Arterial diseases such as atherosclerosis and aneurysms occur in human arteries, and may be induced by local differences in hemodynamic or elastic forces. Both of these forces are tied to the geometry of the lumen. The goal of this study was to measure luminal cross-sectional area variation (area taper) in Y-bifurcations of muscular arteries (porcine renals) and elastic arteries (human aorto-iliac), and compare these bifurcations with data obtained previously on muscular human cerebral bifurcations. Flow accelerates as the arteries taper and decelerates as they flare. Separation zones develop with flaring.;The arteries were pressure-fixed, frozen and sliced at 10-40{dollar}\mu{dollar}m. Luminal area was measured digitally from the block face (this removed histological artifacts). Area taper, defined as the change in luminal area per unit length of artery, was calculated and found to be {dollar}-{dollar}1.4mm ({dollar}\pm{dollar} 0.2 mm S.E.M.) (n = 13) in the parent branch of the renal arterial bifurcations and {dollar}-{dollar}1.5mm {dollar}\pm{dollar} 0.3mm (n = 6) in the parent branch of human cerebral bifurcations. Negative area taper corresponded to an expansion. The maximum cross-sectional area was at the flow divider in both renal and cerebral bifurcations but not in the aorto-iliac bifurcation. The human aorto-iliac had three area tapers: {dollar}-{dollar}0.7 {dollar}\pm{dollar} 0.1mm in the pre-apical region, +2.4 {dollar}\pm{dollar} 0.7mm in the apical region and {dollar}-{dollar}1.1 {dollar}\pm{dollar} 0.3 post-apically.;In vivo geometry of the human abdominal aorta was measured from aortograms and computed topographic (CT) scans from patients with and without aneurysms. CT scans (n = 25) showed the aortic lumen to be circular. Aortograms of normal aortas (n = 8) showed diameter tapers of +0.07 {dollar}\pm{dollar} 0.02 proximal to the throat and {dollar}-{dollar}0.63 {dollar}\pm{dollar} 0.06 proximal to the flow divider. Abdominal aortic aneurysms had three diameter tapers: {dollar}-{dollar}0.6 {dollar}\pm{dollar} 0.2 proximal to the maximum aneurysm area (apex), +0.4 {dollar}\pm{dollar} 0.1 distal to the apex and {dollar}-{dollar}0.6 {dollar}\pm{dollar} 0.1 proximal to the flow divider.;The large tapers make measurements of area ratio reported previously prone to error. Since tapers were not significantly different in porcine renal and human cerebral bifurcations, hemodynamic factors seem less likely than structural factors to explain why aneurysms develop at the apex of cerebral bifurcations
