Body fluid compartments | Renal and urinary system | Physiology

Our body is composed of 50-70% of water. Total body water is distributed between two compartments - ⅔ rd is intracellular fluid or ICF and ⅓ rd is extracellular fluid or ECF. Remember the 60-40-20 rule which indicates that 60% of body weight is water, 40 % is ICF and 20% is ECF. ECF is composed of ¼ th plasma and ¾ th interstitial fluid. Plasma constitutes 55% of blood volume. The interstitial fluid resembles plasma except that plasma proteins and blood cells are absent.

The variation between individuals in the ratio of TBW to total body weight is quite large but the majority of the variation is due to different amounts of adipose tissue as adipose has a low water content. Differences (between individuals) in the amount of bone and plasma are much smaller. Obese adults have a lower ratio because of the greater amount of adipose tissue. Differences in percent body water between males and females are primarily due to differences in amounts of adipose tissue. Neonates contain more water than adults: 75-80% water with proportionately more ECF especially interstitial fluid then 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

ICF can be calculated indirectly by subtracting ECF from the total body water. Interstitial fluid can be calculated by subtracting plasma volume from the ECF.

Fluid shifts between compartments happen to equalize osmolarity on both sides of the cell e.g. between ICF and ECF. Water will freely shift between cell membranes to balance the osmolarity in the ECF and ICF. For example if the ECF becomes hypertonic, water will shift out of the ICF into the ECF till osmolarity is equal in both compartments. Osmolarity of the ECF is determined by the following formula:

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