Page 163 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Renal Circulation Due to autoregulation of renal blood flow, only
slight changes in renal plasma flow (RPF) and
The arcuate arteries (! A1) pass between the glomerular filtration rate (GFR) occur (even in
renal cortex and medulla. They branch towards a denervated kidney) when the systemic blood
the cortex into the interlobular arteries (! A2) pressure fluctuates between 80 and about
from which the afferent arterioles (or vasa af- 180 mmHg (! C). Resistance in the interlobu-
ferentia) arise (! A3). Unlike other organs, the lar arteries and afferent arterioles located up-
kidney has two successive capillary networks stream to the cortical glomeruli is automati-
that are connected with each other by an effer- cally adjusted when the mean blood pressure
Kidneys, Salt, and Water Balance and is regulated by adjusting the width of in- RBF and GFR can also be regulated indepen-
ent arteriole (or vas efferens) (! A, B). Pressure
changes (! B, C). If the blood pressure falls
below about 80 mmHg, however, renal circula-
in the first network of glomerular capillaries
tion and filtration will ultimately fail (! C).
(! p. 148) is a relatively high (! B and p. 152)
dently by making isolated changes in the (se-
terlobular artery, the afferent and/or efferent
rial) resistances of the afferent and efferent
arterioles (! A 3,4). The second network of
arterioles (! p. 152).
peritubular capillaries (! A) winds around the
cortical tubules. It supplies the tubule cells
Non-invasive determination of RBF is
with blood, but it also contributes the to ex-
possible if the renal plasma flow (RPF) is
change of substances with the tubule lumen
known (normally about 0.6 L/min). RPF is ob-
(Fick’s principle) of an intravenously injected
The renal blood flow (RBF) is relatively high,
test substance (e.g., p-aminohippurate, PAH)
ca. 1.2 L/min, equivalent to 20–25% of the car-
7 (reabsorption, secretion; ! p. 154ff.). tained by measuring the amount balance
that is almost completely eliminated in the
diac output. This is required to maintain a high
glomerular filtration rate (GFR; ! p. 152) and urine during one renal pass (PAH is filtered and
results in a very low arteriovenous O 2 differ- highly secreted, ! p. 156ff.). The eliminated
ence (ca. 15 mL/L of blood). In the renal cortex, amount of PAH is calculated as the arterial in-
O 2 is consumed (ca. 18 mL/min) for oxidative flow of PAH into the kidney minus the venous
metabolism of fatty acids, etc. Most of the ATP flow of PAH out of the kidney per unit time.
produced in the process is used to fuel active Since
transport. In the renal medulla, metabolism is Amount/time !
mainly anaerobic (! p. 72). (volume/time) ! concentration [7.1]
.
Around 90% of the renal blood supply goes to the (RPF ! Pa PAH) – (RPF ! Prv PAH) " VU ! U PAH[7.2]
cortex. Per gram of tissue, approximately 5, 1.75 and or
.
0.5 mL/min of blood pass through the cortex, exter- RPF " VU ! U PAH/(Pa PAH – Prv PAH). [7.3]
nal medulla, and internal medulla, respectively. The where Pa PAH is the arterial PAH conc., Prv PAH is
latter value is still higher than in most organs the renal venous PAH conc., U PAH is the urinary
(! p. 213 A). .
The kidney contains two types of nephrons that PAH conc., and V U is the urine output/time.
differ with respect to the features of their second net- Prv PAH makes up only about 10% of the Pa PAH
work of capillaries (! A). and normally is not measured directly, but
! Cortical nephrons are supplied by peritubular is estimated by dividing PAH clearance
.
capillaries (see above) and have short loops of Henle. (= V U · U PAH/Pa PAH; ! p. 152) by a factor of
! Juxtamedullary nephrons are located near the 0.9. Therefore,
.
cortex-medulla junction. Their efferent arterioles RPF ! VU ! U PAH/(0,9 ! Pa PAH). [7.4]
give rise to relatively long (! 40 mm), straight arte-
rioles (vasa recta) that descend into the renal This equation is only valid when the Pa PAH is
medulla. The vasa recta supply the renal medulla and not too high. Otherwise, PAH secretion will be
can accompany long loops of Henle of juxtamedul- saturated and PAH clearance will be much
lary nephrons as far as the tip of the renal papilla smaller than RPF (! p. 161 A).
(! p. 148). Their hairpin shape is important for the RBF is derived by inserting the known he-
concentration of urine (! p. 164ff.). matocrit (HCT) value (! p. 88) into the follow-
Any change in blood distribution to these two
150 types of nephrons affects NaCl excretion. Antidi- ing equation:
uretic hormone (ADH) increases the GFR of the jux- RBF = RPF/(1–HCT) [7.5]
tamedullary nephrons.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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