Body Fluids and Clearance Flashcards
body water fraction in males and females
60 in men, 50 in women
water distribution across comparments
2/3 intracellular
1/3 ECF- 1/4 of which is plasma, 3/4 is interstitial
content difference in plasma and interstitium
much higher protein in plasma, but ions equillibriate
define osmolality
mOsm/kg, number of particles in kg of fluid
calculate total osmolality
osm= 2Na+ glucose/18 + BUN/2/8 + EtOH/3.7
what is tonicity, how to calculate
tonicity=effective osmolality, component of osmolality that contributes to osmotic force (must not permeate thru barrier)
2Na + glucose/18
usual osmolalty of ICF/ECF
usually b/w 275-295, ECF can change transiently in response to changes in osmolality/tonicity from Na or H2O fluctuations- this affects ICF
what is net excretion
filtration + secretion - reabsorption
normal GFR
120 mL/min adult males, 95 in females
filtered load
GFR x Py (plasma concentration)
excreted load
V (urine flow) x Uy (urine concentration)
reabsorption
filtered load- excreted load
secretion
excreted load- filtered load
clearance of y
VxUy/Py= excreted load/plasma concentration
requires steady state of plasma concentration
when does clearance = GFR
when everything filtered is excreted, excreted load =filtered load, no secretion or reabsorption
GFR= UV/P
why is creatinine good for GFR measurement
relatively constant production, disposed by filtration, non invasive (unlike inulin)
describe changes in plasma and urine creatinine following acute change in GFR
early- GFR falls as does filtered and excreted load of Cr, plasma Cr is the same
later- GFR still low, but plasma Cr has increased due to lower excretion (constant production), the proportional rise in plasma Cr brings filtered rate back to where it was
new steady state
para amino hippuric acid approximates which value
renal plasma flow- all that enters kidney is excreted via filtration and secretion (90%)
calculate RPF and RBF for PAH
RPF= (UV/P)/0.9 (extraction ratio)
RBF= RPF/1-Hct
filtration fraction
fraction of plasma that is filtered:
FF= GFR/RPF
3 layers to filtration barrier
from lumen of capillary: endothelial cell, basement membrane, podocytes w/ foot processes
what determines which things are filtered
charge and size: less than 40 A are filtered, 20 A are freely filtered
negative things less filtered (negative basement membrane)
ions freely filtered
how is glomerular hydrodynamics different from other capillaries
high hydrostatic pressure, more than COP throughout (despite increasing COP as plasma becomes more concentrated)
arterioles on both sides, no venous end
how is GFR maintained despite range of blood pressures- 3 main ways
autoregulation- arterioles resist stretching to maintain steady blood flow, RAAS system activated by low pressure in afferent and low Cl delivery to macula densa and sympathetic innervation
increased AII and NE constricts efferent to raise GFR, also stimulates prostaglandins that work in opposite direction
macula densa can dilate the afferent arteriole during low Cl delivery to raise GFR, tubuloglomerular feedback
describe the renal response to hypovolemia/hypervolemia
RAAS activation, AII causes efferent constriction, increased FF, COP in capillary increases, more PCT reabsorption
opposite of all this occurs in hyper
urinary obstruction effect on GFR
increased hydrostatic pressure in bowmans space, opposes filtration so GFR goes down
NSAIDs impact on GFR
block prostaglandin synthesis (normally afferent vasodilation) which oppose AII and NE effects, causes more afferent vasoconstriction and lower GFR
meaning of : CxGFR
clearance greater than GFR= net secretion
less than = net reabsorption
equals GFR= no net secretion or reabsorption
key factors for concentrated urine
medullary gradient: NaCl reabsorption w/o water in ascending limb, slow flow
ADH in plasma, response in CD
nephron lenght and enough solute intake
which segment has most water uptake
CCD
why impaired urinary concentration in sickle cell
loss of vasa recta, effectively shortens loop of nephron and leads to polyuria and nocturnal enuresis in young pts
fractional excretion equation
amount excreted/filtered load
(Ux/Px)/ (Ucr/Pcr) x 100
use of fractional excretion
assess renal handling of specific solute like Na
differentiate AQP1, 2, and 3
1: PCT, descending limb, not ADH regulated,apical and basolateral
2: DCT, CCD and MCD, ADH regulated on apical memrane
3: DCT, CCD and MCD on basolateral, constitutive
why do small increases in CO2 cause major pH drops
change in H+ has bigger effect than the increase in bicarb, way less H+ in circulation
