Page 42 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Renal Physiology 31

C x ¼ GFR þ T x =P x As pressure increases, flow can remain constant only if
resistance increases proportionately. The site of this resis-
Thus, the clearance of a substance experiencing net tance change in the kidneys is the afferent arteriole.
reabsorption is less than GFR (T x is negative), and the Autoregulation is intrinsic to the kidneys and occurs in
clearance of a substance experiencing net secretion is the isolated, denervated kidney and in the adrenalecto-
greater than GFR (T x is positive). The ratio of the clear- mized animal. However, it is impaired by anesthesia in
ance of a substance to inulin clearance gives an indication proportion to the depth of anesthesia. The afferent
of the net handling of that substance by the kidneys. If the arterioles are maximally dilated at mean arterial pressures
ratio is less than 1.0, the substance experiences net of 70 to 80 mm Hg, and at lower pressures, GFR declines
reabsorption; if it is greater than 1.0, it experiences net linearly with RBF (i.e., autoregulation is lost). It is likely
secretion. that autoregulation of RBF is a consequence of the need
to regulate GFR closely and thus maintain tight control
RENAL BLOOD FLOW AND over water and salt balance.Two physiologic mechanisms
RENAL PLASMA FLOW contribute to autoregulation. The myogenic mechanism
is based on the principle that smooth muscle tends to
The kidneys receive 25% or more of cardiac output. The contract when stretched and relax when shortened. As
major sites of resistance within the kidneys are the afferent the afferent arteriole is stretched by increased perfusion
and efferent arterioles, with an approximately 80% to 90% pressure, it constricts, thus limiting transmission of this
decrease in perfusion pressure across this region of the increased pressure to the glomerulus and minimizing
renal vasculature (Fig. 2-6). Blood flow is not uniform any change in glomerular capillary hydrostatic pressure
throughout the kidneys. In dogs, more than 90% of and SNGFR. The myogenic mechanism represents a
RBF is normally directed to the renal cortex, less than coarse control that operates with a delay of 1 to 2 seconds.
10% to the outer medulla, and only 2% to 3% to the inner Tubuloglomerular feedback represents a local
medulla. 51 The actual rate of flow to the renal cortex is intrarenal negative feedback mechanism for individual
approximately 100 times that of resting muscle and is nephrons. The morphologic basis for this physiologic
required for glomerular filtration. Blood flow to the mechanism is the JGA. Increased sodium chloride con-
medulla is similar to that of resting muscle, and this centration or transport in the distal tubule is sensed by
reduced flow is necessary for normal function of the uri- the extraglomerular mesangial cells of the JGA as they
nary concentrating mechanism. monitor sodium chloride transport across the tubular
cells of the macula densa. Transport of NaCl by the tubu-
AUTOREGULATION lar cells of the macula densa requires functional NKCC2
-
þ
þ
Autoregulation refers to the intrinsic ability of an organ to (the Na ,K , 2Cl cotransporter) and ROMK (a potas-
maintain blood flow at a nearly constant rate despite sium channel) in the luminal membranes and functional
þ
þ
changes in arterial perfusion pressure. In the kidneys, Na ,K -ATPase in the basolateral membranes. 46
between perfusion pressures of 80 and 180 mm Hg, Transcellular transport of NaCl causes generation of
GFR and RBF vary less than 10% (Fig. 2-7). Flow (Q) adenosine, which together with angiotensin II causes
is equal to pressure (P) divided by resistance (R). afferent arteriolar constriction in the parent glomerulus.
Renal Afferent Efferent Peritubular Intrarenal Renal
artery arteriole Glomerulus arteriole capillary vein vein

100

80
Pressure (mm Hg) 60

40

20

0
Figure 2-6 Pattern of hydrostatic pressure and vascular resistance in the renal circulation. (Drawing by
Tim Vojt.)