Distribution of body water

[Ref: KB2: p1-2; WG21:p1-3]

Total body water (TBW)
= 60% of total body weight

Distribution of body water in KB2

Using 70 kg man as example

From 42L (100%) - TBW

• 23L (55%) in intracellular (ICF)
• 19L (45%) in extracellular (ECF)
  * 1L (2.5%) in transcellular fluid
  * 3L (7.5%) in bone
  * 3L (7.5%) in dense connective tissue
  * 3L (7.5%) in intravascular (IVF)
  * 9L (20%) in interstitial (ISF)

Since water in bone and dense CT moves slowly

• ICF:ECF ~ 2:1
• ISF:IVF ~ 2:1

Distribution of body water in WG21

• Water content of LEAN body tissues is constant
  * ~ 71-72mL/100g.
• Fat is relatively free of water
  --> TBW:Weight ratio changes with fat %
• Ratio is lower in woman and decrease with age

Body weight

• Protein = 18%
• Mineral = 7%
• Fat = 15%
• Water = 60%

Total body water (as % of BODY WEIGHT)

• ICF = 40%
• ECF = 20%
  * IVF (plasma) = 5%
  * ISF = 15%
• Blood volume = 8%
  * Presumably RBC volume is 3% of body weight and is not included in ECF

Total body water (as % of total body water)

Can be derived from above, assuming TBW = 60% of body weight

• ICF = 66% of TBW
• ECF = 33% of TBW
  * IVF (plasma) = 8% of TBW
  * ISF = 25% of TBW
• Blood volume = 12% of TBW

Other notes

Transcelluar fluid

... fluids in epithelial lined spaces

e.g.

• CSF (150mL)
• Urine
• Joint fluid
• Aqueous humour
• Bile
• Bowel content

ECF

Sodium and its associated anions contribute

• 86% of ECF osmolality
• 92% of ECF tonicity
  * Ineffective osmole not included in tonicity

Functional ECF

• 30% of TBW
• Does not include water in bone or dense CT
• More kinetically active
• ICF: functional ECF = 55:30
  ~2:1

Idiogenic osmoles

Neurones can produce extra osmoles when their cell volume decreases due to extracellular hypertonicity
--> Cell volume restored