18 - Renal Excretory Function Flashcards
How much blood is filtered a day
how much urine is excreted
180 L filtered, 2 L excreted/day
What is produced at bowman’s capsule
Ultrafiltrate of plasma
What factors determine filtrate
o net filtration pressure – higher pressure in venous system to push water to bowmans capsule
o Podocyte slit pores
o Size + charge of molecule
= Negative charge of GBM glycoproteins
What molecules can move freely at the glomerulus
Water, electrolytes e.g na,k, cl, phosphate, glucose, urea, amino acids
What molecules are restricted at the glomerulus
larger solutes/proteins
o MW cut off is 5200 daltons
o Albumin MW 69,000 daltons
What is the glomerular filtration rate
clearance of substance
to assess kidney function
What is used to measure GFR
o Creatinine is used – freely filtered by the glomerulus
Peritubular capillaries secrete small amount
(OVERESTIMATES BY 10-20%)
Measurement of creatinine clearance
o Cr clearance = urine conc x urine volume/ plasma concentration
Collect urine for fixed time to get volume. E.g. 24 hours
• A long time !!
Other measurements of kidney function
Nuclear medicine scan (GS)
eGFR
What is Nuclear medicine scan
o Cr51 – EDTA
o Iothalamate excretion
Estimated GFR
o MDRD equation – Needs creatinine, age, sex, race,
o eGFR (ml/min per 1.73m2) = 186 x ( Creat(mmol/ml)/88.8)-1.154 x (Age) -0.203 X (0.742 if female) x (1.210 if black)
o Cockroft Gault equation – not used
When would GFR underestimate
o Creatinine produced by muscles, muscular people –> raised serum creatinine –> underestimates true GFR. (vice versa for malnourished)
Where is Na/K+ atpase
basolateral
Na out and K+ in
Defect in Apical Na/Cystine cotransporter (PT)
Cystinuria
Defect in Apical Na/glucose cotransporter (PT)
Renal glycosuria
