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Renal blood flow

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Title: Renal blood flow  
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Renal blood flow

Parameter Value
renal blood flow RBF=1000 ml/min
hematocrit HCT=40%
renal plasma flow RPF=600 ml/min
filtration fraction FF=20%
glomerular filtration rate GFR=120 ml/min
urine flow rate V=1 mL/min
Sodium Inulin Creatinine PAH
SNa=150 mEq/L SIn=1 mg/mL SCr=0.01 mg/ml SPAH=
UNa=710 mEq/L UIn=150 mg/mL UCr=1.25 mg/mL UPAH=
CNa=5 mL/min CIn=150 ml/min CCr=125 mL/min CPAH=420 ml/min
ER=90%
ERPF=540 ml/min

In the physiology of the kidney, renal blood flow (RBF) is the volume of blood delivered to the kidneys per unit time. In humans, the kidneys together receive roughly 22% of cardiac output, amounting to 1.1 L/min in a 70-kg adult male. RBF is closely related to renal plasma flow (RPF), which is the volume of blood plasma delivered to the kidneys per unit time.

While the terms generally apply to arterial blood delivered to the kidneys, both RBF and RPF can be used to quantify the volume of venous blood exiting the kidneys per unit time. In this context, the terms are commonly given subscripts to refer to arterial or venous blood or plasma flow, as in RBFa, RBFv, RPFa, and RPFv. Physiologically, however, the differences in these values are negligible so that arterial flow and venous flow are often assumed equal.

Renal plasma flow

Renal plasma flow is the volume of plasma that reaches the kidneys per unit time. Renal plasma flow is given by the Fick principle:

RPF = \frac{U_x V}{P_a - P_v}

This is essentially a conservation of mass equation which balances the renal inputs (the renal artery) and the renal outputs (the renal vein and ureter). Put simply, a non-metabolizable solute entering the kidney via the renal artery has two points of exit, the renal vein and the ureter. The mass entering through the artery per unit time must equal the mass exiting through the vein and ureter per unit time:

RPF_a \times P_a = RPF_v \times P_v + U_x \times V

where Pa is the arterial plasma concentration of the substance, Pv is its venous plasma concentration, Ux is its urine concentration, and V is the urine flow rate. The product of flow and concentration gives mass per unit time.

As mentioned previously, the difference between arterial and venous blood flow is negligible, so RPFa is assumed to be equal to RPFv, thus

RPF \times P_a = RPF \times P_v + U_x V

Rearranging yields the previous equation for RPF:

RPF = \frac{U_x V}{P_a - P_v}

Measuring

Values of Pv are difficult to obtain in patients. In practice, PAH clearance is used instead to calculate the effective renal plasma flow (eRPF). PAH (para-aminohippurate) is freely filtered and it is not reabsorbed within the nephron. Although freely filtered not all PAH crosses into the primary urine within Bowman's capsule. PAH remaining in the vasa recta or peritubular capillaries is taken up actively by epithelial cells of the proximal convoluted tubule and secreted into the tubular lumen. In this way PAH, at low doses, is completely cleared from the blood during a single pass through the kidney. Accordingly, the venous plasma concentration of PAH is approximately zero. Setting Pv to zero in the equation for RPF yields

eRPF = \frac{U_x}{P_a} V

which is the equation for renal clearance. For PAH, this is commonly represented as

eRPF = \frac{U_{PAH}}{P_{PAH}} V

Since the venous plasma concentration of PAH is not exactly zero (in fact, it is usually 10% of the PAH arterial plasma concentration), eRPF usually underestimates RPF by approximately 10%. This margin of error is generally acceptable considering the ease with which PAH infusion allows eRPF to be measured.

Finally, renal blood flow (RBF) can be calculated from a patient's RPF and hematocrit using the following equation:

RBF = \frac{RPF}{1-Hct}

References

Bibliography

External links

  • Renal Clearance Techniques
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