The remainder of the chapter is concerned with the renal handling of specific substances such as Na^{+}, Cl^{−}, HCO3^{−}, K^{+}, and H2O. One level of understanding can be achieved at the level of whole kidney function. For example, Na^{+} is freely filtered across the glomerular capillaries, almost completely reabsorbed, and only a small fraction of the filtered load is excreted. However, *What are the details of the reabsorption process? Is Na*^{+}*reabsorbed throughout the nephron or only in certain segments, and what cellular transport mechanisms are involved?*

To answer these more sophisticated questions, techniques have been developed to study **single nephron function.** In the micropuncture technique, fluid is sampled directly from individual nephrons and analyzed. In the isolated perfused nephron technique, segments of nephrons are dissected out of the kidney and perfused with artificial solutions in vitro. In the isolated membrane technique, vesicles are prepared from luminal or basolateral membranes of renal epithelial cells to study their biochemical and transport properties.

The terms associated with single nephron function are parallel to those used to describe whole kidney function. For example, “U” represents urine in whole kidney terminology and the parallel term for the single nephron, “TF,” represents tubular fluid. “GFR” represents whole kidney glomerular filtration rate, and “SNGFR” is the filtration rate of a single nephron. A summary of terms, abbreviations, and meanings is provided in __Table 6-3__.

**[TF/P]****X**** Ratio**

The [TF/P]x ratio compares the concentration of a substance in tubular fluid with its concentration in systemic plasma. Using the micropuncture technique, the [TF/P]x ratio can be measured at various points along the nephron, beginning in Bowman’s space. *Plasma concentrations are assumed to be constant,* and any changes in the [TF/P]x, therefore, reflect changes in tubular fluid concentration.

To understand how the [TF/P]x ratio is applied, consider a simple example. Assume that the [TF/P]Na^{+} ratio was measured in Bowman’s space and found to be 1.0. A value of 1.0 means that the tubular fluid Na^{+} concentration is equal to the plasma Na^{+} concentration. This value makes perfect sense, based on knowledge of glomerular filtration: Na^{+} is freely filtered across the glomerular capillaries into Bowman’s space, and the Na^{+} concentration of the filtrate should be identical to the plasma concentration (with a small Gibbs-Donnan correction). No reabsorption or secretion has yet taken place. The generalization can be made that *for any freely filtered substance, [TF/P]*x*is 1.0 in Bowman’s space* (before any reabsorption or secretion has taken place to modify it).

The following interpretations can be given for values of [TF/P]x, where x is any solute. Again, the plasma concentration of x is assumed to be constant.

**[TF/P]****x**** = 1.0.** A value of 1.0 can have two meanings. The *first* meaning is illustrated in the preceding example: In Bowman’s space, [TF/P]x for a freely filtered substance is 1.0 because no reabsorption or secretion has yet occurred. The *second* meaning is more complicated. Suppose that tubular fluid is sampled at the end of the proximal tubule, and [TF/P]x is found to be 1.0. *Does that mean that no reabsorption or secretion of the solute has occurred in the proximal tubule?* Not necessarily. It is also possible that reabsorption of the solute has occurred, but reabsorption of water has occurred in exactly the same proportion. If the solute and water are proportionally reabsorbed, the concentration of the solute in tubular fluid does not change. In fact, this is precisely what happens in the case of Na^{+} in the proximal tubule: Na^{+} is reabsorbed, but [TF/P]Na^{+} remains 1.0 along the entire proximal tubule because there is proportionality of Na^{+} and water reabsorption.

**[TF/P]****x**** < 1.0.** A value less than 1.0 has only one meaning. Reabsorption of the solute must have been greater than reabsorption of water, causing the concentration of solute in tubular fluid to decrease below that in plasma.

