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EDTA, or ethylenediaminetetraacetic acid, functions as an anticoagulant by binding calcium ions in the blood. Calcium is an essential element in the coagulation cascade, a series of complex steps that lead to the formation of blood clots. By chelating or binding the calcium, EDTA effectively disrupts this process, preventing clot formation and allowing for the collection of blood samples in a liquid state for laboratory analysis. This property makes EDTA a valuable anticoagulant, particularly in hematology, where it is commonly used for complete blood counts and other blood tests.

Other options are not primarily how EDTA works. For instance, while preventing platelet aggregation is important in some aspects of blood coagulation, EDTA's main action is through calcium chelation. The binding to sodium is not a mechanism through which EDTA acts, as its primary role is in calcium binding. Lowering blood viscosity is not a function of EDTA; rather, it focuses on preventing clotting. Thus, the correct understanding of EDTA's mechanism lies in its ability to bind calcium.