Fluid balance

Question 1

What is the approximate percentage of total body weight that is water in a 70kg male?

Answer 1

Approximately 60%. This is part of the 60-40-20 rule for fluid distribution.

Question 2

Name the three major fluid compartments of the body.

Answer 2

Intracellular fluid (ICF), Interstitial fluid, and Intravascular fluid (plasma).

Question 3

What is the approximate ratio of intracellular to extracellular fluid?

Answer 3

2:1 (ICF:ECF).

Question 4

What are the main determinants of serum osmolality? Provide a mnemonic to help remember.

Answer 4

Sodium (Na+), Potassium (K+), Urea, and Glucose. A simple mnemonic (though not explicitly stated in the document) could be: “S.U.G.A.R.” (though note that Potassium is important too)

Question 5

What is the formula for calculating serum osmolarity?

Answer 5

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

Question 6

What are the key differences between osmolality and osmolarity? mnemonic?

Answer 6

Osmolality: Measures the concentration of dissolved particles (osmoles) per kilogram of solvent (water). Measured directly using an osmometer. Units: mmol/kg.

Osmolarity: Measures the concentration of dissolved particles (osmoles) per liter of solution. Calculated from measured serum values. Units: mmol/L. In dilute solutions, they are approximately equal.

osmoLAlity = lab measured (osmometer)
osmolaRity = litRe - calculatoR

Question 7

What is the normal range for serum osmolality?

Answer 7

The normal range for serum osmolality is generally considered to be 275-295 mmol/kg.

Question 8

What is the osmolar gap?

Answer 8

measured osmolality - calculated osmolality

Question 9

Normal Osmolar Gap?

Answer 9

<10 mmol/kg

Question 10

Elevated Osmolar Gap?

Answer 10

> 10 mmol/kg
Indicated the presence of unmeasured osmotically active substances. The mnemonic “ALPS” (Alcohols, Lipids, Proteins, Sugars) can help recall some possibilities.

Question 11

Describe the key differences in electrolyte composition between ICF and ECF.

Answer 11

ICF is high in potassium (K+) and low in sodium (Na+) and chloride (Cl−). ECF is high in sodium (Na+) and chloride (Cl−) and low in potassium (K+).

Question 12

What is the 60-40-20 rule?

Answer 12

60% of body weight is total body water, 40% is intracellular fluid, and 20% is extracellular fluid.

Question 13

rate the following conditions from highest to lowest osmolality?
DKA, pneumonia, SIADH, HHS, DI

Answer 13

highest –> lowest
HHS, DKA, DI, pneumonia, SIADH