00232_5_Fig-5

A graphic plot of the blood pressure changes observed during gradual exsanguinations of a 14 years old mare recorded by Stephen Hales in his first experiment. Note the plateauing of the blood pressure after bleeding the horse 8 quarts of blood, which Hales attributed to reactive peripheral vasoconstriction to maintain perfusion pressure. Shown in the black boxes are his description of the state of the horse.