JH renal final index cards exam I(1) Flashcards
What is the osmotic coefficient?
function of particle interactions in solutions, which decreases the effective #s of osmoles
What are effective osmoles?
impermeable solutes that can sustain osmosis
What are ineffective osmoles?
permeable solutes that cannot sustain osmosis
reflection coefficient = 100%
particle is reflected back 100% of the time; IMPERMEABLE
reflection coefficient = 0
particle is as permeable as water
Describe the Donnan Effect
behavior of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly across the two sides of the membrane. The usual cause is the presence of a different charged substance that is unable to pass through the membrane and thus creates an uneven electrical charge. Ex: the large anionic proteins in blood plasma are not permeable to capillary walls. Because small cations are attracted, but are not bound to the proteins, small anions will cross capillary walls away from the anionic proteins more readily than small cations.
What is the 60-40-20 rule?
Total Body Water (TBW) = 60% of body weight
ICFV = 40% of body weight
ECFV = 20% of body weight
ISFV = 15% of body weight (or 3/4 of ECFV)
PV = 5% of body weight (or 1/4 of ECFV)
Two reasons that account for the high water permeability of cell membranes
1) lipid bilayer has a small, but not negligible water permeability. Since whole cell surface is available for the transport, there is significant water transport
2) presence of aquaporins, which increase the inherent water property of the cells
What increases the driving force for water entry via Donnan effect in the cell?
1) presence of high intracellular concentrations of macromolecules and metabolic intermediates
2) membrane is impermeable to these molecules, but permeable to water –> results in a significant driving force for osmotic water entry
What is the active process that counters the tendency of cells to swell? What is the net result on ICF and ECF?
Na/K ATPase - net efflux of Na from the cell in order to maintain cell volume; net result: effective osmolality in ICF becomes equal to that in ECF
T/F @ steady state ICF osmolality = ECF osmolality
True.
T/F @ steady state Plasma osmolality = ISF osmolality
False. Plasma is slightly > than ISF due to the presence of plasma proteins
What is the main determinant of plasma osmolality
Na
What are the effective osmoles maintained in the ECF and ICF?
ECF: Na and associated anions
ICF: K and associated anions
T/F Plasma proteins exert a Donnan Effect
True. They’re negatively charged and can attract counterions.
Why is the concentrations of Cl- and HCO3- higher in the ISF?
Donnan Effect. Plasma proteins attract small cations, therefore the concentration of the small cations is 5% higher in the aqueous phase than in the interstitial fluid, and the concentration of small Anions is 5% lower.
How do you figure out the effective osmolality?
2 * [Na]
How do you figure out total plasma osmolality?
2[Na] + [Glucose]/18 + [BUN]/2.8
What is the osmolar gap? What does it mean if it’s increased?
Osmolar gap = measured osmolality - calculated osmolality. An increased osmolar gap indicates the presence of a toxin that contributes to the osmolality
What determines the size of any given compartment (ie ECFV, ICFV, etc)
of osmotically active particles present
What accounts for the decrease in Hct after eating a salt-laden meal?
1) Since Na is restricted to the ECF, this draws fluid out of the cells until ECF=ICF osmolality (but the total number of osmoles is higher in the ECF)
2) ECFV is increased, and this extra fluid is redistributed between the ISFV and plasma in a 3:1 ratio since the endothelium is freely permeable to Na.
3) Hct decreases because a) plasma volume is increased and 2) increased osmolality of the plasma (due to ingestion of Na) results in cell shrinkage
What are the effective osmoles (main determinant of oncotic pressure) in the plasma?
albumin
What are the two opposing forces on fluid movement in the capillaries?
hydrostatic pressure (promotes fluid exit) and oncotic pressure (draws fluid in)
What is the effective oncotic pressure dependent on?
reflection coefficient of the capillary membrane for protein.