Medical Physiology, 3rd Edition

CHAPTER 35. Transport of Sodium and Chloride

Gerhard Giebisch, Erich E. Windhager, Peter S. Aronson

The kidneys help to maintain the body's extracellular fluid (ECF) volume by regulating the amount of Na+ in the urine. Sodium salts (predominantly NaCl) are the most important contributor to the osmolality of the ECF; hence, where Na+ goes, water follows. This chapter focuses on how the kidneys maintain the ECF volume by regulating excretion of Na+ and its most prevalent anion, Cl.

The normal daily urinary excretion of Na+ is only a tiny fraction of the total Na+ filtered by the kidneys (Fig. 35-1). The filtered load of Na+ is the product of the glomerular filtration rate (GFR, ~180 L/day) and the plasma Na+ concentration of ~142 mM (neglecting the small difference from [Na+] in protein-free plasma water; see Table 5-2), or ~25,500 mmol/day. This amount is equivalent to the Na+ in ~1.5 kg of table salt, more than nine times the total quantity of Na+ present in the body fluids. For subjects on a typical Western diet containing ~120 mmol of Na+, the kidneys reabsorb ~99.6% of the filtered Na+ by the time the tubule fluid reaches the renal pelvis. Therefore, even minute variations in the fractional reabsorptive rate can lead to changes in total-body Na+ that markedly alter ECF volume and, hence, body weight and blood pressure. Thus, it is not surprising that each nephron segment makes its own unique contribution to Na+ homeostasis.

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FIGURE 35-1 Distribution and balance of Na+ throughout the body. The values in the boxes are approximations. ICF, intracellular fluid.

Na+ and Cl− Transport by Different Segments of The Nephron

Na+ and Cl−, and Water Transport at the Cellular and Molecular Level

Regulation of Na+ and Cl− Transport

References