The innovation at the heart of the work is a novel geometric model built on a unit-step-function formulation. Unlike classical stenosis models that impose a single cosine, Gaussian, or parabolic profile on a symmetric narrowing, the new framework allows the upper and lower arterial walls to be prescribed independently, capturing true bilateral asymmetry. Surface irregularities, the micro-undulations and protrusions that pathological specimens display, are encoded through independently tunable parameters for each wall. Crucially, phase parameters control the relative axial alignment of the opposing irregularities, permitting systematic exploration of in-phase configurations, where wall peaks align with peaks, and out-of-phase configurations, where a peak on one wall faces a trough on the other. The model also enforces first-order differentiability at the junctions between diseased and healthy lumen, eliminating artificial slope discontinuities that would destabilize analytical and numerical solutions.

Blood itself is treated in an unusually sophisticated way. In vessels 80 to 500 micrometers in diameter, the size of red blood cells, averaging about 7.65 micrometers, becomes comparable to the vessel dimension, and the fluid’s microstructure cannot be ignored. The study therefore models blood as a couple-stress fluid, a formulation first proposed by V.K. Stokes in 1966 that incorporates higher-order stresses associated with particle rotation. This captures the micro-rotation of blood cells and their interaction with surrounding structures, effects that conventional shear-thinning or viscoelastic models miss entirely. The couple-stress parameter, defined as the ratio of vessel radius to an intrinsic microstructural length scale, falls physiologically between roughly 5 and 30 for small arteries, and the analysis employs representative values of 5, 8, and 20.

The flow problem is solved under lubrication and low-Reynolds-number approximations, appropriate because lesion lengths in coronary imaging are several times the local lumen width and because inertial effects are negligible at these scales. The governing equations reduce to a fourth-order differential equation in the transverse direction, and the closed-form solution yields an explicit expression for the pressure gradient as a function of local lumen width and the couple-stress parameter. In the Newtonian limit, as the couple-stress effect vanishes, the solution gracefully recovers the classical plane Poiseuille relation, a rigorous check on the mathematics. The resulting formulas reveal that effective resistance exceeds the classical Newtonian value, particularly in regions where the lumen is most constricted.

The model’s most striking results concern the phase relationship between opposing lesions. When irregular lesions overlap in-phase, with a 47.37 percent axial overlap and mild roughness, the pressure gradient plunges to roughly negative 12 at the throat for the strongest couple-stress case, producing a single broad trough and a predictable bell-shaped rise in wall shear stress. But simply shifting the upper wall by half a unit, inverting the central region to out-of-phase alignment and increasing overlap to 55.55 percent, nearly doubles the peak adverse pressure gradient to negative 24.38, a 99 percent increase relative to the comparable in-phase case. The tortuous channel forces blood through successive accelerations and decelerations, generating sharp, deep spikes in the pressure gradient that dominate the flow field.

Quantitative tables reinforce the message that lesion extent alone is a poor predictor of hemodynamic burden. In one comparison, an out-of-phase configuration with 80 percent overlap and 63.5 percent throat occlusion produced a maximum adverse pressure gradient approximately 231 percent higher and a total pressure drop about 51.8 percent higher than an in-phase baseline, despite having a shorter overlap interval than a rival in-phase case with 89.47 percent overlap. The authors conclude that out-of-phase alignment combined with greater throat occlusion amplifies resistance more strongly than overlap extent alone, helping explain why anatomically similar plaques can impose markedly different functional consequences in patients.

Wall shear stress tells an equally consequential story. In an in-phase configuration with 68.42 percent overlap, shear stress forms a broad bell-shaped peak, reaching a maximum of 9.37 at the throat. In the out-of-phase counterpart with 60 percent overlap, peak shear stress jumps to 12.61, about 34.7 percent higher, while the spatially averaged shear stress remains nearly identical. This divergence means out-of-phase geometries concentrate extreme mechanical loads at discrete micro-constrictions rather than distributing them evenly, imposing localized endothelial injury disproportionate to what aggregate flow indices would suggest. Chronic exposure to such abnormal shear promotes a pro-inflammatory endothelial phenotype, stimulating production of enzymes that degrade elastin and collagen, weakening the arterial wall, and potentially fostering microaneurysm formation with attendant rupture risk.

The downstream clinical cascade modeled by the study is sobering. The marked pressure drop across an irregular stenosis represents irreversible energy loss through viscous dissipation, leaving downstream blood at a permanent pressure deficit even after the gradient normalizes. This weakened driving pressure threatens perfusion of metabolically active organs such as the brain, kidneys, and myocardium, potentially triggering compensatory flow redistribution or vascular steal phenomena. To compensate, the heart must generate higher pressures, a chronic strain linked to cardiac hypertrophy and heightened susceptibility to heart failure. Meanwhile, low and oscillatory shear in the pre- and post-stenotic zones creates athero-prone environments that promote inflammatory cell adhesion and LDL cholesterol infiltration, seeding new lesions or enlarging existing ones in a self-reinforcing cycle of vascular decline.

Context matters here: cardiovascular diseases account for roughly 31 percent of global deaths according to the World Health Organization, and while large-artery atherosclerosis is well studied, small penetrating artery disease remains comparatively neglected. Recent reviews have highlighted persistent challenges in defining, classifying, and treating cerebral small vessel disease, and neuropathological assessment of brain arteriolosclerosis still relies on semi-quantitative scales with only moderate reliability. By providing a mechanistic analytical tool that reflects realistic lesion morphology, the new model could complement imaging-based assessment and contribute to improved risk stratification in atherosclerotic disease, particularly if geometric parameters can one day be extracted from high-resolution clinical imaging.

The study’s broader warning is that smooth-wall simulations may systematically underestimate hemodynamic variations linked to endothelial dysfunction and atherogenesis. Surface irregularities, though modest in amplitude, add remarkable fluctuations to pressure and shear patterns and amplify the damage potential of disturbed flow. Conversely, the couple-stress effect proves nearly negligible in wide, laminar regions but acts as a powerful amplifier wherever geometry concentrates velocity gradients, underscoring the intimate coupling between blood rheology and lesion topology. As a first-order mechanistic foundation for functional lesion assessment, this work marks a significant step toward understanding why the smallest vessels in the body, when diseased, can exert some of the largest influences on human health.

Subject of Research: Mathematical modeling of blood flow in stenosed small arteries with irregular luminal surfaces

Article Title: A novel geometric model of blood flow in stenosed small arteries with luminal surface irregularities

Article References: Elogail, M. (2026). A novel geometric model of blood flow in stenosed small arteries with luminal surface irregularities. Results in Physics, Article 108751. https://doi.org/10.1016/j.rinp.2026.108751

Image Credits: AI Generated

DOI: 10.1016/j.rinp.2026.108751

Keywords: small arteries, stenosis, couple-stress fluid, hemodynamics, wall shear stress, pressure gradient, arteriolosclerosis, cerebral small vessel disease, lubrication approximation, non-Newtonian blood flow, atherosclerosis, mathematical modeling