Renal blood flow
In the physiology of the
kidney, renal blood flow (RBF) is the volume of blooddelivered to the kidneys per unit time. In humans, the kidneys together receive roughly 25% of cardiac output, amounting to 1 L/min in a 70-kg adult male. RBF is closely related to renal plasma flow (RPF), which is the volume of blood plasmadelivered 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 blood flow calculations are based on renal plasma flow and
hematocrit(HCT). This follows from the fact that hematocrit estimates the fractional volume of blood consumed (occupied) by red blood cells. Hence, the fraction of blood that is in the form of plasma is given by 1-"HCT".
Renal plasma flow
Renal plasma flow is given by the
This is essentially a
conservation of massequation which balances the renal inputs (the renal artery) and the renal outputs (the renal veinand 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:
where "Pa" is the arterial plasma concentration of the substance, "Pv" is its venous plasma concentration, "Ux" is its
urineconcentration, 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
Rearranging yields the previous equation for RPF:
Values of "Pv" are difficult to obtain in patients. In practice,
PAH clearanceis used instead to calculate the effective renal plasma flow(eRPF). PAH ("para"-aminohippurate) is freely filtered, and it is not reabsorbed by the kidney so that its venous plasma concentration is approximately zero. Setting "Pv" to zero in the equation for RPF yields
which is the equation for
renal clearance. For PAH, this is commonly represented as
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 eRPF is measured.
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