This investigation demonstrates a decrease in abdominal aortic wall distensibility with age, which occurs at an earlier age in men, and confirms earlier results by use of the indexes Ep and stiffness (beta). This implies that the abdominal aorta in men is more prone to degenerative changes, which may be one of the factors responsible for the sex difference in aortic vascular disease.

Venous compliance in the legs of aging man has been found to be reduced with decreased blood pooling (capacitance response) in dependent regions, and this might lead to misinterpretations of age-related changes in baroreceptor function during orthostasis. The hemodynamic response to hypovolemic circulatory stress was studied with the aid of lower-body negative pressure (LBNP) of 60 cmH(2)O in 33 healthy men [18 young (mean age 22 yr) and 15 old (mean age 65 yr)]. Volumetric technique was used in the study of capacitance responses in the calf and arm as well as transcapillary fluid absorption in the arm. LBNP led to smaller increase in heart rate (P < 0.001) and peripheral resistance (P < 0.01) and reduced transcapillary fluid absorption in the arm (P < 0.05) in old subjects. However, blood pooling in the calf was reduced in old subjects (1.66 +/- 0.10 vs. 2.17 +/- 0.13 ml/100 ml tissue; P < 0. 01). Accordingly, during similar blood pooling in the calf (LBNP 80 cmH(2)O in old subjects), no changes in cardiovascular reflex responses with age were found. The capacitance response in the arm (mobilization of peripheral blood to the central circulation) was still reduced, however (0.67 +/- 0.10 vs. 1.37 +/- 0.11 ml/100 ml tissue; P < 0.01). Thus the reduced cardiovascular reflex response found in the elderly during orthostatic stress seems to be caused by a reduced capacitance response in the legs with age and a concomitant smaller central hypovolemic stimulus rather than a reduced efficiency of the reflex response. With similar hypovolemic circulatory stress, no changes in cardiovascular reflex responses are seen with age. The capacitance response in the arm (mobilization of peripheral blood toward the central circulation) is reduced, however, by approximately 50% in the elderly. This might seriously impede the possibility of survival of an acute blood loss.

The mechanical properties of the aorta play a major role in the regulation of blood pressure and cardiac performance. The effect of sympathetic stimulation on the mechanical properties of the human abdominal aorta was studied in 19 healthy volunteers, divided into young (25 +/- 2 years) and elderly individuals (69 +/- 2 years) of both sexes. A non-invasive ultrasonic echo-tracking system for measurement of systolic/diastolic variation of aortic diameter in combination with intra-aortic pressure measurements was used to determine wall mechanics. The pressure-diameter (P-D) relationship and the distensibility indices, stiffness (beta) and pressure strain elastic modulus (Ep) of the abdominal aorta were obtained. Measurements were made at rest and during sympathetic stimulation induced by lower body negative pressure (LBNP). As a sign of sympathetic activation, the peripheral resistance increased by 74-96% (P < 0.001) during LBNP. However, the mechanical properties of the abdominal aorta remained unaltered, as estimated either from the P-D relationship or from the indices Ep and beta, both in the young (rest: Ep = 0.53 +/- 0.18, beta = 4.5 +/- 1.5; LBNP: Ep = 0.51 +/- 0.15, beta = 4.5 +/- 1.2, NS) and in the elderly (rest: Ep = 2.17 +/- 0.70, beta = 17.6 +/- 5.8; LBNP: Ep = 2.11 +/- 0.60, beta = 16.9 +/- 3.9, NS). In conclusion, this investigation shows that LBNP-induced sympathetic activation does not change aortic wall mechanics. Thus, sympathetic modulation of the aortic smooth muscle contractile activity seems to be unimportant in the blood pressure regulation.

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