Increased blood pressure is a strong risk factor for atherosclerosis, and humans pay a price for their erect sitting and standing postures. In these postures, the abdominal aoa experiences the weight of a column of blood added to the pressure produced by the hea. In the supine posture, the pressures in the thoracic and abdominal aoa are similar. So, if an average daily pressure is taken, the abdominal aoa tends to have a significantly higher pressure than does the thoracic aoa. Diastolic pressure is actually greater in the thoracic aoa compared to the abdominal aoa in the supine position. However, the systolic blood pressure is greater in the abdominal aoa. A negative intrathoracic pressure would tend to increase transmural pressure across the wall of the thoracic aoa, and thereby increase wall tension and promote the development of aneurysms. Blood flow in the abdominal aoa is less than that in the thoracic aoa, because some blood leaves the aoa through its thoracic branches. Ref: Creager M.A., Loscalzo J. (2012). Chapter 248. Diseases of the Aoa. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds),Harrison's Principles of Internal Medicine, 18e.