renal physiology equations Flashcards
renal clearance equations
Cx = UxV/Px = volume of plasma from which a substance is completely cleared per unit time. Cx is clearance of x (mL/min). Ux - urine concentration of x (mg/mL). V - urine flow rate (mL/min). Px = plasma concentration of X (mg/mL).
Cx and its relationship to GFR; implications
Cx less than GFR: net tubular reabsorption of X
Cx greater than GFR: net tubular excretion of X
Cx equal to GFR: no net secretion or reabsorption
glomerular filtration rate: equations, measurement
use inulin clearance (can also be used to measure extracellular volume).
inulin is a good measure of GFR becasue it is freely filtered and not rebasorbed or secreted.
GFR = Uinulin X V/ Pinulin.
GFR = Kf [(Pgc -Pbs)- (oncotic pressure gc - oncotic pressure bs)]
What is the relationship between creatinine clearance and GFR?
creatinine clearance is an approximate measure of GFR. it slightly overestimates GFR becasue there is some secretion of creatinine in the tubules.
effective renal plasma flow; renal blood flow. limitations
ERPF can be estimated using para-aminohippuric acid (PAH) clearance. PAH is both filtered and actively secreted in the proximal tubule. nearly all PAH entering the kidney is excreted.
basically, helps us know about the vol of blood delivered to the kideny per unit time by looking at clearance.
(ie. all PAH that gets to the kidney will leave in the urine).
ERPF - UpahX V/Ppah. Upah is the urine conc. of PAH; Ppah is the plasma concentration.
renal blood flow = renal plasma flow / (1-Hct).
ERPF underestimates true renal plasma flow by about 10%.
filtration fraction; filtered load. what substances are used?
fraction of blood flow to the kidney that is filtered: GFR/RPF. normally around 20%.
remember, GFR is estimated with creatinine (or inulin), RPF is estimated with PAH clearance.
filtered load (mg/min) = GFR (mL/min) X plasma concentration (mg/mL)
describe blood flow into the kidney. What factors influence renal blood flow? How do changes in arterioles affect renal plasma flow, GFR, and filtration fraction
blood comes into the afferent arteriole and leaves via the efferent arteriole.
prostaglandins dilate the afferent arteriole. RPF increases, GFR increases, filtration fraction is constant. NSAIDS will constrict the afferent arteriole.
angiotensin II preferentially constricts the effecent arteriole. RFP decreases, GFR increases, FF increases.
changes in glomerular dynamics on RPF, GFR, and FF. look a incr. and decr. plasma protein conc, constriction of the ureter, and afferent and efferent areteriolar constriction
incr plasma protein: no change, decr, decr.
decr. plasma protein: no change, incr., incr.
ureter constriction: no change, decr. decr.
afferent constriction: decr., decr, no change
efferent constriction: decr., incr, increase
calculation of reabsorption and secretion rate
filtered load = GFR X Px
excretion rate = V X Ux (v is urine flow rate)
reabsorption = filtered load - excretion rate
secretion = excretion rate - filtered load
amino acid clearance and pathology
sodium-dependent transporters in PCT reabsorb amino acids. this is secondary active transport. Na is co-transported.
hartnup disease: autosomal recessive disorder with a deficiency of neutral amino acid transporters in the PCT and on enterocytes. there is decreased absoprtion from the gut and decreased resorption from the kidney. causes a neutral aminoaciduria and pellagra like symptoms (niacin is derived from tryptophan).
treat with a high protein diet and nicotinic acid.
glucose clearance; normal variations.
glucose at normal plasma levels is completely reabsorbed in the proximal tubule by Na/glucose cotransport. at a plasma glucose of about 200 mg/dL, glucosuria begins. at 375, all glucose transporters are fully saturated.
normal pregnancy cause decreased reabsorption of gluocse and amino acids in the proximal tubule. patients may have glucosuria and aminoaciduria.