**[TF/P]****x**** > 1.0.** A value greater than 1.0 has two possible meanings. The *first* meaning is that there has been net reabsorption of the solute, but solute reabsorption has been less than water reabsorption. When solute reabsorption lags behind water reabsorption, the tubular fluid concentration of the solute increases. The *second* meaning is that there has been net secretion of the solute into tubular fluid, causing its concentration to increase above that in plasma.

**[TF/P]****Inulin**

The previous discussion about values of [TF/P]x emphasizes how their interpretation requires a simultaneous knowledge of water reabsorption. Recall one of the questions asked: *Does [TF/P]*x*equal 1.0 because there has been filtration but no reabsorption or secretion? Or, does [TF/P]*x*equal 1.0 because there has been proportional reabsorption of the solute and water?* These two very different possibilities can be distinguished only if water reabsorption is measured simultaneously.

**Inulin,** the substance used to measure GFR, can also be used to **measure water reabsorption** in the single nephron. Recall that once inulin is filtered across glomerular capillaries, it is inert—that is, it is neither reabsorbed nor secreted. Thus, the concentration of inulin in tubular fluid is not affected by its own reabsorption or secretion, and it is only affected by the volume of water present. For example, in Bowman’s space, the tubular fluid inulin concentration is identical to the plasma inulin concentration (because inulin is freely filtered). As water is reabsorbed along the nephron, the inulin concentration of tubular fluid steadily increases and becomes higher than the plasma concentration.

Water reabsorption can be calculated from the value of [TF/P]inulin. Consider an example in which tubular fluid is sampled and the measured **[TF/P]****inulin** = **2.0**. In words, this means that the tubular fluid inulin concentration is twice the plasma inulin concentration. Water must have been reabsorbed in earlier portions of the nephron to cause the tubular fluid inulin concentration to double. *How much water was reabsorbed to achieve this value of [TF/P]*inulin? This simple example can be analyzed intuitively: If the tubular fluid inulin concentration doubles, then 50% of the water must have been removed (i.e., reabsorbed).

Other values of [TF/P]inulin can be used to measure water reabsorption by the following equation:

The equation can be understood by comparing it with the intuitive solution for **[TF/P]****inulin** = **2.0.** In that example, the fraction of the filtered water reabsorbed = 1 − 1/2 = 0.5 or 50%. The mathematical solution provides exactly the same answer as the intuitive approach, which also concluded that 50% of the water was reabsorbed.

Other values of [TF/P]inulin are not as easy to solve intuitively, and it may be necessary to use the equation. For example, if **[TF/P]****inulin** = **100,** the fraction of the filtered water reabsorbed = 1 − 1/100 = 1 − 0.01 = 0.99 or 99%. Incidentally, this is the value of [TF/P]inulin that could occur at the end of the collecting ducts, at which point 99% of the filtered water has been reabsorbed back into blood.

**[TF/P]****X****/[TF/P]****Inulin**

The [TF/P]inulin ratio provides a tool for correcting [TF/P]x for water reabsorption. With this correction, it can be known with certainty whether a substance has been reabsorbed, secreted, or not transported at all. [TF/P]x/[TF/P]inulinis a **double ratio** that makes this correction. The exact meaning of the double ratio is this: **fraction of the filtered load of substance x remaining** at any point along the nephron. For example, if [TF/P]x/[TF/P]inulin = 0.3, then 30% of the filtered load of the solute remains in the tubular fluid at that point in the nephron, or 70% has been reabsorbed. This is approximately the situation for Na^{+} at the end of the proximal tubule: [TF/P]Na^{+}/[TF/P]inulin = 0.3, which means that 30% of the filtered Na^{+} remains at that point and 70% has been reabsorbed. From the earlier discussion, recall that at the end of the proximal tubule [TF/P]Na^{+} = 1.0, which led to confusion about whether Na^{+} was reabsorbed in the proximal tubule. Now, using the double ratio to correct for water reabsorption, the answer is clear: A large fraction of the filtered Na^{+}*has been* reabsorbed, but because water is reabsorbed along with it, [TF/P]Na^{+}does not change from its value in Bowman’s space.