Page 37 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 2

Applied Renal Physiology

Stephen P. DiBartola

“Superficially it might be said that the function of the kidneys is to make urine; but in a more considered view, one
can say that the kidneys make the stuff of philosophy itself.”
Homer W. Smith

Each day the glomeruli of the kidneys filter an enormous any substance may be calculated, but the clearance of
volume of plasma water, and the tubules must reabsorb certain substances (e.g., inulin, p-aminohippuric acid
most of this water along with vital solutes so that only a [PAH], and creatinine) provides important information
small volume of water and unneeded solutes are excreted about renal function (see later discussion of measurement
as urine. For example, a normal 10-kg dog may have a of glomerular filtration rate and measurement of renal
glomerular filtration rate (GFR) of 4 mL/min/kg. In blood flow and renal plasma flow).
the course of one day, this dog would filter 57.6 L of
plasma water in its kidneys. If 60% of body weight is GLOMERULAR FILTRATION
water, this volume represents almost 10 times the dog’s
total body water. The same dog may have a urine output
of 33 mL/kg/day. Thus, more than 99% of plasma water GLOMERULAR MORPHOLOGY
filtered by the glomeruli is reabsorbed by the tubules. The The glomerular capillary wall or filtration barrier consists
proximal tubules and loops of Henle reabsorb approxi- of three components: the capillary endothelium, base-
mately 85% of the filtered water and solutes, whereas ment membrane, and visceral epithelium (Fig. 2-2).
the collecting ducts adjust the final composition of urine The glomerulus is a unique vascular structure consisting
to compensate for fluctuations in intake and prevent of a capillary bed interposed between two arterioles: the
changes in the volume and composition of body fluids. afferent and efferent arterioles. The glomerular capillary
The major functions of the various segments of the neph- divides into several branches, each of which forms a lob-
ron are depicted in Figure 2-1. ule of the glomerulus. The capillary endothelium of the
glomerulus is fenestrated by openings 50 to 100 nm in
CONCEPT OF RENAL diameter. These openings exclude cells from the ultrafil-
CLEARANCE trate, but macromolecules are not restricted based on
size. The luminal surface of the endothelium is covered
An appreciation of the concept of clearance is crucial to by negatively charged sialoglycoproteins that contribute
understanding how renal function is evaluated clinically. to the charge selectivity of the filtration barrier.
The renal clearance of a substance is the volume of plasma The glomerular basement membrane is composed of
that contains the amount of the substance excreted in the the lamina rara interna on the endothelial side, the central
urine in 1 minute. It is the volume of plasma that must be lamina densa, and the lamina rara externa on the epithelial
filtered each minute to account for the amount of the side. The lamina rara interna and lamina rara externa con-
substance appearing in the urine each minute under tain polar noncollagenous proteins that contribute to the
steady-state conditions. If the concentration of the sub- negative charge of the filtration barrier. The lamina densa
stance in urine is U x and the urine flow rate is V, the contains nonpolar collagenous proteins that contribute
amount of the substance excreted in the urine per minute primarily to the size selectivity of the filtration barrier.
is U x V. If the concentration of the substance in plasma is The filtration barrier is permeable to molecules with
P x , the volume of plasma that contains the same quantity effective molecular radii less than 2 nm and impermeable
of that substance or the volume of plasma that must be fil- to those with radii greater than 4 nm.
tered per minute to account for that amount in the urine is The visceral epithelial cells or podocytes constitute
U x V/P x , the standard clearance formula. The clearance of the outermost portion of the filtration barrier. They cover

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