H4i: Describe the principles of measurement of renal blood flow
- Renal blood flow (RBF) is calculated from RENAL PLASMA FLOW & HCt
- \( RBF = \frac{RPF} {1-HCt}\)
- RENAL PLASMA FLOW is estimated from clearance of PAH
- PAH is a ‘water soluble anion’ of 194 Da
- Freely filtered, but also secreted up to a TMAX
- ∴ The clearance of PAH can estimate RPF
- \( Clearance= \frac{\text{Urinary [PAH] & Volume of urine} } {\text{Plasma [PAH]}}\)
= GFR → normally 125mL/min
- HCt = 0.45
- \( ∴RBF = \frac{RPF} {1-HCt}\)
- Normal RBF = 600mL/kidney/minute = 20% CO!
- Far exceeding metabolic demands of kidneys, but rather emphasising its role in filtration & regulation of plasma content