The preferential reabsorption of bicarbonate represents a sizable fraction of the anion reabsorption that accompanies Na+ reabsorption in the proximal tubule. As a consequence of the fall in the luminal bicarbonate concentration from 25 mmol/L to 3.8 mmol/L, the luminal Cl- concentration rises a corresponding amount from about 105 mmol/L to 126 mmol/L so that the total concentration of these major anions in the luminal fluid remains nearly constant. Given our approximation that the average transepithelial electrical voltage is zero, the increase in the luminal Cl- concentration gives rise to a driving force that favors the passive diffusion of Cl-from the lumen to the blood side of the proximal tubule epithelium. The rate at which Cl- is reabsorbed depends on the magnitude of its transepithelial concentration difference and its relatively high permeability across the epithelium. Therefore, the rate of Cl- reabsorption increases as the Cl- concentration in the lumen rises until, in the late proximal tubule, HCO3- reabsorption has ceased and the reabsorption of Cl- equals that of Na+. By means of this passive mechanism, about 60% of the filtered Cl- is reabsorbed. This is less than the fractional volume reabsorption of 66%, accounting for the rise in the Cl- concentration.
The same passive mechanism serves to drive K+ reabsorption—as fluid is reabsorbed, the K+ concentration of the luminal fluid rises, leading to the diffusion of K+ from the lumen. Because K+ is also relatively able to permeate the epithelium, this mechanism accounts for the reabsorption of about 60% of the filtered load of K+, and the K+ concentration in the lumen also rises slightly.
There is also no active reabsorptive mechanism for urea. Urea is a metabolic by-product of protein metabolism that must be eliminated from the body, therefore, active reabsorption would be counterproductive. Nevertheless, passive urea absorption does occur. Because the ability of urea to permeate the epithelium is less than that of either Cl- or K+, it is reabsorbed somewhat more slowly, and its tubular fluid (TF) concentration increases as water is reabsorbed. The normal concentration of urea in plasma and, thus, in the initial filtered fluid is 4 mmol/L [in clinical units this is equivalent to a blood urea nitrogen concentration (BUN) of 11.2 mg/dL]. By the end of the proximal tubule, the urea concentration rises to about 6 mmol/L, meaning that slightly less than 50% is reabsorbed. Because urea reabsorption is passive along the entire nephron, the extent to which it is reabsorbed depends on the rate of fluid flow along the nephron and, thus, on the GFR and final urine flow (UF). This flow dependence occurs because the urea is in contact with the tubular epithelium for a longer time at slower flow rates, giving more time for diffusional equilibration and, thus, increased reabsorption. At high rates of UF, the urea clearance approaches 60-70% of the GFR but it decreases markedly with slower UF rates. This effect gives rise to the fact that urea clearance by the kidney increases with more rapid UF.
Water reabsorption leads to an even greater increase in the luminal concentration of solutes that permeate the tubular epithelium poorly. Any such solute that is filtered in appreciable quantities becomes concentrated in the lumen and opposes fluid reabsorption because of its osmotic effect. Normally, no such solute is filtered in any appreciable concentration at the glomerulus. However, the effect can be used to advantage clinically by introducing a poorly reabsorbed substance into the circulation so that it will be filtered into the proximal tubule and oppose reabsorption. The result is the development of an osmotic diuresis as described in the following Clinical Note.
In mannitol diuresis, the reabsorption of Na+ is decreased because its concentration in the lumen falls due to the water retained by the osmotic effect of the mannitol. This decrease in luminal Na+ concentration has little effect on the active transport of Na+ out of the lumen; however, there is a concentration gradient that favors the backflow of Na+ from the interstitium to the lumen because of the reduced Na+ concentration in the latter compartment. Historically, mannitol diuresis was used to demonstrate experimentally that Na+ reabsorption must be active. It was difficult to establish that Na+ was actively transported under normal conditions because the Na+ concentration in the lumen remained about the same as that in the blood and interstitial fluid. However, in the presence of mannitol it was readily shown that net Na+ reabsorption occurred against a concentration gradient and against an electrochemical potential gradient.
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