Architecture of small arteries during vasoconstriction.

• A fundamental characteristic of blood vessels is the ability to change in caliber in response to neural and chemical stimuli. The effects of such changes on the architecture of the vessel wall cannot be appreciated in studies on physical models, such as soap bubbles and balloons, which have negligible wall thickness and uniform composition. The familiar Laplace relationship (T = PR) states that the tension in the wall of a cylinder is related to the product of the transmural pressure and the radius of the tube. This relationship is true, however, only when the wall thickness is negligible. Peterson et al. recently analyzed the importance of wall thickness in blood vessels and used the expression T = PR/8, where S is the wall thickness. Arterial walls are relatively thick in proportion to the diameter of the lumen and contain both active and passive elements having varying degrees of resistance to deformation. Any change in the caliber of arteries must necessarily be accompanied by an internal rearrangement of the components of their walls as well as by variations in their thickness. Most investigations of the architecture of blood vessels have not been concerned with the functional state of their walls. Many such studies have been conducted on specimens obtained from post-mortem material. Others have been carried out on arteries excised from living tissue but with loss of the distending blood pressure. Further, inadequate attention lias been given to the effects of fixation on the various mural components; during prepProm the Department of Physiology and verphysics and the Department of Pathology, University of Washington School of Medicine, Seattle, Washington.