Page 44 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Renal Physiology 33

In the dog and cat, normal values for RPF are 7 to 20 Transepithelial potential difference
−
mL/min/kg and 8 to 22 mL/min/kg, respectively. 38,39,45 +
If all of the plasma were filtered in one pass of blood
Transmembrane potential difference
through the glomeruli, an immovable mass of red blood + − − +
cells would be all that remained behind at the efferent
arteriole of the glomerular capillary. This does not occur
−4 mV −66 mV −70 mV Interstitial
because p GC increases along the length of the capillary Tubular fluid fluid Blood
and, in conjunction with P T , effectively opposes
Lateral
further filtration. The filtration fraction is the fraction intercellular
space
of plasma flowing through the kidneys that is filtered Luminal membrane Basolateral
membrane
into the Bowman space. It is determined by the
following equation: Tight junction
FF ¼ GFR=RPF
Transcellular
In the dog and cat, values for FF are 0.32 to 0.36 and
0.33 to 0.41, respectively. These values are higher than
those observed in humans in whom FF is approximately
0.20. Paracellular

RENAL TUBULAR FUNCTION

The terms reabsorption and secretion refer to the direc-
tion of transport across an epithelium. In the kidneys, Basement
membrane Endothelium
reabsorption refers to movement of water and solutes Figure 2-8 Diagram demonstrating selected terminology as
from the tubular lumen to the peritubular interstitium. applied to the renal tubular epithelium: luminal versus basolateral
Secretion refers to movement of water and solutes from membranes, transmembrane versus transepithelial potential
the peritubular interstitium to the tubular lumen. Some difference, and transcellular versus paracellular
substances experience reabsorption in one part of the transport. (Drawing by Tim Vojt.)
nephron and secretion in another part (e.g., urate and
potassium). The term reabsorption often is used to
denote net reabsorption, which is the algebraic sum of negative. The transepithelial PD affects movement of
the fluxes in both directions across the renal tubular charged solutes across the renal tubular epithelium and
epithelium. contributes to the electrochemical gradient for such
The luminal membranes separate the cytoplasm of the solutes.The paracellular route refers to movement of
tubular cell from the tubular fluid. The basolateral solutes and water between cells (i.e., from the tubular
membranes separate the cytoplasm of the tubular cell lumen to the lateral intercellular space across tight
from the lateral intercellular spaces and the peritubular junctions connecting epithelial cells). The transcellular
interstitium. The transmembrane potential difference route refers to movement of solutes and water through
(PD) refers to the electrical PD between the outside the cytoplasm of the tubular cells. The junctions between
and inside of the cell. The transepithelial or renal epithelial cells at the luminal surface are classified as
transtubular PD is the electrical PD between the tubular leaky (proximal tubules) or tight (distal convoluted
lumen and the peritubular interstitium and is the alge- tubules, collecting ducts). Leaky epithelia do not gener-
braic sum of the transmembrane PD between the tubular ate large transepithelial concentration gradients, exhibit
lumen and cell cytoplasm, and the transmembrane PD a small transepithelial PD, and have high water perme-
between the peritubular interstitium and cell cytoplasm. ability, whereas tight epithelia can generate large
These relationships are depicted in Figure 2-8. Trans- transepithelial concentration gradients, exhibit a large
membrane PD usually is 60 to 70 mV (cell interior transepithelial PD, and have low basal water permeability.
negative), whereas transepithelial PD is only a few The paracellular route allows movement of ions (e.g.,
millivolts. In the early proximal tubule, the tubular lumen potassium, chloride) and large, nonpolar solutes by pas-
is a few millivolts negative relative to the peritubular sive diffusion and solvent drag. Electrochemical, hydro-
interstitium, whereas in the later proximal tubule, the static, and oncotic gradients are important driving
tubular lumen is a few millivolts positive relative to the forces for reabsorption by the paracellular route. The
peritubular interstitium. In the thick ascending limb of paracellular route accounts for only 1% of the surface area
Henle’s loop, the transepithelial PD is lumen positive, available for reabsorption and 5% to 10% of water trans-
but in the distal tubule, the transepithelial PD is lumen port, whereas the transcellular route accounts for 99% of