A better understanding of how hemodynamic factors affect the integrity and function of the vascular endothelium is necessary to appreciate more fully how atherosclerosis is initiated and promoted. A novel technique is presented to assess the relation between fluid dynamic variables and the permeability of the endothelium to macromolecules. Fully anesthetized, domestic swine were intravenously injected with the albumin marker Evans blue dye, which was allowed to circulate for 90 min. After the animals were euthanized, silicone casts were made of the abdominal aorta and its iliac branches. Pulsatile flow calculations were subsequently made in computational regions derived from the casts. The distribution of the calculated time-dependent wall shear stress in the external iliac branches was directly compared on a point-by-point basis with the spatially varying in vivo uptake of Evans blue dye in the same arteries. The results indicate that in vivo endothelial permeability to albumin decreases with increasing time-average shear stress over the normal range. Additionally, endothelial permeability increases slightly with oscillatory shear index.

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The purpose of this work was to investigate the effects of physiologically realistic cardiac-induced motion on hemodynamics in human right coronary arteries. The blood flow patterns were numerically simulated in a modeled right coronary artery (RCA) having a uniform circular cross section of 2.48 mm diam. Arterial motion was specified based on biplane cineangiograms, and incorporated physiologically realistic bending and torsion. Simulations were carried out with steady and pulsatile inflow conditions (mean ReD=233, alpha=1.82) in both fixed and moving RCA models, to evaluate the relative importance of RCA motion, flow pulsation, and the interaction between motion and flow pulsation. RCA motion with a steady inlet flow rate caused variations in wall shear stress (WSS) magnitude up to 150% of the inlet Poiseuille value. There was significant spatial variability in the magnitude of this motion-induced WSS variation. However, the time-averaged WSS distribution was similar to that predicted in a static model representing the time-averaged geometry. Furthermore, the effects of flow pulsatility dominated RCA motion-induced effects; specifically, there were only modest differences in the WSS history between simulations conducted in fixed and moving RCA models with pulsatile inflow. RCA motion has little effect on time-averaged WSS patterns. It has a larger effect on the temporal variation of WSS, but even this effect is overshadowed by the variations in WSS due to flow pulsation. The hemodynamic effects of RCA motion can, therefore, be ignored as a first approximation in modeling studies.

The Dynamics Explorer (DE) pair of spacecraft provide a unique opportunity to search for the presence of electric fields aligned parallel to magnetic field lines by sampling, nearly simultaneously, the velocity‐space distribution functions of ions and electrons at two points on auroral field lines: DE 1 at high altitudes (9000–15,000 km in this study) and DE 2 at low altitudes (400–800 km). Three independent techniques are used to infer the auroral electrostatic potential difference from the particle distributions: (1) the energy of the precipitating electrons at DE 2 (compared to that at DE 1), (2) the energy of the upgoing ions at DE 1, and (3) the widening of the loss cone for electrons at DE 1. The three estimates are in general agreement, confirming the long‐standing, but not fully accepted, hypothesis that parallel electrostatic fields of 1–10 kV potential drop at 1–2 RE altitude are an important source for auroral particle acceleration. The upflowing ion distribution typically can be characterized by a sharp peak and a falloff at high energies of the form exp‐{(E‐Epeak)/Eo}, with Epeak being the peak energy and Eo the characteristic energy. This is the functional dependence one expects if a Maxwellian of thermal energy Eo is accelerated upward by a parallel electric field with eΦ=Epeak. The fact that the peak energies and not the flow velocities of the various ion species are in agreement also lends strong credence to the parallel electric field hypothesis. The acceleration mechanism cannot be a simple parallel electric field, however, for two reasons: first, the characteristic energy Eo is considerably larger than the ionospheric thermal energy (Eo is typically hundreds of electron volts and 20–30% of Epeak), and second, the energy Epeak is typically 30–50% smaller than that inferred by the two other independent techniques. The distribution does appear to be consistent with an ionospheric source, heated within (or above) the acceleration region, since the ion average energy is comparable to eΦ. The average energies of O+ and He+ are comparable to, but typically somewhat larger than, that of H+, indicating that a two‐stream instability may be the heating mechanism. Electron heating is also detected within the auroral acceleration region, with gains in characteristic energies of 10–15% of eΦ. From the high‐altitude electron measurements, we can determine a minimum potential distribution as a function of altitude in order to overcome the mirror force. We find that in one case at least 9M V of the 2300‐V potential drop must occur above 7000 km, and at least 960 V above 2000 km.

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