- compares the concentration of a substance in tubular fluid at any point along the nephron with the concentration in plasma.
a. If TF/P = 1.0, then either there has been no reabsorption of the substance or reabsorption of the substance has been exactly proportional to the reabsorption of water.
- For example, if TF/PNa+ = 1.0, the [Na+] in tubular fluid is identical to the [Na+] in plasma.
- For any freely filtered substance, TF/P = 1.0 in Bowman's space (before any reabsorption or secretion has taken place to modify the tubular fluid).
b. If TF/P < 1.0, then reabsorption of the substance has been greater than the reabsorption of water and the concentration in tubular fluid is less than that in plasma.
- For example, if TF/PNa+ = 0.8, then the [ Na4] in tubular fluid is 80% of the [Na+] in plasma.
c. If TF/P > 1.0, then either reabsorption of the substance has been less than the reabsorption of water or there has been secretion of the substance.
- is used as a marker for water reabsorption along the nephron.
- increases as water is reabsorbed.
- Because inulin is freely filtered, but not reabsorbed or secreted, its concentration in tubular fluid is determined solely by how much water remains in the tubular fluid.
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