PRINCIPLES OF TOXICOLOGY

6.1 BASIC KIDNEY STRUCTURES AND FUNCTIONS 131
Figure 6.2 Cortical and juxtamedullary nephrons. Enlargement of representative kidney section in Figure 6.1c.
(Based on B. Brenner and F. Rector, The Kidney, Saunders, Philadelphia, 1976.)
into the dilated, blind end of the nephron (Bowman’s capsule). The capillaries are supplied by an
afferent arteriole and drained by an efferent arteriole. These vascular elements deliver waste and other
materials to the tubular element for excretion, return reabsorbed and synthesized materials from the
tubular element to the blood circulation, and deliver oxygen and nourishment to the nephron.
The Glomerulus and Glomerular Filtration The glomerulus behaves as if it were a filter with
pores 100 Å in diameter, or about 100 times more permeable than the capillaries in skeletal muscle.
Substances as great as 70,000 daltons can appear in the glomerular filtrate, but most proteins in
the plasma are still too large to pass through the glomerulus. Therefore, a substance that is, for
example, 75 percent bound to plasma proteins has an effective filterable concentration of 25
percent its total plasma concentration. Small amounts of protein, principally the albumins, which
are important chemical-binding proteins, may appear in the glomerular filtrate, but these are then
normally reabsorbed. The glomerular filter can be made more permeable in certain disease states
and by actions of certain nephrotoxicants. Both circumstances may result in the appearance of
protein in the urine (proteinuria). If damage to the glomerular element is severe, the result is a
loss of a large amount of the plasma proteins. If this occurs at a rate greater than the rate at which
the liver can synthesize the plasma proteins, the result will be hypoproteinemia (lower than normal
levels of proteins in the blood) and a concomitant edema due to the reduction in osmotic pressure.
This clinical picture is sometimes referred to as the nephrotic syndrome. However, transient but
significant proteinuria occurs normally after prolonged standing or strenuous exercise, so a single
measurement of high protein levels in the urine may not indicate kidney damage.
Nephron Tubules and Tubular Reabsorption The tubular element of the nephron selectively reabsorbs
98–99 percent of the salts and water of the initial glomerular filtrate. The tubular element of the