Excretion = GFR x Pr

Excretion = GFR x Pr


Figure 31-5

Figure 31-4

Effect of reducing glomerular filtration rate (GFR) by 50 per cent on serum creatinine concentration and on creatinine excretion rate when the production rate of creatinine remains constant.

is that substances such as creatinine and urea depend largely on glomerular filtration for their excretion, and they are not reabsorbed as avidly as the electrolytes. Creatinine, for example, is not reabsorbed at all, and the excretion rate is equal to the rate at which it is filtered.

Creatinine filtration rate = GFR x Plasma creatinine concentration = Creatinine excretion rate

Therefore, if GFR decreases, the creatinine excretion rate also transiently decreases, causing accumulation of creatinine in the body fluids and raising plasma concentration until the excretion rate of creatinine returns to normal—the same rate at which creatinine is produced in the body (Figure 31-4). Thus, under steady-state conditions, the creatinine excretion rate equals the rate of creatinine production, despite reductions in GFR; however, this normal rate of creatinine excretion occurs at the expense of elevated plasma creatinine concentration, as shown in curve A of Figure 31-5.

Representative patterns of adaptation for different types of solutes in chronic renal failure. Curve A shows the approximate changes in the plasma concentrations of solutes such as creatinine and urea that are filtered and poorly reabsorbed. Curve B shows the approximate concentrations for solutes such as phosphate and urate. Curve C shows the approximate concentrations for solutes such as sodium and chloride.

Some solutes, such as phosphate, urate, and hydrogen ions, are often maintained near the normal range until GFR falls below 20 to 30 per cent of normal. Thereafter, the plasma concentrations of these substances rise, but not in proportion to the fall in GFR, as shown in curve B of Figure 31-5. Maintenance of relatively constant plasma concentrations of these solutes as GFR declines is accomplished by excreting progressively larger fractions of the amounts of these solutes that are filtered at the glomerular capillaries; this occurs by decreasing the rate of tubular reabsorption or, in some instances, by increasing tubular secretion rates.

In the case of sodium and chloride ions, their plasma concentrations are maintained virtually constant even with severe decreases in GFR (see curve C of Figure 31-5). This is accomplished by greatly decreasing tubular reabsorption of these electrolytes. For example, with a 75 per cent loss of functional nephrons, each surviving nephron must excrete four times as much sodium and four times as much volume as under normal conditions (Table 31-6).

Part of this adaptation occurs because of increased blood flow and increased GFR in each of the surviving nephrons, owing to hypertrophy of the blood vessels and glomeruli, as well as functional changes that cause the blood vessels to vasodilate. Even with large decreases in the total GFR, normal rates of renal

Table 31-6

Total Kidney Excretion and Excretion per Nephron in Renal Failure

75% Loss of

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