Body Fluid Compartments

Question 1

Define osmolarity?

Answer 1

Concentration of osmotically active particles present in a solution

Question 2

Units of osmolarity?

Answer 2

osmol/l

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

Question 3

2 factors that must be known to calculate osmolarity?

Answer 3

1. Molar conc. of the solution

2. No. of osmotically active particles present

Question 4

Osmolarity of 150mM NaCl?

Answer 4

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)

Question 5

Osmolarity of 100mM MgCl2?

Answer 5

Osmolarity = 3 x 100 = 300 mosmol/L

Question 6

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

Answer 6

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

Question 7

Difference between osmolarity and osmolality?

Answer 7

Osmolarity has units of osmol/kg water

Osmolarity has units of osmol/l

Question 8

Osmolarity of body fluids?

Answer 8

~300 mosm/l

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

Question 9

Define tonicity?

Answer 9

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

Question 10

Effect of an isotonic solution on a rbc?

Answer 10

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

Question 11

Effect of a hypertonic solution on a rbc?

Answer 11

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

Question 12

Effect of a hypotonic solution on a rbc?

Answer 12

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

Question 13

Factor that affects tonicity?

Answer 13

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

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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

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

Question 17

2 major compartment of TBW?

Answer 17

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)

Question 18

How can the body fluid compartments be measured?

Answer 18

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

Question 19

Dilution principle to measure volume of distribution?

Answer 19

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

Question 20

How to measure the distribution volume of a tracer?

Answer 20

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)

Question 21

Water balance equation?

Answer 21

Input(s) - output(s)

Water imbalance manifests as changes in body fluid osmolarity

Question 22

Contributions to fluid input?

Answer 22

Fluid intake - 1200
Food intake - 1000
Metabolism - 300

(ml/day)

Question 23

Contributions to fluid output?

Answer 23

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

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

(ml/day)

Question 24

Maintenance of water balance?

Answer 24

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

Question 25

Ionic composition of the ICF and ECF?

Answer 25

``` 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

Question 26

Define fluid shift?

Answer 26

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

Question 27

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

Answer 27

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

Question 28

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

Answer 28

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

Question 29

Factors that change the fluid osmolarity?

Answer 29

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

Question 30

Factors that change the ECF volume only?

Answer 30

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

Question 31

Why is regulation of ECF volume important?

Answer 31

Vital for long-term control of plasma volume and BP

Question 32

Why is electrolyte balance important?

Answer 32

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

Question 33

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

Answer 33

Na+ and K+

Question 34

Why is Na+ a major determinant of ECF volume?

Answer 34

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

Question 35

Role of K+?

Answer 35

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

Question 36

Consequences of changes in plasma K+ conc. ?

Answer 36

Muscle weakness can cause paralysis Cardiac arrhythmia can lead to arrest

Question 37

Intake and output of salt?

Answer 37

Intake: • Fluids and food - 10.5g Output: • Sweat and faeces - 0.5g • Urine - 10g

Question 38

How does salt imbalance manifest?

Answer 38

Changes in ECF volume Regulation is important for long-term BP control