Significance of macrorheology and microrheology in atherogenesis

Atheromatous lesions represent a disturbance of homeostasis in the arterial wall, where the rates of uptake and production of atherogenic materials exceed the rates of egress and metabolic removal. The pressure and flow in the arterial system generate circumferential stress in the wall and shear stress on the wall. The magnitude and distribution of these stresses are affected by local vascular geometry and microrheologic behavior of blood. Circumferential stress is borne primarily by the media and adventitia, and shear stress has a greater influence on the endothelial cells, which form the principal barrier to transport of macromolecules into the arterial wall. Shear stress, turbulence, and longitudinal stretch cause an increase of macromolecular uptake by arterial wall, especially when the mechanical disturbances are periodic. These effects may be explained by an enhanced diffusion of plasmalemmal vesicles in the endothelial cells. Such short-term effects of rheologic factors should be considered together with their long-term influences on the structure and function of the arterial wall in order to elucidate the role of rheology in atherogenesis.

The propulsive mechanism of the dinoflagellate transverse flagellum reconsidered

As a result of the recent discovery that the transverse flagellum of dinoflagellates does not beat helically within the cingulum it has been necessary to formulate a new model for the manner in which this flagellum can impart a forward motion to the cell. It is theorised that the contribution to forward propulsion by the transverse flagellum can only arise directly from an asymmetry of the flagellar wave. In addition we have drawn attention to topographic features of the cell, and particularly of the left sulcal list, which may augment this thrust by redirecting cingular flow down the sulcus.

Quantitative Studies of Microcirculatory Structure and Function

A modified version of the Wiederhielm electronic servo system for micropressure recordings was used to establish the details of the pressure drop across successive segments of the microcirculation from arterioles (150µ) to venules (200µ) in the mesentery of the cat; most of the readings were made in vessels less than 25jz in diameter. Micropressures correlated with central pressure only in arterial vessels larger than 50µ. Frequencydistribution plots of micropressures showed a wider spread in the vessels that were 50-1Oµ in diameter than they did in capillary vessels that were less than 20µ in diameter.Therefore, it appears that classification of vessels on the basis of diameter, which is at present the only practical way of comparing pressure measurements in presumably identical segments of a continuously branching system, is not adequate and makes statistical analysis difficult. In the present study, micropressures remained pulsatile throughout the bed, although the pulse pressure could be narrowed to 2-4 mm Hg in the capillaries. Simultaneous measurement of pressure in two vessels showed that the greatest pressure drop occurred in the region where arterioles (40-50)u) distributed numerous precapillary branches (10-15/x); in this region arteriolar pressures were reduced from a level of 45-65 mm Hg to about 30-35 mm Hg. These data suggest that application of the resistance term for Poiseuille flow in the microcirculation should take into account the relative sizes of the parent vessel and the branch, the entry dimensions into the branch and the length of the neck or junctional segment. Evidence found in this investigation indicated that precapillary branching configurations were self-regulatory. A small number of low-resistance, shunt pathways between arterioles and venules were regularly present, as shown by the extremely small pressure drop in 15-18% of the vessels. The data indicate that much more caution is needed in interpretation of pressure-related phenomena in the microcirculation because of their strong dependence on the anatomical and physical features characteristic of individual tissues.