ACTIVE CHANGES IN SIZE AND DISTENSIBILITY OF THE AORTA DURING ACUTE HYPERTENSION

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Abstract
The expansion of the thoracic aorta was studied by means of an optically recording aortagraph during the development and recedence of acute hypertension, produced by nervous and hormonal means. Distensibility curves plotted from consecutive aortagrams and corresponding optical pressure curves (recorded by a calibrated manometer) indicate that dogs'' aortae fall into three groups, viz., those with high, low and intermediate degrees of distensibility. In the first 2 groups the aorta was definitely less distensible during life than shortly after death. The reason for this is not clear. Except in aortae classed as having low degrees of distensibility, a reversal of diminishing into increasing pulse pressures with rising pressures caused by increased peripheral resistance is not probable on the basis of physical studies recently reported. During the rise and reces-sion of pressures in acute hypertension produced by nervous and hormonal agents, the initial passive expansion is followed by an active diminution in size which outlasts the peripheral constriction, but from which recovery occurs in 3 to 5 mins. after pressures have returned to lower levels. The exciting agent and ultimate mechanisms remain to be detd. but the natural conclusion would be that muscular contraction is involved. The diminution in size of the thoracic aorta is followed after a definite latent interval by increased distensibility of the walls. Whether this is an effect of the same muscular action that causes diminution in size or to some other obscure mechanism cannot be decided at present. Dynamically, decrease in vol. and increase in extensibility act as opposites in determining the pulse pressure amplitude and height of systolic pressures. Whether they neutralize each other or whether one dominates cannot be detd. by data available.

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