Question

The extracellular fluid volume varies with the
size of the person. Suppose in an individual we
determine that the ECF is 14 L. The average
[Na1] in the ECF is about 143 mM.
A. What is the total amount of Na1 in the ECF,
in moles? In grams?
B. Suppose this person works out and sweats
1.5 L with an average [Na1] of 50 mM.
During this time the urine output is 30 mL
with an average [Na1] of 600 mM. How
much Na1 is lost during the workout?
C. If the person does not drink fluids at all during the workout, what will be the [Na1 ] in
the plasma at the end of the workout?
Assume that all of the fluid in the sweat and
urine originated from the ECF.

Answer 1

Total volume of ECF = 14 L

Concentration of Na = 143 mM = 0.143 M

(a). Moles of Na = Molarity * Volume

= 14 * 0.143

= 2.002 moles

Molar mass of Na = 23 g/mol

Mass of Na = 2.002 * 23

= 46.046 grams

(b). Volume of sweat = 1.5 L

Molarity of Na1 in sweat = 50 mM = 0.050 M

Moles of Na1 in sweat = 1.5 * 0.050

= 0.075 moles

Volume of urine = 30 mL = 0.030 L

Molarity of Na1 in urine = 600 mM = 0.6 M

Moles of Na1 in urine = 0.030*0.60

= 0.018 moles

Total moles of Na1 lost during workout = 0.075 + 0.018

= 0.093 moles

Mass of Na1 lost = 0.093 * 23

= 2.139 grams

(c). Moles of Na1 left in the plasma = 2.002 - 0.093

= 1.909 moles

Volume of ECF left in the body = 14 - 1.53

= 12.47 L

[Na1]  in the plasma at the end of the workout = 1.909 / 12.47

[Na1] = 0.153 M