Salt And Water Reabsorption In The Proximal Tubule

As shown in the following sections, it is important to think of the transport of each substance along the nephron in terms of its rate of movement rather than by its concentration in the tubular fluid. The same considerations of mass balance that apply to the kidney as a whole also apply to the proximal tubule. In the case of the proximal tubule, the rate of input of substances is determined by the product of the glomerular filtration rate (GFR) and the concentration of the substance in the glomerular ultrafiltrate. The rate at which the substance is delivered to the next segment of the nephron, the loop of Henle, is determined by the rate of filtration minus the rate at which the substance is reabsorbed. In the case of substances that are secreted, the rate of delivery to the loop of Henle will be greater than the rate of filtration by an amount equal to the rate of secretion. Thus, comparison of the rate at which a substance is delivered to the loop of Henle with its rate of filtration indicates whether there has been net addition (secretion) or loss (reabsorption) of the substance along the proximal tubule. However, as in the case of the whole nephron, it does not indicate whether only reabsorption or only secretion is occurring, or the mechanism by which these processes occur.

Isosmotic Volume Reabsorption

Quantitatively, the most impressive event in the proximal tubule is the reabsorption of more than two-thirds of the filtered load of salt and water. Thus, with a GFR of ~130 mL/min, the proximal tubule reabsorbs more than 85 mL/min and passes less than 45 mL/min on to the loop of Henle. This rate of reabsorption is substantially larger than occurs across any other epithelium in the body. The reabsorption is driven by a massive transport of solutes from the lumen of the proximal tubule to the interstitial fluid and, thus, to the peritubular capillary circulation. The cells of the proximal tubule exhibit morphology consistent with these high transport rates, as shown in Fig. 3. On the apical surface of the cells, the luminal membrane forms a dense carpet of microvilli, which, as in the small intestine, greatly expands the surface area available for transport. The surface area of the basolateral membrane is also dramatically amplified by being folded into long undulating ridges like a very full drapery, and the basal membrane has finger-like microvilli that abut the basement membrane. As in the case of other leaky epithelia such as the distal small intestine and the gallbladder (see Chapter 36), the reabsorption occurs with virtually no change in the osmolality of the luminal fluid. Thus, the process has come to be referred to as isosmotic volume reabsorption.

If there is no change in the osmolality of the tubular fluid, how can one determine that any solutes or water have been reabsorbed? This question is answered most

Mass Flow Balance Proximal Tubule

FIGURE 3 Scanning electron micrograph showing several cells in the midportion of a proximal tubule. A frozen segment of a proximal tubule was fractured to expose the lateral aspects of the cells with their luminal surfaces toward the upper right and the basement membrane toward the lower left. These cells possess an elaborate apical (luminal) surface of microvilli (AM). The lateral cell membrane is folded into long lateral ridges (LR), and the basal membrane also forms microvilli that lie on the basement membrane. (Courtesy of Dr. Andrew P. Evan, Indiana University Medical Center, Indianapolis, IN.)

FIGURE 3 Scanning electron micrograph showing several cells in the midportion of a proximal tubule. A frozen segment of a proximal tubule was fractured to expose the lateral aspects of the cells with their luminal surfaces toward the upper right and the basement membrane toward the lower left. These cells possess an elaborate apical (luminal) surface of microvilli (AM). The lateral cell membrane is folded into long lateral ridges (LR), and the basal membrane also forms microvilli that lie on the basement membrane. (Courtesy of Dr. Andrew P. Evan, Indiana University Medical Center, Indianapolis, IN.)

easily if we consider what would happen to a solute that is not reabsorbed or secreted. Inulin is ideal in this regard, as discussed in connection with the determination of the GFR (see Chapter 25). If inulin were present in the plasma at a concentration of 1 mg/dL (0.01 mg/mL), it would enter all the proximal tubules collectively at a rate equivalent to the product of the GFR times the plasma concentration, or (130 • 0.01) = 1.3 mg/min. Because inulin is neither reabsorbed nor secreted, it must leave the proximal tubule to enter the loop of Henle at the same rate as it is filtered. If the rate of volume flow to the loop of Henle is given as VL, and the concentration of inulin in the tubular fluid at the end of the proximal tubule is TFin, the mass flow equality can be expressed as:

where Pin is the plasma inulin concentration. From this relation, it can be seen that as VL is reduced due to fluid reabsorption, there must be an inversely proportional rise in the inulin concentration. If the fluid flow to the loop of Henle is only one-third of the GFR, the inulin concentration must become three times higher than that in the plasma.

Historically, this is how the rate of fluid reabsorption was determined experimentally by micropuncture experiments in individual proximal tubule segments. In these experiments, inulin was infused into an experimental animal to maintain a constant plasma concentration. Very fine glass micropipettes were then used to collect samples of proximal tubule fluid, and the inulin concentration (TFin) in these samples (1-5 nl in volume) was measured and compared with the inulin concentration in the plasma (Pin). From the ratio of these concentrations, the rate of fluid delivery to the point of sampling (Vx) could be calculated using Eq. [1] as:

Thus, because inulin is neither reabsorbed nor secreted, volume reabsorption in the proximal tubule is matched by a proportional increase in inulin concentration. It was found that the inulin concentration in the proximal tubule fluid samples was always greater than that in the plasma, indicating that the volume flow at the point of sampling was less than the GFR. In other words, the higher concentration of inulin in the proximal tubule fluid sample indicated that some of the volume flow had been reabsorbed between the glomerulus and the point of sampling. Furthermore, as shown in Fig. 4, it was observed that the ratio of the inulin concentration in the

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