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Body fluid compartments & Water Balance

The body fluid compartments in the body can be divided into:

1.Extracellular fluid compartment

2. Intracellular fluid compartment

Distribution of compartments:

•Average 70 kg adult human – 42 liters

•60% of total body weight

–ICF contains 28L

–ECF contains 14 L

interstitial fluid is 11 litres.

plasma is 3 litres.

Total body water depends upon:

  • Age
  • Sex
  • Degree of obesity

With Increasing Age

•Percentage of total weight that is water decreases

•Aging – associated with increased percentage of body water that is fat

•Fat – decreases percentage of water in body

Women – More body fat than men

•Contain slightly less water than men in proportion to their weight

Body water

Importance/significance Solvent



–Intracellular medium

–Water deprivation leads to DEATH

Physiological variation


–Male (60%) 54 – 70%

–Female (50%) 45 – 60%


–More in children

–Gradual decrease


–Less body water

Water balance

•In health total body water is kept reasonably constant in spite of wide fluctuations in daily intake.

–Daily intake = Daily output • (20 ºC, No exercise)

Intracellular fluid Compartment

– 28 liters inside 75 trillion cells of body

Constitutes 40% of body weight

Composition of cell fluids – remarkably similar even in different animals ranging from most primitive micro-organisms to humans

•ICF of all different cells regarded as one large fluid compartment

Extracellular fluid compartment:

•All fluid outside cells

•14 liters in normal 70 kg adult •20% of body weight

Compartments of ECF

  1. Interstitial fluid
  2. Plasma

Interstitial Fluid

3/4th of ECF

•11 liters


•1/4th of ECF

•3 liters

Blood Volume

•Both Intracellular & Extracellular fluids are involved

•Separate fluid compartment –contained in a chamber of its own –circulatory system

•5 liters / 7% of body weight

Plasma – 60% of blood

Red blood cells – 40%

•Vary depending upon

  • Age
  • Sex
  • Weight etc

Water Balance

Daily Intake of Water

  • Fluids Ingested (liquids/water in diet) – 2100 ml
  • From metabolism – 200 ml

Net Intake = 2300 ml

Daily output of Water

  • Insensible (skin) – 350 ml •
  • Insensible (lungs) – 350 ml
  • Sweat – 100 ml
  • Feces – 100 ml
  • Urine – 1400 ml

Net Output = 2300 ml

Water balance

•Water intake=water output


–Determined by social considerations rather than physiological needs.

–Thirst safeguards fluid intake.

•Water loss


•Exercise(3300 ml/day)

•Hot weather(6600 ml/day) •Pathological(4_8 L/day)

–Urinary volume —only variable that controls

Positive water balance

•Increased water intake than water loss



–Growing children



Negative water balance

•Decreased water intake than water loss


–Fatty meal

–Diuretics use


Measurement of different body fluid compartments;
Indicator Dilution Method
Depends upon law of conservation of mass.

Total mass of the indicator after distribution in the compartment is the same as the mass before distribution.

Indicator properties

•Disperses evenly throughout the compartment

•Disperses only in the compartment that is being measured

•Not metabolized or excreted

Indicators used:

1. Total Body Water

•Radioactive water (Tritium, 3H2O)

•Heavy water (Deuterium, 2H2O)


2. ECF Volume



•125I-iothalamate thiosulfate

3. ICF Volume

ICF Vol = Total Body water – ECF Vol

4. Plasma Volume

•Evans Blue dye (T-1824)


5. Interstitial Fluid Volume

Interstitial Fluid vol = ECF vol – Plasma Vol

Osmotic equilibrium

•Maintained between ECF and ICF.

•Free flow of water through cell membrane.

•TBW – Final distribution determined by osmotic and hydrostatic forces

•Osmolality of body fluids-300 m osm /L

•Isotonicity, hypotonicity, hypertonicity

•Isotonic solutions

–0.9% NaCl

–5% Glucose

Abnormal states and changes in vol and osmolalities (ECF and ICF)

  • Dehydration
  • I / V infusions
  • Fluid loss (Physiological,pathological) •Osmolalities do not change for longer duration after addition of a solution

There is excellent osmolality regulation

Water depletion / dehydration


–Low intake

–Poor absorption

–Increased loss




Clinical features
  • Thirst
  • Dryness of mouth
  • Dry loose skin
  • Oliguria
  • Sunken eyes
  • Hypotention
  • Delerium
  • Haemoconcentration (↑ PCV )

Water Excess ( water toxicity)


  • Excessive intake
  • Renal retention / dysfunctional nephrosis
  • Nephrotic syndrome
  • Liver damage _ hypoproteinemia
  • Raised ADH
  • Drugs e.g. narcotics
  • Hypothyroidism


  • Headache
  • Nausea / vomiting
  • Cramps
  • Raised BP
  • Polyuria
  • Cardiac overload
  • Haemodilution


–Accumulation of excessive fluid in body tissues



–Extracellular (more common)

Intracellular edema


  • Depression of metabolic systems of tissues (cells)
  • Lack of adequate nutrition
  • Na+-K+ ATPase pump failure
  • Inflammed tissues

Extracellular edema


  1. Abnormal fluid leakage from blood capillaries
  2. Failure of lymphatic drainage system


  • Increased capillary pressure
  • Decreased plasma proteins
  • Increased capillary permeability
  • Lymphatic blockage
Increased capillary pressure

Excessive kidney retention of salt/water

  • Acute / chronic kidney failure
  • Mineralocorticoid excess

High venous pressure/constriction

  • Heart failure
  • Venous obstruction
  • Failure of venous valves

↓arteriolar resistance

  • Excessive body heat
  • Insufficient sympathetic nervous sys
  • Vasodilator drugs

Decreased plasma proteins

  • Loss of proteins in urine (nephrotic syndrome)
  • Loss of protein from denuded skin



  • Failure to produce proteins

Liver disease


Increased capillary permeability

  • Immune reactions causing release of histamine/immune products
  • Toxins
  • Bacterial infection
  • Vitamin C deficiency
  • Blockage of lymph return


Infections (filaria nematodes)


  • Congenital absence of lymphatic vessels
edema safety factors

§ Low compliance in negative ISF pressure range

§ Increased lymphatic flow

§ Protein wash out from ISF

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