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Positive balance Production

Days

Figure 27-18

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

to the total renal plasma flow. In other words, the amount of the substance delivered to the kidneys in the blood (renal plasma flow x Ps) would be equal to the amount excreted in the urine (Us x V). Thus, renal plasma flow (RPF) could be calculated as

Because the GFR is only about 20 per cent of the total plasma flow, a substance that is completely cleared from the plasma must be excreted by tubular secretion as well as glomerular filtration (Figure 27-20). There is no known substance that is completely cleared by the kidneys. One substance, however, PAH, is about 90 per cent cleared from the plasma. Therefore, the clearance of PAH can be used as an approximation of renal plasma flow. To be more accurate, one can correct for the percentage of PAH that is still in the blood when it leaves the kidneys. The percentage of PAH removed from the blood is known as the extraction ratio of PAH and averages about 90 per cent in normal kidneys. In diseased kidneys, this extraction ratio may be reduced

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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