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Year 2 Physiology > Body Fluid Compartments > Flashcards

Body Fluid Compartments Flashcards

1
Q

Define osmolarity?

A

Concentration of osmotically active particles present in a solution

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2
Q

Units of osmolarity?

A

osmol/l

mosmol/l (this used to describe weak salt solutions, i.e: body fluids)

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3
Q

2 factors that must be known to calculate osmolarity?

A
  1. Molar conc. of the solution

2. No. of osmotically active particles present

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4
Q

Osmolarity of 150mM NaCl?

A

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)

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5
Q

Osmolarity of 100mM MgCl2?

A

Osmolarity = 3 x 100 = 300 mosmol/L

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6
Q

Differences and similarities between 150 mM of NaCl and 100 mM of MgCl2?

A

The 2 solutions are very different (in terms of molar conc. and solutes) but they have the same osmolarities

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7
Q

Difference between osmolarity and osmolality?

A

Osmolarity has units of osmol/kg water

Osmolarity has units of osmol/l

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8
Q

Osmolarity of body fluids?

A

~300 mosm/l

Cell membranes are selectively permeable; despite this, the osmotic conc. of both ECF and ICF are identical (~300 mosm/l)

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9
Q

Define tonicity?

A

Effect a solution has on cell volume:
• Isotonic
• Hypotonic
• Hypertonic

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10
Q

Effect of an isotonic solution on a rbc?

A

No effect on cell volume (there is still movement across the cell membrane but there is no net movement in one direction)

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11
Q

Effect of a hypertonic solution on a rbc?

A

Cell volume DECREASES as water leaves the cell by osmosis; result is CELL SHRINKAGE

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12
Q

Effect of a hypotonic solution on a rbc?

A

Cell volume INCREASES as water enters the cell by osmosis; result is CELL LYSIS

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13
Q

Factor that affects tonicity?

A

Also takes into consideration the ability of a solute to cross the cell membrane (permeability)

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14
Q

Effect of 300mM urea on a rbc and why this occurs?

A

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

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15
Q

Effect of 300mM sucrose on a rbc and why this occurs?

A

300mM sucrose is isotonic, as the cell membrane is relatively impermeable to it

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16
Q

Total body water (TBW) in males and females? Why are the different?

A

Male - ~60% of body weight
Female - ~50% of body weight

Females have more adipose tissue, which does not hold much water

17
Q

2 major compartment of TBW?

A

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)

18
Q

How can the body fluid compartments be measured?

A

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

19
Q

Dilution principle to measure volume of distribution?

A
  1. Imagine adding a known dose of tracer, e.g: D; = 42mg, to a container holding a large and unknown volume of water (V)
  2. Mix the tracer/allow it to equilibrate with the water.
  3. Take a small sample volume from the container (5ml) and measure the concentration of the tracer (C) in this sample
  4. 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

20
Q

How to measure the distribution volume of a tracer?

A
  1. Add a known quantity of tracer X (QX; mol or mg) to the body
  2. Measure the equilibration volume of X in the body ([X])

Distribution volume (litres) = Qx (mol) / [X] (mol/litre)

21
Q

Water balance equation?

A

Input(s) - output(s)

Water imbalance manifests as changes in body fluid osmolarity

22
Q

Contributions to fluid input?

A

Fluid intake - 1200
Food intake - 1000
Metabolism - 300

(ml/day)

23
Q

Contributions to fluid output?

A

Insensible losses (no physiological control over these):
• Skin - 350
• Lungs - 350

Sensible loss:
• Sweat - 100
• Faeces - 200
• Urine - 1500

(ml/day)

24
Q

Maintenance of water balance?

A

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

25
Q

Ionic composition of the ICF and ECF?

A 
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

26
Q

Define fluid shift?

A

Movement of water between the ICF and ECF in response to an osmotic gradient.

27
Q

What would happen to ECF and ICF volumes if the osmotic concentration of the ECF increases?

A

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

28
Q

What would happen to ECF and ICF volumes if the osmotic concentration of the ECF decreases?

A

May occur if too much fluid has been ingested; so, the ECF has become hypotonic and water moves into the cell (increased ICF volume)

29
Q

Factors that change the fluid osmolarity?

A
  1. Gain or loss of water
  2. 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
30
Q

Factors that change the ECF volume only?

A

Gain or loss of isotonic fluid - does not change the fluid osmolarity; changes the ECF volume only

31
Q

Why is regulation of ECF volume important?

A

Vital for long-term control of plasma volume and BP

32
Q

Why is electrolyte balance important?

A
  1. Total electrolyte concentrations can directly affect water balance (via changes in osmolarity)
  2. Concentrations of individual electrolytes can affect cell function
33
Q

Which ions are the main contributors to the osmotic conc. of the ECF and ICF?

A

Na+ and K+

34
Q

Why is Na+ a major determinant of ECF volume?

A

> 90% of the osmotic conc. of the ECF results from the presence of Na+ salts; Na+ is mainly present in the ECF

35
Q

Role of K+?

A

Key role in establishing membrane potential; >95% is intracellular

36
Q

Consequences of changes in plasma K+ conc. ?

A

Muscle weakness can cause paralysis

Cardiac arrhythmia can lead to arrest

37
Q

Intake and output of salt?

A

Intake:
• Fluids and food - 10.5g

Output:
• Sweat and faeces - 0.5g
• Urine - 10g

38
Q

How does salt imbalance manifest?

A

Changes in ECF volume

Regulation is important for long-term BP control

Year 2 Physiology (21 decks)

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