CHAPTER 38
Renal Function & Micturition 643facilitated transporter for urea, and the ascending vasa recta
have a fenestrated endothelium, consistent with their function
in conserving solutes.
The efferent arteriole from each glomerulus breaks up into
capillaries that supply a number of different nephrons. Thus,
the tubule of each nephron does not necessarily receive blood
solely from the efferent arteriole of the same nephron. In
humans, the total surface of the renal capillaries is approxi-
mately equal to the total surface area of the tubules, both
being about 12 m
2
. The volume of blood in the renal capillar-
ies at any given time is 30 to 40 mL.
LYMPHATICS
The kidneys have an abundant lymphatic supply that drains
via the thoracic duct into the venous circulation in the thorax.
CAPSULE
The renal capsule is thin but tough. If the kidney becomes
edematous, the capsule limits the swelling, and the tissue pres-
sure
(renal interstitial pressure)
rises. This decreases the glo-
merular filtration rate and is claimed to enhance and prolong
anuria in acute renal failure.
INNERVATION OF THE RENAL VESSELS
The renal nerves travel along the renal blood vessels as they en-
ter the kidney. They contain many postganglionic sympathetic
efferent fibers and a few afferent fibers. There also appears to be
a cholinergic innervation via the vagus nerve, but its function is
uncertain. The sympathetic preganglionic innervation comes
primarily from the lower thoracic and upper lumbar segments
of the spinal cord, and the cell bodies of the postganglionic neu-
rons are in the sympathetic ganglion chain, in the superior mes-
enteric ganglion, and along the renal artery. The sympathetic
fibers are distributed primarily to the afferent and efferent arte-
rioles, the proximal and distal tubules, and the juxtaglomerular
cells (see Chapter 39). In addition, there is a dense noradrener-
gic innervation of the thick ascending limb of the loop of Henle.
Nociceptive afferents that mediate pain in kidney disease
parallel the sympathetic efferents and enter the spinal cord in
the thoracic and upper lumbar dorsal roots. Other renal affer-
ents presumably mediate a
renorenal reflex
by which an
increase in ureteral pressure in one kidney leads to a decrease
in efferent nerve activity to the contralateral kidney, and this
decrease permits an increase in its excretion of Na
- and water.
RENAL CIRCULATION
BLOOD FLOW
In a resting adult, the kidneys receive 1.2 to 1.3 L of blood per
minute, or just under 25% of the cardiac output. Renal blood
flow can be measured with electromagnetic or other types of
flow meters, or it can be determined by applying the Fick
principle (see Chapter 33) to the kidney; that is, by measur-
ing the amount of a given substance taken up per unit of time
and dividing this value by the arteriovenous difference for
the substance across the kidney. Because the kidney filters
plasma, the
renal plasma flow
equals the amount of a sub-
stance excreted per unit of time divided by the renal arterio-
venous difference as long as the amount in the red cells is
unaltered during passage through the kidney. Any excreted
substance can be used if its concentration in arterial and re-
nal venous plasma can be measured and if it is not metabo-
lized, stored, or produced by the kidney and does not itself
affect blood flow.
Renal plasma flow can be measured by infusing
p-
amino-
hippuric acid (PAH) and determining its urine and plasma
concentrations. PAH is filtered by the glomeruli and secreted
by the tubular cells, so that its
extraction ratio
(arterial con-
centration minus renal venous concentration divided by arte-
rial concentration) is high. For example, when PAH is infused
at low doses, 90% of the PAH in arterial blood is removed in a
single circulation through the kidney. It has therefore become
commonplace to calculate the “renal plasma flow” by dividing
the amount of PAH in the urine by the plasma PAH level,
ignoring the level in renal venous blood. Peripheral venous
plasma can be used because its PAH concentration is essen-
tially identical to that in the arterial plasma reaching the kid-
ney. The value obtained should be called the
effective renal
plasma flow (ERPF)
to indicate that the level in renal venous
plasma was not measured. In humans, ERPF averages about
625 mL/min.Example:
Concentration of PAH in urine (U
PA H
): 14 mg/mL
Urine flow (V- ): 0.9 mL/min
Concentration of PAH in plasma (P
PA H
): 0.02 mg/mL
= 630 mL/min
It should be noted that the ERPF determined in this way is
the
clearance
of PAH. The concept of clearance is discussed
in detail below.
ERPF can be converted to actual renal plasma flow (RPF):
Average PAH extraction ratio: 0.9ERPFUPAHV ̇
PPAH==------------------- Clearance of PAH C()PAHERPF^14 ×0.9
0.02=-------------------ERP
Extraction ration
-----------------------------------------
630
0.9==-------- Actual RPF=700 mL/min