Dependence of wall stress in the human thoracic aorta on age and pressure.

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The tangential wall stress and the tangential elasticity modulus at pressures from 0 to 200 mm Hg were calculated from pressure-volume diagrams of 27 human thoracic aortas aged 22 to 85 years. The relative volume increase of the aortas between 0 and 100 mm Hg decreased linearly from about 190% at age 20 to about 40% at age 85, whereas the elasticity modulus at 100 mm Hg increased linearly from 10 kg/cm 2 at age 20 to 42.5 kg/cm 2 at age 85. The decrease of the distensibility of the aortic wall with age was partly compensated by an increase of the internal volume of the aorta but at higher ages the distensibility was less than the theoretical values necessary for a proper buffering chamber activity. The tangential wall stress decreased linearly with age from about 2.5 kg/cm 2 at age 20 to about 1.2 kg/cm 2 at age 85. Since the pressoreceptors in the wall of the elastic vessels respond to the wall stress, this result means either that smooth muscles must increase the stress of the elastic fibers continuously, or the pressoreceptors must adapt their sensitivity throughout life to lower wall stress so that it will be kept constant. Weakness of the smooth muscles or a failure of the pressoreceptors to adapt themselves to lower stresses at higher ages may be the cause of essential hypertension.