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The arterial system of intact anesthetized dogs and a physical model, which consisted of a “heart,” an elastic “arterial” reservoir, and a “peripheral resistance,” have both been studied in comparison. In both systems, the volume contained in the arterial reservoir was rhythmically altered by means of a slow sinusoidal piston pump which resulted in slow oscillations of the mean arterial pressure with the arterial pressure pulsations superimposed. It was found that in the physical model, the systolic pressure pulse was larger during the phase of rising pressure (increasing volume) than during the phase of falling pressure (decreasing volume), while in the animal the systolic pressure pulses were equal in either phase. The aortic pressure pulse of the animal was a linear function of the presystolic aortic pressure during pressure oscillation and independent of the induced volume alteration. Recordings of the stroke output of the heart revealed that the heart of the animal adjusted its output in such a manner that the induced systolic changes of the arterial contents were quantitatively compensated. The mechanism by which this is accomplished is of particular interest, since it would also enable the heart to compensate instantaneously for physiological alterations of the volume in the arterial system as caused by changes in peripheral runoff. This mechanism of instantaneous adjustment of the stroke output can be explained on the basis of Starling's Law of the Heart if the procedure of the calculation of the external work is modified.

Work of the Heart Under the Effect of Induced Arterial Pressure Oscillations Studied in Intact Anesthetized Dogs