permeable to charged molecules
i.e cells are full of proteins which are osmotically active but impermeable to the membrane
Define hypotonic, isotonic, hypertonic
Hypotonic: Makes cells swell (water moves into the cells)
Isotonic: Water equilibrium, cell remains same
Hypertonic: Makes cells shrink (Water moves out of the cell)
What is the gibbs-donnan equilibrium:
In the presence of a non-diffusible ion i.e protein, charged particles can fail to distribute evenly across a semi-permeable membrane
What is the gibbs-donnan equilibrium responsible for?
Competing electrical and concentration gradients mean that at equilibrium the side with the proteins is more negatively charged = voltage gradient
More osmotically active molecules are on the protein side (greater osmolality) therefore water flows into the protein side (oncotic pressure)
How does ECF and ICF osmolality compare?
Different compositions but osmolality identical
ICF: Lots of negatively charged protein and K+
ECF: Lots of Na
What does hypotonic ECF cause?
Cells to swell via osmosis
What is ECF osmolality dominated by?
Na
What does ECF hypertonicity cause? and what is the implication of this
Cells to shrink via osmosis
ECF osmolality control is critical. ECF osmolality is largely regulated by altering water levels
What is the variation of osmolality of the ECF?
1-2%, tightly regulated via water
Is ECF volume tightly controlled?
Less tightly controlled, 15% varitiation….
i.e gain Na = gain water and vice versa
What is the major regulator of salt and water? and what is an additional regulator..
The kidney i.e ECF osmolality and volume
Additional: Starling forces = movement b/w vasculature and ECF, oncotic and osmotic pressures..
What is an example of abnormal interstitial fluid compartment expansion?
Oedema, localised or general - results from a change in starlings forces
What happens following a five day high salt diet?
Salt excretion is delayed compared to intake therefore water is retained to maintain ECF osmolality
Summise the impacts of a five day high salt diet:
- Transient increase in plasma osmolality
- Increased renal salt excretion (but few days lag)
- Increased thirst
- Plasma osmolality returns to normal but at expense of larger ECF volume
- Larger ECF continues while high salt diet continues
- ECF volume returns to normal if less Na is ingested or renal Na excretion increases i.e diuretic
Describe how increased ECF volume contributes to Na regulation
- As ECF volume increases, BP increases and renal Na increases (Pressure natriuresis) (helps resotre ECF volume back to baseline)
Describe the rate of clearance of free water vs saline
Free water is cleared very rapidly,
saline takes a long time to be cleared.
What is the value of giving 5% dextrose?
Glucose is metabolised or becomes bound to glycogen. So infusing 1L of 5% dextrose will ultimately dilute all compartments (as it causes water to be shifted into cells)
Whats the value of isotonic saline?
Temporarily expands ECF (doesnt go into ICF because isotonic) (given to expend circulating volumes and replace actual fluids losses)
What is the value of hypotonic saline fluids such as 0.45fi NaCl?
Hypotonic saline fluids such as 0.45% NaCl dilute the ECF first then expand the intracellular compartment causing cells to swell.
What is normal plasma osmolarity?
285-295 mOsm/L
What is GFR normally?
~125ml/min
Whats the action of aldosterone and what stimulates it?
- Increase Na reabsorption in DCT and CD
- Stimualted by ANG2 and Potassium
Whats the actions of ANG2?
- Vasocon
- ADH release stim
- Na reabsorb PCT
- Thirst
- Lowers GFR by contracting mesangial cells (reducing area for glom filtration)
- BUT also increases GFR by constricting afferent art.
- Stimulates aldo release
What are some NA reference ranges?
135-145 mmol/L norm
What can result in hypernatreamia?
- Impaired thirst / level on consciousness
- No access to fluids
- Burns/diarrhoea/blood loss
- Solute diuresis (HONK/DKA)
What is diabetes insipidus?
Central = no ADH (thus dilute urine)
Nephrogenic = ADH receptors or ADH doesnt work.
What can cause hyponatraemia?
- Excessive Na loss
- Excessive water retention
How can you tell if someone has psuedohyponatraemia?
- Serum osmolarity is normal despite apparent low Na
- Can occur in hyperglyceamia which draws water into bloodstream and ‘dilutes’ Na
What should you check in hyponatraemia?
Urine osmolarity, very low osmolarity indicates water intoxication/polydipsea.
What happens in hypovolaemia to Na and water?
Na loss but relatively less water loss.
- Urine Na <20mmol/L
i.e gives appearance of hyponatraemia
What can lead to hypovolaemia?
- DnV
- Bowel obstruction
- Skin loss, sweating
- Urinary loss b/c
-> Diuretics
-> Addisons
-> Osmotic diuresis
What happens in hypervolaemia to Na and water?
Na retention but relatively more water retention
i.e gives appearance of hyponatraemia
What can result in hypervolaemia?
- Cirrhosis
- Nephrotic syndrome
- Heart failure
- Renal failure
What euvolaemic conditions can result in hyponatramia?
- Syndrome of inappropriate ADH
- Endocrinopathies
- Diuretics
- Fluid replacement
What is SIADH?
- Inappropriate ADH produced in absence of stimuli
- Lowers plasma osmolarity
What can cause SIADH?
- Trauma
- Tumors
- Chronic lung disease
- Head injury
- Medications (SSRIs)
What do the symptoms of hyponatraemia vary with the speed of onset?
Slow -> Brain adaptation (corrects cerebral oedema), confused, not quite self
Rapid -> Cerebral oedema, confusion, seizures, coma
How do you treat a rapid onset hyponatramia?
Rapid onset:
- Fluids restrict
- Normal saline or 3% if in ICU
How do you treat slow onset hyponatraemia?
- Correct gradually
- Generally just fluid restriction
- No more than 8mmol/day
Whats a potential consequence of rapidly correct hyponatraemia?
Brain dehydration -> Central pontine myelinolysis
Compression of myelin sheaths - Rapid demyelination
-> Quadraparesis
-> Pseudobulbar palsy
-> Locked in syndrome
-> Irreversible
When someone is hyponatraemic what is the first step of analysis?
Check their volume status!
