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Body Fluid Compartments
and
Fluid Balance
Water gain must = water loss
Source of metabolically
formed water
Rate of formation of metabolic water is not regulated

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Osmolarity of plasma is about 300 mOsm/L
(Measured by a method called “Freezing Point
Depression)
TONICITY is Osmolarity in relation to that of
plasma. It has the ability to change the
VOLUME of a cell

Isotonic solution: same osmolarity as of plasma

Hypertonic solution

Hypotonic solution?
Osmotic equilibrium is maintained rapidly between ICF and ECF
 Isotonic fluid gain: e.g Intravenous infusion of isotonic
saline (0.9 % NaCl, 5% glucose solution)
 Isotonic fluid loss: Hemorrhage
No net fluid shift occur
What happens ?
Water moves into the cells from ECF until osmotic equilibrium is achieved.
Pronounced swelling of neurons leads to brain dysfunction
Swelling of muscle leads to muscle weakness.
Hypertension and edema caused by expansion of plasma volume.
What Causes this?
Over hydration:
Renal failure
with inability
to excrete
diluted urine
Excessive drinking
(transient)
Increase in ADH
secretion
Water
intoxication:
condition of over
hydration,
hypotonicity and
cellular swelling
from excess
water.
What happens ?
 Water moves out of the cells to ECF until osmotic equilibrium is achieved
 Cells shrink as water leaves them
 Neurons dehydration causes disturbances in brain function manifested as
mental confusion to coma.
Dehydration
due
to:
What Causes
this?
 Insufficient
water intake
 Excessive
water loss
(vomiting or
diarrhea)
 Diabetes
Insipidus
(deficiency of
ADH)
Accumulation
of high levels
of osmotically
active solutes
(urea in kidney
failure)
Tonicity & RBC
Baroreceptor
Rate of
formation of
metabolic
water is not
regulated to
maintain
homeostasis
2% decrease in
body mass due
to fluid loss
causes mild
dehydration
Water gain is
regulated
mainly
by drinking
through thirst
mechanism
Dye-dilution metod
(Indicator-dilution)
Measurements of various body volumes
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Total body wateruse of a substance which when inserted in
to the blood, will spread evenly through out all the body
compartments.
Substances used i. Radio active water
ii. Antipyrine –because it is lipid soluble
Extra cellular volume Substance should disperse easily
through water/plasma but NOT enter cells
Substances used I Radioactive Na, Inulin
These rapidly spread through out extra cellular space30-60 min
Extra cellular volume is also called SODIUM space or INULIN space
ICF = TBW-Extra cellular volume
Plasma & blood volumes

Plasma Volume Substance must stay INSIDE blood
compartment after it is injected in to the blood stream
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Evans Blue (Dye) is the best choice

Blood volume = Plasma volume + hematocrit (cell volume)
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Then BV = Plasma Volume/ (1-Hct)
Given Plasma vol as 3 litres, and
Hct as 0.45
BV = 3/(1-0.45) = 3/0.55
= 5.45 L
Measurement of various body fluid compartments
Summary
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1. Osmolarity, its normal value, and osmotic
pressure
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2. Hydrostatic pressure
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3. Distribution of body water
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4. Intra and extra cellular fluids and their
composition
5. Tonicity of plasma and learnt what is iso,
hypo and hyper tonicity
6. Measurements of body fluid compartments