SM 187a - Body Fluid Compartments

Question 1

What is the equation for estimating serum osmolality?

Answer 1

Question 2

Describe the changes to volume and osmolaltiy of ECF and ICF fluid when a person loses pure water

Answer 2

• ECF volume decreases due to loss of pure water
• This increases ECF osmolality
• Water from ICF moves to ECF until the compartments are equal in osmolality
• This decreases ICF volume
• This increases ICF osmolality

In the end, both compartments increase in osmolarity and decrease in volume. ICF volume decreases more than ECF volume (volume loss is proportional to the size of the compartment)

Question 3

Give 2 examples of hypoosmotic fluid loss

Answer 3

Diuretics

Excessive sweating

Question 4

How do the osmolarity and volume of intracellular and extracellular fluid change with hypoosmotic fluid loss?

Answer 4

Hypoosmotic fluid loss can be thought of as losing some isosmotic fluid and some pure water

• Isosmotic fluid loss
  
  • ECF volume decreases
  • No change in ECF osmolality, ICF volume, or ICF osmolality
• Pure water loss
  
  • ECF and ICF volume decrease, but in proportion to their compartment sizes
    
    • => ICF volume decreases 2x as much as ECF volume
  • ECF and ICF osmolality increase the same amount

For example…

Losing 1L of fluid with 1/2 of the osmolality of the ECF = losing 0.5 L isotonic fluid and 0.5 L pure water

• Losing 0.5 L isotonic fluid
  
  • ECF volume loss of 1/2 L
• Losing 0.5L pure water
  
  • ECF volume loss = 0.5 L * 1/3 = 1/6 L
  • ICF volume loss = 0.5 L * 2/3 = 1/3 L
• Total ECF volume loss = 1/2L * 1/6L = 2/3L
• Total ICF volume loss = 1/3L