PRINCIPLES OF TOXICOLOGY - Biology East Borneo

6.1 BASIC KIDNEY STRUCTURES AND FUNCTIONS 131Figure 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 anafferent arteriole and drained by an efferent arteriole. These vascular elements deliver waste and othermaterials to the tubular element for excretion, return reabsorbed and synthesized materials from thetubular 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 withpores 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 inthe plasma are still too large to pass through the glomerulus. Therefore, a substance that is, forexample, 75 percent bound to plasma proteins has an effective filterable concentration of 25percent its total plasma concentration. Small amounts of protein, principally the albumins, whichare important chemical-binding proteins, may appear in the glomerular filtrate, but these are thennormally reabsorbed. The glomerular filter can be made more permeable in certain disease statesand by actions of certain nephrotoxicants. Both circumstances may result in the appearance ofprotein in the urine (proteinuria). If damage to the glomerular element is severe, the result is aloss of a large amount of the plasma proteins. If this occurs at a rate greater than the rate at whichthe liver can synthesize the plasma proteins, the result will be hypoproteinemia (lower than normallevels 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 butsignificant proteinuria occurs normally after prolonged standing or strenuous exercise, so a singlemeasurement of high protein levels in the urine may not indicate kidney damage.Nephron Tubules and Tubular Reabsorption The tubular element of the nephron selectively reabsorbs98–99 percent of the salts and water of the initial glomerular filtrate. The tubular element of the