The extracellular fluid (ECF) consists of all the body fluid outside of cells. It includes the intravascular fluid (blood plasma) and the interstitial fluid (fluid between cells). The intravascular fluid is the portion of the ECF contained within blood vessels, principally composed of blood plasma.
In a healthy adult, the intravascular fluid volume is about 25% of the total extracellular fluid volume. Thus, if we look at the options given for this question:
A. 75% of ECF - This is too high; the majority of the ECF is outside the blood vessels, in the interstitial space.
B. 25% of ECF - This is the accurate representation of the portion of ECF that's intravascular.
C. 60% of TBW (Total Body Water) - This is not accurate; total body water includes intracellular and extracellular fluid, and the intravascular portion of the ECF is a much smaller portion of TBW.
D. 55% of Total blood volume - The phrase seems ambiguous, but it doesn't fit the context since we're looking for the relation to ECF.
Therefore, the correct answer is B. 25% of ECF. This reflects the approximate proportion of the extracellular fluid that is contained within the vascular system of a healthy individual.
