Answer: " Water. "
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Water (H₂O) is the most common molecule in "blood."
In fact, water has a "relatively low viscosity"; or just the right level of viscosity; and blood is composed mainly of waters. This property of water (relatively low—or "low enough"—viscosity) —and the fact that blood is composed primarily of "water (H₂O) " — allows for the ease of transport and circulation through the body (of mammals).
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Hemoglobin -The most abundant cells in vertebrate blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates oxygen transport by reversibly binding to this respiratory gas and greatly increasing its solubility in blood.
