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
