Final answer:
Approximately 25% of the extracellular fluid is composed of fluid in the blood.
Step-by-step explanation:
Extracellular Fluid Composition
The extracellular fluid (ECF) is composed of fluid in the blood and interstitial fluid. It represents about 20% of the body weight. Out of this, 5% of the ECF is in blood plasma. The remaining 15% is in the interstitial fluid, which surrounds cells that are not blood cells. In comparison, intracellular fluid makes up about 50% of the body weight in lean individuals.
Therefore, approximately 25% of the extracellular fluid is composed of fluid in the blood.