What fraction of blood fluid is represented by extracellular fluid?

Extracellular fluid comprises approximately one-third of the total body water. When considering the entirety of blood volume, the extracellular fluid itself can be further subdivided into interstitial fluid and plasma. Plasma, which is the liquid component of blood, accounts for about 55% of the total blood volume, while interstitial fluid is the fluid that surrounds individual cells.

When calculating the total body water, it's useful to remember that approximately two-thirds (or 67%) of the body's water is found inside the cells (intracellular fluid), while the remaining one-third (or 33%) is the extracellular fluid. This balance is vital for maintaining homeostasis and facilitating various physiological functions. Therefore, the fraction of blood fluid represented by extracellular fluid being one-third aligns with established physiological principles about fluid distribution in the human body.