Pressure-Circumference Relations in the Aorta

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Abstract
The pressure and circumference of the aorta at the arch were simultaneously measured in dogs. The pressure and circumference tended to fluctuate together, but the patterns differed slightly in configuration. A continuous plot of aortic pressure-circumference relationships on the face of a cathode ray oscilloscope inscribed loops which implied that the aorta was imparting energy to the blood. Explanations offered for this phenomenon included (a) technical artifacts altering the phase relations between the two records; (b) active contraction of smooth muscle in the aortic wall during each systole phase of the cardiac cycle; (c) changes in length of the aorta during each cycle. If the over-all distensibility of the aorta can be altered by active contraction in the walls, as indicated in these experiments and by those of Wiggers and W6gria, significant errors might be produced in computations of cardiac output from analysis of arterial pressure pulses. If changes in aortic length play a significant role in establishing the pressure-volume relations of the aorta of intact animals, this factor should be carefully regulated in experiments on isolated segments of the vessel.

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