Fluid Balance

Question 1

What is the 60-40-20 rule regarding fluid?

Answer 1

60% body weight is water
40% body weight intracellular
20% body weight extracellular

Question 2

Give examples of transcellular compartment fluids

Answer 2

Within epithelial lined spaces:

CSF
Joint fluid
Bladder urine
Aqueous humour

Question 3

What is osmolality?

Answer 3

Total number of particles in solution - measured with osmometer, units are mmol/kg

Question 4

What is osmolarity?

Answer 4

Calculated, units are mmol/L

depends on the volume of the solution, which can change with temperature and pressure.

Question 5

How to calculate osmolarity?

Answer 5

2 (Na+ + K+) + urea + glucose

Question 6

Osmolarity and osmolality should roughly equate, what is the difference between the two called?

Answer 6

Osmolar gap

MeasuredOsmolality − CalculatedOsmolarity

If osmolarity is lower than osmolality you can assume there are extra unmeasured solutes dissolved in serum

Question 7

What is the normal range for serum osmolality?

Answer 7

275 - 295mmol/kg