Renal circulation

Some kidney-specific figures

• 0.5% of body weight
  * ~300g
  * same as heart
• 25% of cardiac output [KB2:p73]
  * 20% of CO [BL8:p258; AV6:p25]
  * 23.3% of CO [WG21:p615]
• 7.2% of body oxygen consumption

 

Renal blood flow

Renal blood flow

= 25% of cardiac output

= 1250mLs/min

Cortex vs medulla

• Cortex receives 95% of the blood flow
• Medulla receives 5% of the blood flow

 

Renal oxygen consumption

Renal oxygen consumption

= 7.2% of body oxygen consumption

= 18mLs O2/min

 

Oxygen extraction ratio

Oxygen extraction ratio
= 7.2%

This is coincidental because

Oxygen flux
= 250mLs O2
= body oxygen consumption

However,

Using slightly different numbers,

Blood flow is 24% of cardiac output
= 1200mL

Oxygen flux
= 240mL O2/min

Oxygen extraction ratio
= 18/240
= 7.5%

Measurement

Renal blood flow is measured using para-aminohippurate (PAH)

• PAH is excreted almost completely in urine
  * No tubular resorption, secretion
• Renal clearance of PAH is almost equal to the renal plasma flow
  * Renal blood flow can be calculated if Hct is known

Equations

Renal plasma flow
= PAH uptake / [PAH] difference in renal arterial and venous blood

But because clearance is not 100%, therefore...

Effective plasma flow
= Urinary [PAH] x urine volume per unit time / venous [PAH]

Extraction of PAH is about 90%
--> Plasma flow x 0.9 = Effective plasma flow

Other notes

• pO2 in renal medulla = 20mmHg
• See renal for more info on regulation of renal blood flow
• Pressure autoregulation is between MABP of 75mmHg to 160mmHg [KB2:p74] OR 90mmHg and 200mmHg [Vander]