Effects of dietary sodium (Na) intake on extracellular fluid (ECF) volume in chronic renal failure (CRF) [75]. Compared with normal persons, patients with CRF have expanded ECF volume at normal Na intake. Furthermore, the time necessary to return to neutral balance on shifting from one to another level of Na intake is increased. Thus, whereas urinary Na excretion equals dietary intake of Na within 3 to 5 days in normal persons, this process may take up to 2 weeks in patients with CRF. This time delay means that not only are these patients susceptible to volume overload, but also to volume depletion. This phenomenon can be modeled simply by reducing the time constant (k) given in the equation in Figure 2-2, and leaving the set point (A0) unchanged. The curves here represent time constants of 0.79 ±0.05 day-j (normal), 0.5 day-i (mild CRF), and 0.25 day-i (severe CRF).

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