1. Remember that the conductance of the distal tubule to small ions is fairly low. The permeability to water of the early segment is low, that of the later segment is under the control of ADH. The transport systems can establish high gradients across the tubular epithelium but transport at a slow rate. See Table 3-1.

2. From the hypotonic fluid entering it, the tubule reabsorbs additional NaCl. In the absence of ADH only a small amount of water is reabsorbed and the osmotic concentration tends to fall further. In the presence of ADH, the permeability of the epithelium to water is higher and the rate of water reabsorption increases so that the tubular fluid comes into osmotic equilibrium with the cortical ISF.

3. In the early segment of the tubule, Na and Cl are reabsorbed via a NaCl cotransporter in the apical membrane and the Na-K ATPase system in the basolateral membrane. Cl exits the cell via a Cl channel in the basolateral membrane. The NaCl cotransporter is inhibited by the thiazide class of diuretic agents, one of which is hydrochlorothiazide.  A genetic disease, Gitelman's Syndrome, results in loss of function of the cotransporter, causing a mild degree of salt wasting with other complications.

Fig. 6-7. Mechanisms of NaCl reabsorption in the distal convoluted tubule.

4. Cells in the late segment of the tubule resemble those of the collecting tubule and have similar properties (see below). Salt and water are reabsorbed by the principal cells under the influence of aldosterone and ADH.

1. The collecting tubule in the cortex and in the outer medulla contains two cell types, the principal (light) cells and the intercalated (dark) cells. Only principal cells are found in the inner medullary segment. These two cell types have differing transport capabilities. In general the epithelium has a low conductance and, in the absence of ADH, a low water permeability. The transport systems are all low rate-high gradient mechanisms.

2. Na is reabsorbed primarily by the principal cells (fig. 6-8). Na crosses the apical membrane via a Na channel (the ENaC, epithelial Na channel) and is pumped out of the cell across the basolateral membrane by Na-K ATPase. Na reabsorption is stimulated by aldosterone and to a lesser extent by ADH. This is a low rate-high gradient system. The paracellular pathway has a very low conductance. Low concentrations of amiloride can inhibit Na reabsorption by blocking the Na channels. A genetic disease, Liddle's Syndrome, causes a gain of function of the ENaC channel resulting in salt retention and potassium wasting.

Fig. 6-8. Na+ and Cl- reabsorption by the principal cells in the collecting tubule.

3. The mechanisms for Cl reabsorption in the collecting tubule are poorly understood. Significant Cl reabsorption may occur via the paracellular pathway driven by the transepithelial electrical gradient (lumen negative) which can be fairly high. There is evidence that the terminal section of the collecting tubule may secrete Cl in certain circumstances.

4. The intercalated cells possess the capability to secrete both protons and HCO₃ and to reabsorb K. The principal cells can secrete K. These mechanisms will be described in Sections X and XI.

5. In the absence of ADH the tubular epithelium is poorly permeable to water. Na reabsorption and the reabsorption of other solutes can reduce the osmotic concentration of the tubular fluid to a low of 40 mOsM/kg H₂O. When ADH is present, water reabsorption occurs in response to whatever osmotic gradient exists across the tubular epithelium. In the cortex this will maintain the osmolality of the tubular fluid at about 290 mOsM/kg H₂O. In the medullary segments the osmotic concentration of the medullary ISF can be quite high and this will drive water reabsorption until the tubular fluid reaches the same osmotic concentration. This may rise to a maximum of 1200 to 1400 mOsM/kg H₂O.

QUESTIONS:  
10. Compare and contrast the processes of salt and water reabsorption in the proximal tubule and in the collecting tubule.

Summary. The salt reabsorptive mechanisms are the primary transport processes involved in the control of the volume and osmotic concentration of the ECF. Throughout the nephron, they serve to generate the osmotic gradients that bring about the passive and secondary reabsorption of water. The largest bulk of the filtrate is reabsorbed in the proximal tubule. Here, the volume reabsorbed can be altered by a variety of factors, but the osmotic concentration never varies more than a slight degree from that of plasma and the glomerular filtrate. Thus the fluid returned to the circulation can vary greatly in volume but varies little in composition. In the more distal structures, the variable permeability of the tubular epithelium to water, the low permeability to electrolytes, and the variable rates of salt transport permit reabsorption of salt and water in varying proportions so that the electrolyte and osmotic concentrations, as well as the volume of the urine and of the fluid returned to the circulation, can fluctuate over a wide range.

The vertebrate brain is a most wonderful mechanism for wresting freedom out of necessity, and the vertebrate brain attains its greatest efficiency in man. But the evolution of the vertebrate brain parallels the evolution of the neuromuscular system, and the evolution of the neuromuscular system in all its complexities has been made possible by the evolution of an internal environment of constant composition. I need not emphasize that this internal environment is in effect synthesized by the kidneys, that every drop of this environment is resynthesized by the kidneys some sixteen times a day. Historically and physiologically, consciousness and urine formation are inseparable.

Homer W. Smith, "De Urina", Kaiser Foundation Medical Bulletin, 6:1, 1958.

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