BACKGROUND Aneurysms occur most frequently in infra-renal aorta. In this study we try to reveal the underlying cause of this peculiar phenomenon. Generally, abdominal aortic aneurysms (AAAs) arise in the late stage of lifecycle, which result from the gradual degeneration of aortic wall for decades. Based on its degenerative essence, the pathophysiology of AAAs should be discussed in scale of whole lifetime. Attention should also be paid to those factors with weak influence on degraded process of aortic wall, since accumulated damages of several decades will be of significant to the process of aneurismal formation. In this study we observed the general principles of degenerative process of aortic wall and the effect of anatomical defect in infra-renal aorta on promoting the degenerative process. MATERIALS AND METHODSGeneral Principles of Degenerative Process of Aortic WallContent and turnover of extracellular matrix of different stage in arterial wall degenerative process was observed in this study. Tissues specimens of 10 normal aorta, 135 atherosclerosis and 7 AAAs were stained for elastin-collagen and immunohistologically colorated for pCOL-Ⅰ, pCOL-Ⅲ, MMP-2, MMP-9 and TIMP-2.Contents of elastin and collagen of 102 arterial tissue specimens without wall inflammation were measured by computer image analysis. Mathematic analyses were performed to determine the relationship among elastin, collagen and arterial diameter.The relationship between structure-stress imbalance of artery and wall inflammation were determined by comparing the extracelluar matrix contents in inflammatory arterial tissue with those in non-inflammatory arterial tissue. To explore the influence of cyclic stress on the progression of AAA. AAA tissues were cultured with different stress load in vitro. 2 Cause of Media Structure Defect in Infra-Renal Aorta and Its Effect on Degeneration Process Diameters of 55 infra-renal aorta, 55 common carotid arteries and 50 thoracic descending aorta age from 1 to 30 years were scaled and the amplitude of diameter increase in this period were calculated. Pulsatile wall motion (PWM) of infra-renal aorta and common carotid artery of 74 cases in different age was determined by echo tracing duplex ultrasonic scan.To validate the hypothesis that media deficiency will contribute to intimal proliferation, the ratio of elastic laminae to arterial diameter (EDR) of different location in arterial tree was calculated. EDR reflects the instinctive structure of the arterial wall. Statistic analysis was made to verify the possible relationship between EDR and intimal thickness. To confirm the hypothesis that media structure defect and amplified PWM will result in cyclic stress convergence in inner aortic wall and thus lead to intimal proliferation, stress-strain curves for separated layer of canine aortic wall were measured. RESULTSGeneral Principles of Degenerative Process of Aortic WallDuring the whole lifecycle, the elastin in arterial wall decreased gradually combing with collagen increasing, which results in collagenization at last. Both intima atheroma and media inflammation accelerated the elastin degradation. Based on the characters of extracellular matrix modification, the lifetime evolvement of arterial wall was staged as five periods including stage Ⅰ developmental period, stage Ⅱ physiological remodeling period, stage Ⅲ medial sclerosis period, stageⅣ inflammation stage and, stage Ⅴ aneurismal stage. Structure-stress balance was a basic rule directing arterial matrix remodeling. The content of elastin (Xe) and collagen (Xc) mathematically correlated with the arterial diameter (φ), which manifested as the formula: φ=17.026 Xc+20.721Xe. We defined (17.026 Xc+20.721Xe)/φ as arterial wall remodeling coefficient (Cr). Cr reflected the balance of structure-stress of arterial wall. Arteries with wall inflammation have lower Cr value than that of normal group. Cr value of iliac arteries from 5 AAA patients was also lower than nor