Our body is made up of about 50-70% water. This total body water (TBW) is distributed between two main compartments:

• Intracellular fluid (ICF): about 2/3 of TBW (fluid inside cells)
• Extracellular fluid (ECF): about 1/3 of TBW (fluid outside cells)

A helpful way to remember these proportions is the 60-40-20 rule:

• 60% of body weight is water (TBW)
• 40% of body weight is ICF
• 20% of body weight is ECF

The ECF is further divided into:

• Plasma: 1/4 of ECF
• Interstitial fluid: 3/4 of ECF

Plasma makes up about 55% of blood volume. Interstitial fluid is similar to plasma, except it lacks plasma proteins and blood cells.

The ratio of TBW to total body weight varies widely between individuals. Most of this variation is explained by differences in adipose tissue, which has a low water content. Differences in the amounts of bone and plasma contribute much less.

• Obese adults have a lower percent body water because they have more adipose tissue.
• Differences in percent body water between males and females are mainly due to differences in adipose tissue.
• Neonates have more water than adults: about 75-80% of body weight is water, with proportionately more ECF, especially interstitial fluid, than adults.

Markers for body fluid compartments

Compartment	Marker
Total body water	D2O, antipyrine, THO or tritiated water
ECF	Mannitol, inulin, sulfate
Plasma	Radiolabeled albumin, Evan’s blue

You can’t measure ICF directly using a marker, so it’s calculated indirectly:

• ICF = TBW − ECF

You can also calculate interstitial fluid indirectly:

• Interstitial fluid = ECF − plasma volume

Fluid shifts between compartments occur to equalize osmolarity across cell membranes (especially between ICF and ECF). Water moves freely across cell membranes to balance osmolarity.

For example, if the ECF becomes hypertonic, water shifts out of the ICF and into the ECF until osmolarity becomes equal in both compartments.

ECF osmolarity is estimated using plasma osmolarity:

Plasma osmolarity = 2 X Plasma Na+ + Glucose/ 18 + BUN/2.8

Where BUN is blood urea nitrogen.

Changes in body fluid compartments in different conditions

Type	Example	Osmolarity	ECF	ICF	Hematocrit	Plasma protein conc.
Isosmotic* volume contraction	Diarrhea, burns	No change	Decreases	No change	Increases	Increases
Hyposmotic volume contraction	Hypoadrenalism	Decreases	Decreases	Increases	Increases	Increases
Hyperosmotic volume contraction	Sweating, fever, DI**	Increases	Decreases	Decreases	No change	Increases
Isosmotic volume expansion	Infusion of isotonic saline	No change	Increases	No change	Decreases	Decreases
Hyposmotic volume expansion	SIADH***	Decreases	Increases	Increases	No change	Decreases
Hyperosmotic volume expansion	High salt intake	Increases	Increases	Decreases	Decreases	Decreases

*ICF is not changing as there is no water shift owing to constant osmolarity.

**Diabetes insipidus

***Syndrome of inappropriate ADH secretion