Page 39 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

28 APPLIED PHYSIOLOGY

1.0
Albumin
(3.6 nm)
EA
0.8
Fractional clearance (C dextran /C inulin ) Cationic
N MD 0.6

G 0.4

B Neutral
M 0.2
Anionic
BM 0
EN
1.8 2.2 2.6 3.0 3.4 3.8 4.2
F
Effective molecular radius (nm)
EP Figure 2-3 Effect of electrostatic charge on filtration of
macromolecules across the glomerular capillary wall. (Drawing by
BS Tim Vojt.)

PT
SNGFR ¼ K f ½ðP GC P T Þ ðp GC p T ފ
Figure 2-2 Schematic representation of the glomerulus
demonstrating the afferent and efferent arterioles, juxtaglomerular where P GC is the hydrostatic pressure in the glomerular
apparatus, and glomerular capillary loops. At the vascular pole, an
afferent arteriole (AA) enters and an efferent arteriole (EA) leaves capillary, which falls slightly along the length of the glo-
the glomerulus. At the urinary pole, the Bowman space (BS) merular capillary, averaging 55 mm Hg; P T is the hydro-
becomes the tubular lumen of the proximal tubule (PT). The static pressure in the Bowman space, which is higher than
epithelial cells composing the Bowman capsule (B) enclose the systemic interstitial pressure, averaging 20 mm Hg; p GC is
Bowman space. Smooth muscle cells proper of the arterioles and all the oncotic pressure in the glomerular capillary, which
cells derived from smooth muscle are shown in black, including the increases along the length of the capillary because of loss
granular cells (G).The afferent arteriole is innervated by sympathetic of protein-free ultrafiltrate into the Bowman space, aver-
nerve terminals (N). The extraglomerular mesangial cells are aging 20 mm Hg; and p T is the oncotic pressure in the
located at the angle between AA and EA and continue into the Bowman space and is negligible because the ultrafiltrate
mesangial cells (M) of the glomerular tuft. The glomerular capillaries is nearly protein free. If p T is neglected, the formula for
are outlined by fenestrated endothelial cells (EN) and covered from SNGFR simplifies to:
the outside by the epithelial cells (EP) with foot processes (F). The
glomerular basement membrane (BM) is continuous throughout the
glomerulus. At the vascular pole, the thick ascending limb touches SNGFR ¼ K f ðP GC P T p GC Þ
the macula densa (MD), the extraglomerular mesangium. 31
These relationships are depicted in Figure 2-4, in which
average pressure values are those reported for dogs 36
7
or no penetration into the renal medulla. These nephrons and cats. If the average pressures just described are con-
tend to excrete relatively more solute and water. sidered alone, it can be seen that the net filtration pressure
Juxtamedullary nephrons have long loops of Henle that in the glomerulus is approximately 15 mm Hg, which is
penetrate the inner medulla, and these nephrons tend similar to values obtained for systemic capillaries. The fact
to conserve solute and water. All of the nephrons in the that GFR is so much higher than the movement of fluid
canine and feline kidneys are thought to have long loops across systemic capillaries is explained by different values
of Henle. for K f .
The glomerular ultrafiltrate is a protein-free ultrafil- The ultrafiltration constant, K f , is dependent on the
trate of plasma containing water and all of the crystalloids surface area available for filtration and the permeability
of plasma in concentrations similar to those in plasma. per unit area of capillary to crystalloids and water. The
The concentrations are not exactly the same because of morphology of the glomerulus is such that the surface
the Gibbs-Donnan effect. The same Starling forces that area available for filtration is much greater than that
govern the movement of fluid across other capillaries in found in the capillary beds of skeletal muscle, and the unit
the body determine SNGFR, but there are some impor- permeability of the glomerular endothelium is more than
tant differences in the glomerulus that account for the 100 times that of skeletal muscle capillaries. This much
relatively high rate of filtration: higher value for K f in glomerular capillaries than in