Body Fluid Compartments

List the main fluid compartments

Estimate the volume of the main fluid compartments based on sex & mass

Compare & contrast the composition of the main body fluid compartments

Explain how body fluid compartments can 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 & tonicity

TOTAL BODY WATER (TBW)

Intracellular Fluid (ICF)

Extracellular Fluid (ECF)

found
surrounding all cells

found
INSIDE all cells

divided into:

  • interstitial fluid (ISF) = 3/4
  • plasma = 1/4
  • transcellular fluid = 0.5L

total amount of body fluid in weight of a 70kg man

60%

∴ 70kg = 70L
so 60% = 42L

factors affecting TBW

  • age
  • gender
  • percentage body fat

TBW in:

  • MALES = 60%
  • FEMALES = 50%
  • INFANTS = 80%

why difference between male & female?

  • females higher % of adipose tissue

In EXTRACELLULAR FLUID

higher %

  • Na+
  • Cl-
  • HCO3+

in INTRACELLULAR FLUID

higher %

  • K+
  • PO4 3-
  • protein

similar levels of

  • Ca 2+
  • Mg 2+

volume of compartments measured using technique....
INDICATOR (DYE) DILUTION METHOD

indicator properties:

  • even distribution through relevant compartments
  • no leak, loss or metabolism
  • non-toxic

method

  1. known amount of indicator = X
    injected into body compartments
  2. unknown volume of fluid (body compartments) = V
  3. final concentration = C

image

DIFFERENT INDICATORS used to calculate volumes of DIFFERENT COMPARTMENTS

some spaces CANNOT be measured
with markers

  • must use other values calculated from


    other compartments


  • e.g. ICF = TBW-ECF

  • ISF = ECF - plasma volume

OSMOSIS

requires:

  • solute conc gradient
  • semipermeable membrane

D: water moves from LOW solute conc to HIGH solute conc

primary force that drives fluid exchange between different compartments

OSMOLARITY

  • total no. of particles in solution
  • mosmol/L

OSMOTIC PRESSURE

  • opposing pressure required to prevent osmotic movement of H2O across given membrane
  • kPa or mmHg

TONICITY

  • osmotic effect of a solution at cell membrane relative to normal extracellular fluid
  • isotonic, hypertonic, hypotonic

what causes fluid exchange?

difference in hydrostatic & osmotic pressure

hydrostatic pressure

  • pressure of fluid within compartment
  • e.g. plasma within capillary exert pressure on capillary wall

osmotic pressure

  • pressure required to prevent osmosis occurring

process of
fluid exchange

1.fluid leaves plasma

  • (at arteriolar end of capillary)

why?

  • outward force hydrostatic pressure predominates
  1. fluid returns to plasma
  • (at venular end of capillary)

why?

  • inward force of colloid osmotic pressure predominates
  1. fluid enters lymph capillaries

why?

  • hydrostatic pressure in interstitial spaces forces fluid
  1. interstitial fluid now in EQUILIBRIUM with transcellular & intracellular fluid

plasma, ISF & ICF have SAME OSMOLARITY at equilibrium

  • 278-300 mOsmol/kg H2O

to maintain equilibrium in body...
water intake must equal water output

water gain = 2.5L/day

  • food & drink = 2.2L
  • metabolism = 0.3L

water loss = 2.5L/day

  • skin & lungs = 0.9L
  • urine = 1.5L
  • faeces = 0.1L

how body increases body water/retains water?

  1. plasma volume more concentrated
    (↑ plasma osmolarity or ↓ blood volume = dehydrated)


  1. ↑ thirst = ↑ water ingestion
    or
  2. ↑ ADH released = ↓ water excretion


  1. plasma volume returns to normal conc
    (↓ plasma osmolarity or ↑ blood volume)


  1. NEGATIVE FEEDBACK = decrease ADH & decrease thirst

how can body decrease body water?

increases urine output as decrease in ADH

  • limits amount of water reabsorbed into blood

excessive intake of water =

  • water intoxication
  • hyponatremia
    (important electrolytes diluted - leads confusion, headaches, swelling of brain)

excessive loss of water =

  • dehydration
  • hypovolaemia
    (when body loses fluid - leads to dizziness, dry mouth, fatigue)
  • sweating (exercise/hot weather)
  • blood loss
  • diarrhoea
  • vomiting
  • burns (seeps out of body)
  • alcoholic beverages

4 different types of intravenous fluids

WATER
exact same effect as hypotonic saline

HYPOTONIC SALINE

  • ECF volume = increase
  • ICF volume = increase - water moves in via osmosis
  • ECF osmolality = decreases
  • ICF osmolality = decreases

HYPERTONIC SALINE

  • ECF volume = dramatically increase - water pulled out of cells from ICF
  • ICF volume = decreases
  • ECF osmolality = increases (because added hypertonic solution w/ lots of solutes)
  • ICF osmolality = increase

ISOTONIC SALINE

  • ECF volume = increase
  • ICF volume = stay same
  • ECF osmolality = stay same
  • ICF osmolality = stay same