181: Body Fluid Compartments Flashcards
What fraction of body fluid is intracellular?
2/3
What fraction of body fluid is extracellular?
1/3
Extracellular fluid = plasma + interstitial fluid
What are the two compartements of extracellular fluid?
- Plasma
- 25% of extracellular fluid
- 1/12 of total body fluid
- Extracellular fluid = 1/3 of body fluid
- 1/4 of 1/3 = 1/12
- Interstitial fluid
- 75% of extracellular fluid
- 1/4 of total body fluid
What is the major solute of intracellular fluid?
K+ ions
What is the major solute of extracellular fluid?
Na+ ions
What is the difference between primary active transport and secondary active transport?
Primary active transport hydrolyzes ATP to move a substance across the membrane
Secondary active transport moves substances across the membrane using the concentration gradient set up by primary active transport
What is the difference between osmolarity and osmolality?
Which one is preferred clinically?
Osmolality = number of osmoles/Kg H2O
Osmolarity = number of osmoles/L H2O
Osmolality is preferred becasue weight is not sensitive to tempearature changes but volume is
What is the equation for estimating serum osmolality?
Hypertonic vs. hypotonic?
_________ solutions cause cell shrinkage
_________ solutions cause cell swelling
hypertonic solutions cause cell shrinkage
hypotonic solutions cause cell swelling
Why isn’t pure water administered intravenously to patients who are dehydrated?
Pure water = hypotonic
Coulc cause cell lysis
How do the volume and osomolality of ECF and ICF change with isosmotic fluid loss?
- ECF
- Volume decreases
- No change in osmolaity
- ICF
- No change in volume
- No change in osmolality
ECF volume will decrease, but there will be no change in the osmotic or concentration gradient (and therefore no net fluid movement) across the cell membranes
Describe the changes to volume and osmolaltiy of ECF and ICF fluid when a person loses pure water
- 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)
What is the most common type of fluid loss in clinical medicine?
Hypoosmotic fluid loss
Caused by excessive sweating/diuresis
Give 2 examples of hypoosmotic fluid loss
Diuretics
Excessive sweating
How do the osmolarity and volume of intracellular and extracellular fluid change with hypoosmotic fluid loss?
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
- ECF and ICF volume decrease, but in proportion to their compartment sizes
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
How do the volume and osmolality of ICF and ECF change with an infusion of 0.9% NaCl?
0.9% NaCl is isosmotic
- ECF
- Increased volume
- No change in osmolality
- ICF
- No change in volume
- No change in osmolality
An IV infusion of 0.9% NaCl is…
- Hypoosmotic
- Isosmotic
- Hyperosmotic
b. Isosmotic
An IV infusion of 0.45% NaCl is…
- Hypoosmotic
- Isosmotic
- Hyperosmotic
a. Hypoosmotic
How do the volume and osmolality of ICF and ECF change with an infusion of 0.45% NaCl?
- ECF
- Volume increase (more than ICF)
- Osmolality decrease
- ICF
- Volume increase (less than ECF)
- Osmolality decrease
Think of this like giving 1/2 isosmotic fluid and 1/2 pure water
- Isosmotic fluid infusion
- ECF volume increase
- No change to other compartments
- Pure water infusion
- ECF
- Volume increase
- Osmolality decrease
- ICF
- Volume increase - ICF is hyperosmotic compared to ECF, water will move ECF -> ICF until osmolality equilibrates
- Osmolality decrease
-
An infusion of pure water will increase ICF volume more than ECF volume
- Volume increase is proportional to compartment size
- ECF
- HOWEVER, when you account for the increase in ECF only due to isosmotic fluid infusion, ECF volume expands more than ICF volume
Explain the process of acute water intoxication
Too much water ->
- ECF fluid increases in volume and decreases in osmolality
- Water moves from the ECF to the ICF
- ICF expands
- Cells swell and potentially lyse
- Swelling is especially bad in the brain
Describe (sequentially) what happens to ECF and ICF if 3% NaCl is administered intravenously to a patient
3% NaCl is hyperosmotic
- ECF immediatly increases in volume and osmolality
- Water moves from ICF to ECF until osmolality equilibrates
Result
- ECF
- Volume increases
- Osmolality increases
- ICF
- Volume decreases
- Osmolality increases
Which organ is most important for regulating blood pressure over a long time period?
How does it do so?
Kidney
Regulates Na+ excretion
Suppose you have two chambers of water separated by a selectively permeable membrane. The chambers are labeled A and B respectively and initially the equal-sized chambers are both half-full with pure water. The membrane is permeable to water and sodium, but impermeable to potassium and chloride.
What will happen with respect to net movement of water if chamber A suddenly has 100 mM potassium chloride?
Water will move from chamber B to chamber A
Suppose a person is given a 1 liter intravenous infusion of isotonic NaCl solution.
How much of this fluid will remain in the extracellular space?
All of it
There will be no osmotic gradient for water movement from the ECF to the ICF
The sodium-potassium pump hydrolyzes one molecule of ATP each time it moves 3 sodium ions out of the cell and 2 potassium ions into the cell.
What type of transport does this carrier protein perform?
Primary active transport
