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?