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
Defect in Basolateral Na/HCO3 contr - PT
Proximal RTA
Defect in Apical Na/K/2CL co tran (TAL)
Bartter t1
Defect in Apical Na-Cl contra on distal dubule
Gitelman’s
Function of proximal tubule
o Bulk of reabsorption of solutes to up to 80%
Water 65% reabsorbed
AA, low molecular weight proteins – 100%
Thick ascending limb
active transport of sodium takes place, impermeable to water
Solute transport in descending limb
water by osmosis
Solute transport in ascending limb
secondary active transport of Na, K and Cl
Paracellular transport of Na, Ca and Mg down an electrochemical gradient
Loop diuretics
TAL
- Inhibits 2cl
Function of distal nephron
o K excretion
o Regulation of Na delivery to collecting duct
o Urine acidification – maintain acid-base balance
Function of ADH
In the collecting tubule
o Aquaporins become permeable to water
o Passage of water from collecting tubule to interstitium down a concentration gradient
o Production of concentrated urine
ADH deficiency
copious dilute urine i.e Diabetes insipidus
Renal threshold
concentration of a substance dissolved in the blood above which the kidneys begin to remove it into the urine
