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Past measurements of arterial branching geometry have indicated that the branching geometry is somewhat consistent with an optimal trade-off between the work needed to build and maintain the arterial tree and the work needed to operate the tree as a transport system. The branching geometry is also consistent with the mechanism that acutely adjusts the lumen diameter by way of maintaining a constant shear stress by dilating (or constricting) the arteries via the nitric oxide mechanism. However, those observations also indicate that there is considerable variation about the predicted optimization, both within any one individual and between individuals. Possible causes for this variation include: (1) measurement noise -- both due to the imprecision of the method but also the preparation of the specimen for applying the measurement technique, (2) the fact that the measurement task presents a major logistic problem, which increases as the vessel size decreases (but the number of branches correspondingly doubles at each branching) and results in progressive under-sampling as the vessel size decreases, (3) because of the logistic task involved the number of arterial trees analyzed is also greatly limited, and (4) there may indeed be actual heterogeneity in the geometry which is due to slight variation in implementation of the 'rules' used to construct a vascular tree. Indeed, it is this latter possibility that is of considerable physiological interest as it could result in the observed heterogeneity of organ perfusion and also provide some insight into the relative importance of 'initial ' conditions (i.e., how the vascular tree initially develops during embryogenesis) and the adaptive mechanisms operative in the maturing individual. The use of micro-CT imaging to provide 3D images of the intact vascular tree within the intact organ overcomes or minimizes the logistic problems listed above. It is the purpose of this study to examine whether variability in the branching geometry is constant over the length of an artery or whether this progressively amplifies along the length of the artery.
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