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
- known amount of indicator = X
injected into body compartments - unknown volume of fluid (body compartments) = V
- final concentration = C
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
- fluid returns to plasma
- (at venular end of capillary)
why?
- inward force of colloid osmotic pressure predominates
- fluid enters lymph capillaries
why?
- hydrostatic pressure in interstitial spaces forces fluid
- 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?
- plasma volume more concentrated
(↑ plasma osmolarity or ↓ blood volume = dehydrated)
- ↑ thirst = ↑ water ingestion
or - ↑ ADH released = ↓ water excretion
- plasma volume returns to normal conc
(↓ plasma osmolarity or ↑ blood volume)
- 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