Renal Physiology: Body Fluid comp. Flashcards
How much water is in each compartment?
2/3 is intracellular fluid
1/3 is extracellular fluid, where 80% of this is in the interstitum and 20% in the plasma
Osmolality vs Osmolarity
Osmolality: the number of osmotically active particles per unit of solvent (water) (mosmol/kg). Determines the osmotic pressure.
Osmolarity: the number of osmotically active particles per litre of total solution.
What is tonicity
Describes the osmotic pressure a solute exerts.
Only accounts for osmotically active impermeable solutes. Tonicity is thus a property of a solution in reference to a particular membrane.
Thus hypotonic, low pressure, cell swells
hypertonic, high pressure, water leaves cell, and it shrinks
What is the Gibbs Donnan equilibrium
Charged particles separated by a partially permeable membrane can fail to distribute evenly across the membrane in the presence of a non diffusable ion (eg a protein) (diagrams pg 15/15)
However essentially due to the negative proteins in a cell, more negative charges outside will go into cell down conc. gradient, meaning a more negative inside of the cell. A voltage gradient.
ALSO more osmotically active particles are inside the cell, this creates the ONCOTIC pressure
How does the cell counteract the osmotic pressure?
The Na+/K+ ATPase pumps out OSMOTICALLY active ions. This means that inside potassium and proteins are balanced by extracellular sodium, ISOTONIC
How does the osmolality differ between ICF and ECF?
despite different compositions, the osmolalities are the same, everything is balanced.
How do our bodies resolve hypotonic and hypertonic solutions (ECF) (osmolality maintenance)
Hypotonic(removed Na+): remove water to conc. Na+
Hypertonic (added Na+): add water
So osmolality is largely regulated by renal water levels.
Osmolality is tightly regulated, if salt levels change by one or two percent ded.
Maintained by ADH
How is ECF volume changed
ECF volume less tightly controlled
volume dependent on amount of renal sodium
i.e if we eat more salt, more water to balance
if we eat less salt, less water drunk to balance
maintained by RAAS
to maintain ECF volume, Na+ excretion must match sodium input
High sodium diet explained
Transient increase in plasma osmolality
increased renal salt excretion few days lag.
increased thirst (water retained to maintain osmolality)
osmolality returns but at expense of greater volume
larger ECF volume stay with whilst high sodium diet
ECF volume will return to normal if sodium intake lowers or renal excretion increases
as ECF volume increases, renal Na+ loss increases and BP increases to return volume to baseline
IV water, saline, glucose
IV water, will add to the blood volume, lower osmolality, so body gets rid of the water
IV saline: same osmolality as the body, so no change in body osmolality. So increase in total volume but not osmolality. No need to lose liquid as 15% volume buffer
IV glucose: metabolised to water so will dilute all compartments
