Body Fluid Compartments Flashcards
Define osmolarity?
Concentration of osmotically active particles present in a solution
Units of osmolarity?
osmol/l
mosmol/l (this used to describe weak salt solutions, i.e: body fluids)
2 factors that must be known to calculate osmolarity?
- Molar conc. of the solution
2. No. of osmotically active particles present
Osmolarity of 150mM NaCl?
Molar conc. = 150mM (this is the same as 150 mmol/L)
No. of osmotically active particles = 2
Osmolarity = 2 x 150 = 300 mosmol/L (as the molar conc. is already in mM)
Osmolarity of 100mM MgCl2?
Osmolarity = 3 x 100 = 300 mosmol/L
Differences and similarities between 150 mM of NaCl and 100 mM of MgCl2?
The 2 solutions are very different (in terms of molar conc. and solutes) but they have the same osmolarities
Difference between osmolarity and osmolality?
Osmolarity has units of osmol/kg water
Osmolarity has units of osmol/l
Osmolarity of body fluids?
~300 mosm/l
Cell membranes are selectively permeable; despite this, the osmotic conc. of both ECF and ICF are identical (~300 mosm/l)
Define tonicity?
Effect a solution has on cell volume:
• Isotonic
• Hypotonic
• Hypertonic
Effect of an isotonic solution on a rbc?
No effect on cell volume (there is still movement across the cell membrane but there is no net movement in one direction)
Effect of a hypertonic solution on a rbc?
Cell volume DECREASES as water leaves the cell by osmosis; result is CELL SHRINKAGE
Effect of a hypotonic solution on a rbc?
Cell volume INCREASES as water enters the cell by osmosis; result is CELL LYSIS
Factor that affects tonicity?
Also takes into consideration the ability of a solute to cross the cell membrane (permeability)
Effect of 300mM urea on a rbc and why this occurs?
Urea does not dissociate and remains as a single particle and thus it would be expected to be isotonic
However, urea causes rbc lysis, as the cell membrane is very permeable to urea
Effect of 300mM sucrose on a rbc and why this occurs?
300mM sucrose is isotonic, as the cell membrane is relatively impermeable to it
Total body water (TBW) in males and females? Why are the different?
Male - ~60% of body weight
Female - ~50% of body weight
Females have more adipose tissue, which does not hold much water
2 major compartment of TBW?
Intracellular fluid (ICF) - 67% of TBW
Extracellular fluid (ECF) = ~33% of TBW; this consists of:
• Plasma (~20%)
• Interstitial fluid (~80%)
• Lymph (negligible)
• Transcellular fluid, e.g: CSF, pleural fluid (negligible)
How can the body fluid compartments be measured?
Use tracers and obtain the distribution volumes of these tracers; useful tracers are:
• TBW: titriated water (3H2O with the 3 superscripted)
• ECF: inulin
• Plasma: labelled albumin
TBW = ECF + ICF so ICF can be calculated if the TBW and ECF are known
Dilution principle to measure volume of distribution?
- Imagine adding a known dose of tracer, e.g: D; = 42mg, to a container holding a large and unknown volume of water (V)
- Mix the tracer/allow it to equilibrate with the water.
- Take a small sample volume from the container (5ml) and measure the concentration of the tracer (C) in this sample
- On analysis, C = 0.005mg/5ml = 0.001mg/ml = 1mg/litre
The volume of the water in the container (V) can be calculated as:
V (litres) = Dose (D) / Sample conc. (C)
42 mg / 1mg/litres = 42 litres
How to measure the distribution volume of a tracer?
- Add a known quantity of tracer X (QX; mol or mg) to the body
- Measure the equilibration volume of X in the body ([X])
Distribution volume (litres) = Qx (mol) / [X] (mol/litre)
Water balance equation?
Input(s) - output(s)
Water imbalance manifests as changes in body fluid osmolarity
Contributions to fluid input?
Fluid intake - 1200
Food intake - 1000
Metabolism - 300
(ml/day)
Contributions to fluid output?
Insensible losses (no physiological control over these):
• Skin - 350
• Lungs - 350
Sensible loss:
• Sweat - 100
• Faeces - 200
• Urine - 1500
(ml/day)
Maintenance of water balance?
Increased water ingestion
Decreased excretion of water by the kidneys alone is insufficient to maintain water balance; the kidneys must always produce a small amount of urine in order to excrete waste products
Ionic composition of the ICF and ECF?
ICF (mM): • Na+ 10 • K+ 140 • Cl- 7 • HCO3- 10
ECF (mM): • Na+ 140 • K+ 4.5 • Cl- 115 • HCO3- 28
There are more Na+, Cl- and HCO3- ions in the ECF; there are more K+, Mg2+ ions and negatively-charged proteins in the ICF
Define fluid shift?
Movement of water between the ICF and ECF in response to an osmotic gradient.
What would happen to ECF and ICF volumes if the osmotic concentration of the ECF increases?
If ECF osmotic conc. increases, this means they have lost water from the ECF; so, the ECF is hypertonic and water moves from inside the cell into the ECF
This result is an increase in ECF and a decrease in ICF
What would happen to ECF and ICF volumes if the osmotic concentration of the ECF decreases?
May occur if too much fluid has been ingested; so, the ECF has become hypotonic and water moves into the cell (increased ICF volume)
Factors that change the fluid osmolarity?
- Gain or loss of water
- Gain or loss of NaCl:
• ECF NaCl gain - increased ECF volume and decreased ICF volume
• ECF NaCl loss - decreased ECF volume and increased ICF volume
Factors that change the ECF volume only?
Gain or loss of isotonic fluid - does not change the fluid osmolarity; changes the ECF volume only
Why is regulation of ECF volume important?
Vital for long-term control of plasma volume and BP
Why is electrolyte balance important?
- Total electrolyte concentrations can directly affect water balance (via changes in osmolarity)
- Concentrations of individual electrolytes can affect cell function
Which ions are the main contributors to the osmotic conc. of the ECF and ICF?
Na+ and K+
Why is Na+ a major determinant of ECF volume?
> 90% of the osmotic conc. of the ECF results from the presence of Na+ salts; Na+ is mainly present in the ECF
Role of K+?
Key role in establishing membrane potential; >95% is intracellular
Consequences of changes in plasma K+ conc. ?
Muscle weakness can cause paralysis
Cardiac arrhythmia can lead to arrest
Intake and output of salt?
Intake:
• Fluids and food - 10.5g
Output:
• Sweat and faeces - 0.5g
• Urine - 10g
How does salt imbalance manifest?
Changes in ECF volume
Regulation is important for long-term BP control
