Question

A person has an intracellular fluid (ICF) volume of 34 L, an extracellular fluid (ECF) volume of 16 L and an original osmolarity of 295 mOsM. The person eats a meal that contains 6 g of NaCl (MW = 58.4 g mol^-1) and 0.5 L of water. Assume it is all absorbed and distributed. Calculate the estimated new ICF and ECF volume and osmolarity.

Answer 1

Ans:

ICF = 34 L, ECF = 16 L, Original osmolarity = 295 mOsM

So, Total Body Water (TBW) = 50 L

Original Total Osmolarity = 50 L x 295 mOsM = 14750 mosmole

Ingesting 6 g NaCl adds 2 osmoles mol-1 x 6 g / 58.4 g mol-1 = 205.5 mosmole to the total.

The final osmolytes in the body after meal absorbed and distributed = 14750 + 205.5 = 14955.5 mosmole

New volume = 50 L + 0.5 L = 50.5 L

So, New osmolarity = 14955.5 mosmole / 50.5 L = 296.2 mOsM

The ICF osmolytes = 34 L x 295 mOsM = 10030 mosmole

So, New ICF volume = 10030 mosmole / 296.2 mOsM = 33.86 L

The ECF osmolytes = 16 L x 295 mOsM + 205.5 mosmole = 4925.5 mosmole

So, New ECF volume = 4925.5 mosmole / 296.2 mOsM = 16.63 L