Body Fluid Compartments

Question 1

Learning objectives

Answer 1

list the main fluid compartments
• estimate the volume of the main fluid compartments based on sex and mass
• compare and contrast the composition of the main body fluid compartments
• explain how these compartments may be measured
• explain the mechanisms responsible for fluid exchange between the main compartments
• appreciate that there are mechanisms that regulate body fluid volume and osmolality
• describe the effects of intravenous infusion of solutions of differing osmolality and tonicity

Question 2

What are the two main compartments of TBW (total body water)?

Answer 2

Intracellular fluid (ICF)
• Inside all cells
• 2/3 of TBW

Extracellular fluid (ECF)
• Surrounds all cells
• 1/3 of TBW
• Divided into:
• Interstitial Fluid (ISF) = ¾ of ECF
• Plasma = ¼ of ECF

Question 3

What factors does TBW depend on?

Answer 3

Age, gender and % body fat.
Total body water (TBW) constitutes ~60% of body weight in young men & ~50% of body weight in young women- difference as % fat is higher in females

TBW= 0.6 x BM
1/3 ECF, 2/3 ICF
ECF- 1/4 plasma, ISF 3/4 - transcellular fluid 0.5l

Question 4

How do the two main body fluid compartments differ in terms of electrolytic composition?

Answer 4

Sodium, chloride, calcium (ionised) and bicarbonate ions more abundant in ECF
Potassium, magnesium, phosphate and organic ions and proteins more abundant in ICF (Na/K ATPase pump regulates)

Question 5

Measurement of body fluid compartments

Answer 5

Amount of indicator= x (e.g sucrose)
Volume of fluid = V
Final concentration= C

V= X/C
Indicator properties- even distribution throughout relative compartment
No leak, loss or metabolism
Non- Toxic
- Intracellular water and Interstitial water (“extravascular water”) cannot be measured with dye/dilution technique

Question 6

What is the role of osmosis in fluid exchange of differnent compartments?

Answer 6

Low solute conc to high solute conc, across partially permeable membrane
Units of osmotic activity
Osmolality- total no. of all particles in solution/ mosmol/kg solvent
Osmolarity- total no. of particles in solution mosmol/l solution

Question 7

How is the net movement of fluids between compartments achieved?

Answer 7

Net movement of fluids between compartments results from differences in hydrostatic and osmotic pressures

The pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity.

The plasma, ISF & ICF have the same osmolality at equilibrium: 278–300 mOsmol/kg H2O

Question 8

What is tonicity?

Answer 8

Tonicity refers to the osmotic effect of a solution at the cell membrane, relative to normal physiological solution
RBC in isotonic solution- no change. Conc inside and outside in eqm
In hypertonic solution, RBC crenates- water drawn out by osmosis (attempt to create osmotic eqm)
In hypotonic solution, RBC lyses (haemolysis)- water taken in, higher conc inside than out, swelling

Question 9

How can the body decrease body water?

Answer 9

How can the body decrease body water?
-It increases the urine output (￬ ADH)
However,
- Excessive loss can lead to hypovolaemia or dehydration
-Other causes of water loss:
• Sweating in hot climates
• Sweating during exercise: loss can be up to 12 L/day when intensive
• Blood loss
• Diarrhoea
• Vomiting
• Burns
• Alcoholic beverages