Chemical Pathology - Fluid Balance

Question 1

What is the 60-40-20 rule?

Answer 1

60% total body weight = water
40% body weight = intracellular
20% body weight = extracellular

Question 2

What is the volume (L) and percentage of total volume of intracellulalar water?

Answer 2

• 28L
• 60-65%

Question 3

What are the different types of extracellular water, volumes and percentages of total volumes?

Answer 3

14L overall, 35-40%
- Interstitial (between cells): 10L, 24%
- Intravascular: 3L, 5%
- Transcellular (within epithelial-lined spaces): 1L, 3%

Question 4

What is osmolality?

Answer 4

The total number of particles in a solution
- Measured with an osmometer
- Units = mmol/kg

• MASS

Question 5

What is osmolarity?

Answer 5

The measure of solute concentration/the number of particles of solute per litre of solution - calculated
- Units = mmol/L

• VOLUME

Question 6

What are the phsyiological + pathological determinants in serum/plasma?

Answer 6

Physiological = Na, K, Cl, HCO3, Urea, Glucose
Pathological = Endogenous (e.g. glucose), exogenous (e.g. ethanol, mannitol)

Question 7

What is the equation for calculating osmolarity?

Answer 7

= 2(Na + K) + Urea + Glucose

Question 8

What is the osmolar gap?

Answer 8

The difference in osmolarity and osmolality (should be roughly equal)

Question 9

What is the normal range of osmolality?

Answer 9

275-295 mmol/Kg

Question 10

What does it mean if the osmolarity is lower than the osmolality?

Answer 10

Assume there are extra (unmeasured) solutes that are dissolved in the serum