Key Points continued

membrane, it is constantly transporting Na+ actively from the cell toward the interstitial fluid and the plasma.

• The luminal membrane contains specialized transporters that allow Na+ entry into the epithelial cell down its electrochemical potential gradient. This polarization of Na+ transporters—passive influx at the luminal membrane and active efflux at the basolateral membrane—imparts the directionality, i.e., reabsorption, to net Na+ transport all along the nephron.

• Solute reabsorption slightly reduces the osmo-lality of the tubular fluid and increases the osmolality of the interstitial fluid, which provides a small osmotic pressure difference to drive nearly isosmotic volume reabsorption.

• The active reabsorption of solutes and water by the proximal tubule causes a positive hydrostatic pressure of ~7 mm Hg in the renal interstitium.

• Protons (H+) are actively secreted in the proximal tubule and in the most distal nephron segments. Intracellular HCO3~ rises due to the loss of H+ from the cytoplasm, and produces an electrochemical gradient for HCO3~

transport across the basolateral membrane, thus returning this base to the plasma.

• In the proximal tubule, an Na+/H+ antiporter couples the energy from downhill Na+ entry across the luminal membrane to drive secondary active H+ secretion.

• As water is reabsorbed from the proximal tubule fluid, the concentrations of passively reabsorbed solutes rise, which favors their passive reabsorption by diffusion. The rate of passive reabsorption of a given solute is proportional to its permeability across the proximal cell membranes or through the junctional complexes.

• The preferential reabsorption of many solutes such as glucose and amino acids is driven by secondary active transport driven by Na+ cotransport down its electrochemical potential gradient. Each of these transporters is characterized by a transport maximum (Tm), which limits the amount of solute that can be reabsorbed.

• Active secretion of metabolic by-products, drugs, and potential toxins in the proximal straight tubule accelerates their excretion.

Glomerular filtration delivers approximately 130 mL of plasma ultrafiltrate to the nephrons every minute. Approximately two-thirds of this volume is reabsorbed by the proximal tubules and returned to the blood via the peritubular capillaries. Although it is important that the proximal tubule reabsorb most of the filtered load of water and solutes, this is a selective process: Useful substrates for metabolism, such as glucose and amino acids, are rapidly reabsorbed, whereas some by-products of metabolism, such as creatinine and urea, are reabsorbed slowly or not at all. The proximal tubule also actively secretes many organic solutes that must be excreted rapidly from the body, including metabolic byproducts and "foreign substances" such as toxins and drugs.

The proximal tubule is a "mass transporter" in that it transports the largest fraction of most substances handled by the nephron. Segments that are more distal are better adapted for the fine regulation of reabsorption and secretion that matches the final excretion rate to the rate of intake plus production for each substance.

The proximal tubule is the major reabsorber of the massive quantities of salts and water that are filtered and must be returned to the plasma. The rate at which the proximal tubule reabsorbs NaCl and water is a major determinant of the rate of their excretion. Thus, the regulation of this reabsorptive process is of utmost importance in maintaining a normal extracellular fluid volume (as discussed in Chapter 29).

In this chapter, the general function of the proximal tubule is presented as if this segment of the nephron were a homogeneous structure. Thus, the convoluted and straight segments of superficial and juxtamedullary nephrons are referred to collectively as the proximal tubule when considering both their morphology and function. In fact, the proximal tubule segments differ in length and size between superficial and juxtamedullary nephrons. There are also three cell types in the proximal tubule. However, these details are ignored here, except in those instances in which the localization of certain reabsorptive or secretory processes may be important. In addition, this chapter examines the collective operation of all proximal tubules, in other words, the total effect of all two million proximal tubules in reabsorbing substances from the total glomerular ultrafiltrate or adding substances to it.

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