A 67-year-old man with an 18-year history of type 2 diabetes mellitus presents for a routine physical examination. His temperature is 36.9 C (98.5 F), his blood pressure is 158/98 mm Hg and his pulse is 82/minute and regular. On examination, the physician notes a non tender, pulsatile, mass in the mid-abdomen. A plain abdominal x-ray film with the patient in the lateral position reveals spotty calcification of a markedly dilated abdominal aoic walI. Which of the following physiologic observations helps to account for the fact that 75% of the aneurysms of this patient’s type are found in the abdomen and only 25% principally involve the thorax?
Correct Answer: The average blood pressure in the abdominal aoa is higher than that in the thoracic aoa
Description: 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.
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