Renal Blood Flow
The kidneys receive approximately 15% to 25% of total cardiac
output, or 1.0 to 1.25 L of blood per minute through the renal arteries, depending
on the state of the body. Most of the blood is received by the renal cortex, with
only 5% of cardiac output flowing through the renal medulla, which makes the renal
papillae vulnerable to ischemic insults. Renal blood flow is regulated by a variety
of mechanisms that control the activity of vascular smooth muscle and thereby alter
vascular resistance. The sympathetic tone of renal vessels increases during exercise
to shunt renal blood flow to exercising skeletal muscle; similarly, renal blood vessels
relax during the resting condition of the body. Sympathetic stimulation as a result
of surgery can increase vascular resistance and thereby reduce renal blood flow,
whereas anesthetics may reduce renal blood flow by decreasing cardiac output.
Glomerular capillaries separate afferent arterioles from efferent arterioles. Glomerular
capillaries are high-pressure systems, whereas peritubular capillaries are low-pressure
systems. Consequently, the glomerular capillaries are a fluid-filtering system,
whereas the peritubular capillaries are a fluid-absorbing system. The vasa recta,
a specialized portion of peritubular capillaries formed from efferent arterioles,
are important in the formation of concentrated urine by a countercurrent mechanism.
An intrinsic mechanism that causes vasodilation and vasoconstriction of renal afferent
arterioles regulates the autoregulation of renal blood flow. A fall in mean arterial
pressure will also decrease renal blood flow and eventually affect the glomerular
filtration rate (GFR) when the pressure decreases below 60 mm Hg. A persistently
low mean arterial pressure above 60 mm Hg affects renal blood flow but does not affect
the GFR because of the intrinsic mechanism of autoregulation ( Fig.
54-3
). Autoregulation maintains mean arterial pressure between 60 and
160 mm Hg in intact as well as denervated kidneys.[4]
Although knowledge of neuroanatomy and renal blood flow is essential
to provide adequate anesthesia, a thorough understanding of renal physiology and
pharmacology is equally important. Frequently, genitourinary surgical patients have
mechanical or functional renal disease. Anesthetics and surgery will significantly
alter renal function. Conversely, renal dysfunction will significantly affect the
pharmacokinetics and pharmacodynamics of anesthetics and adjuvant drugs. Evaluation
of a patient with renal disease is discussed later.
Figure 54-3
Autoregulation of renal blood flow (RBF) and the glomerular
filtration rate (GFR). The relationships between RBF, GFR, and urine flow rate (UFR)
and mean renal arterial pressure in dogs are shown as renal arterial pressure is
varied from 20 to 280 mm Hg. Autoregulation of RBF and GFR is observed between about
80 and 180 mm Hg. (Redrawn from Hemmings HC: Anesthetics, adjuvants and
drugs and the kidney. In Malhotra V [ed]: Anesthesia
for Renal and Genitourinary Surgery. New York, McGraw-Hill, 1996, p 18.)
