The distribution of body fluids depends on

• intracellular fluid (fluid inside the cell), and
• extracellular fluid (fluid found outside or in between cells).

Total body water is the sum of intracellular and extracellular water (volume) making up about 60-70% of total body weight for normal adults.

Age affects the distribution of body fluids:

At birth, TBW accounts for 75%-80% of body weight, infants 67%-70%, children 65%, adults 60%, and elderly 50%.

INTRACELLULAR FLUID:

• fluid inside the cells.

EXTRACELLULAR FLUID:

• interstitial fluid (solution that bathes and surrounds the tissue cells),
• intravascular fluid (blood plasma) and
• other fluid that shifts between the two spaces—"third space fluids."

Third space shifts become clinically meaningful when they are not in balance with the interstitial and intravascular fluid volumes.

THIRD SPACE: in certain situations, such as trauma, obstruction, crush injuries, infection, or obstruction to a large venous system, there can be volume depletion by the loss of interstitial and intravascular fluid into a third-space that is not in equilibrium with the extracellular fluid.

For example, a fracture can isolate a large amount of blood around it, and although this fluid will be re-absorbed back into the extracellular fluid over a period of time, the acute reduction in blood volume can lead to significant volume depletion that can cause hypovolemic shock.

In the field, the distribution of body fluids is relevant only in that intervention for an uncompensated shift will require the next best thing until proper blood products can be restored at an appropriate facility. This is usually 500 cc boluses of normal saline, serially, until the systolic pressure reaches/maintains at 90 mmHg.