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The answer to the question in the title is not because the first person to make the measurement used a mercury manometer. In 1733, Rev. Stephen Hales was the first to measure blood pressure directly in an unanesthetized horse. It is interesting to note that Hales had used the U-tube manometer to measure the pressure of sap in plants. Meanwhile, Poiseuille successfully applied to blood flow in capillaries and veins, to air flow in lung alveoli, and to the flow through a drinking straw or a hypodermic needle.