Creatinine is a by-product of muscle metabolism and is cleared from the body fluids almost entirely by glomerular filtration. Therefore, the clearance of creatinine can also be used to assess GFR. Because measurement of creatinine clearance does not require intravenous infusion into the patient, this method is much more widely used than inulin clearance for estimating GFR clinically. However, creatinine clearance is not a perfect marker of GFR because a small amount of it is secreted by the tubules, so that the amount of creatinine excreted slightly exceeds the amount filtered. There is normally a slight error in measuring plasma creatinine that leads to an overestimate of the plasma creatinine
Measurement of glomerular filtration rate (GFR) from the renal clearance of inulin. Inulin is freely filtered by the glomerular capillaries but is not reabsorbed by the renal tubules. Pinulin, plasma inulin concentration; Uinulin, urine inulin concentration; V, urine flow rate.
concentration, and fortuitously, these two errors tend to cancel each other. Therefore, creatinine clearance provides a reasonable estimate of GFR.
In some cases, it may not be practical to collect urine in a patient for measuring creatinine clearance (CCr). An approximation of changes in GFR, however, can be obtained by simply measuring plasma creatinine concentration (PCr), which is inversely proportional to GFR:
If GFR suddenly decreases by 50%, the kidneys will transiently filter and excrete only half as much creatinine, causing accumulation of creatinine in the body fluids and raising plasma concentration. Plasma concentration of creatinine will continue to rise until the filtered load of creatinine (PCr x GFR) and creatinine excretion (UCr x V) return to normal and a balance between creatinine production and creatinine excretion is reestablished. This will occur when plasma creatinine increases to approximately twice normal, as shown in Figure 27-18. If GFR falls to one-fourth normal, plasma creatinine would increase to about 4 times normal, and a decrease of GFR to one-eighth normal would raise plasma creatinine to 8 times normal. 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 Figure 27-19.
PAH Clearance Can Be Used to Estimate Renal Plasma Flow
Theoretically, if a substance is completely cleared from the plasma, the clearance rate of that substance is equal
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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.