Interstitial fluid is a major component of the liquid environment in the body and fills the spaces between the body’s cells. In contrast, blood circulates only within the circulatory vessels of the body and is composed of blood cells and the liquid part of the blood, plasma. Both fluids contain special components called biomarkers, which are valuable indicators of bodily health. These biomarkers include various types of molecules such as proteins, hormones or DNA, and can also include drugs and metabolites.
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Source: Phys.org