Stephen Hales was a theologian, botanist and physiologist. He invented the first manometer and was the first to measure the blood pressure in animal model. His work is considered the greatest contribution to cardiovascular physiology after Harvey.
Hales had a great interest in the study of circulation and quantitative measurements of the blood pressure, blood flows and vascular resistance in animals including horse, ox, sheep and dog. His observations were published in "Statical essays" in the second volume entitled, "haemastaticks", in 1733. They were described through a series of twenty-five experiments. He inserted a cannula into the femoral artery of a horse after placing a temporary ligation. The cannula was connected to a glass tube of the same diameter and the ligature of the artery was untied. He then observed the height of the rise and fall of the blood above the level of the left ventricle allowing him to estimate the blood pressure (experiment one). Several pages of "haemastaticks" are printed here to show how during his measurements, Hales was correlating blood volume with the blood pressure. Hales showed that the peak levels of blood pressure correlated with the output of the heart. He also showed that the lowest levels of pressure resulted from a resistance to flow in the arterial system.
He did similar experiments on quantitative measurements of pressure in the venous system and showed that the ratio between arterial versus venous pressure was about ten to one. He also did several experiments on the velocity of the blood in the aorta, left ventricular volume (see above) and cardiac output in animals. He calculated that in man the velocity of the blood flow in the proximal aorta was 146 feet per minutes and the cardiac output about 4 L/min.
The entire text of the first experiment is displayed here.
Hales also did extensive studies on the physiology of plants. The results of his experiments were published in the first volume of "Statical essays" entitled, "Vegetable staticks" (1727).
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Hall WD. Profiles in cardiology: Stephen Hales. Clin Cardiol 1987;10:487-489