## Examples

1. What is the mass flow of Na+ excreted in the urine if the rate of urine flow (UF) is 1.5 mL/ min and the Na+ concentration in that urine is 78 mmol/L? To apply Eq. [1], it is first necessary to convert the volume flow and concentration into consistent volume units. The Na+ concentration can be expressed as 0.078 mmol/mL, or the UF can be expressed as 0.0015 L/min. The rate of

Na+ mass flow in the urine, referred to as the excretion rate, can then be calculated: 1.5 mL/ min • 0.078 mmol/mL or 0.117 mmol/min. 2. If 2 L of urine are produced over a 24-hr period, and the creatinine concentration in that urine volume is 1.0 mg/mL, then the rate of creatinine excretion is (2 L/day) • (1000 mg/L) = 2000 mg/day or 83.3 mg/hr or 1.4 mg/min.

other substances may not normally be ingested but are produced by metabolism. In this case, the rate at which a particular substance is eliminated must equal the rate at which it is produced in order for the total amount of the substance in the body to remain constant. In the most general terms, balance for any given substance can be maintained only when the rate of output from the body is equal to the sum of the rates at which the substance is ingested (or administered paren-terally) and produced by metabolism:

Rate of output = rate of intake + rate of production.

This equality is referred to as a condition of steady state and is shown diagrammatically in Fig. 1. Although there may be a continuous flow of a substance into the body, a steady state (and, thus, a constant total amount of the substance in the body) will occur as long as the rate of output equals this intake plus any additional metabolic production.

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