Arteriosclerosis is known to be a multi-factorial disease but the fact that it appears at very specific locations suggests that proximity to the heart and the local geometry of the arteries may be common factors in producing disturbances in the blood flow which place excess mechanical stress on the artery walls.
Merab Beraia, in his paper “Frequency dispersion of the surface wave on the vessel wall”, explains the architecture of the arterial system and gives a lesson in fluid dynamics.
“As a result of their unique location, endothelial cells experience three primary mechanical forces: pressure, created by the hydrostatic forces of blood within the blood vessel; circumferential stretch or tension, created as a result of defined intercellular connections between the endothelial cells; and wall shear stress, the dragging frictional force created by blood flow. Of these forces, shear stress appears to be a particularly important hemodynamic force because it stimulates the release of vasoactive substances and changes gene expression, cell metabolism, and cell morphology.” – M. Beraia
The aorta forms a resonance chamber, trapping and accumulating pulse waves from the heart. Standing waves are formed and energy is concentrated at certain points more than others with the Aortic Isthmus (shown) being particularly affected and in resonance with the pulse.
Shock waves bounce around in this environment with some merely glancing off the walls of the blood vessel and others hitting it almost perpendicularly, giving maximum energy transfer and causing maximum vibration of the tissue and instability of the blood flow.
Flow separation occurs at the site of the vibrating blood vessel. Show here on an aeroplane wing, steady ‘laminar’ flow at the boundary layer degenerates and vortices are formed which break away from the surface of the solid, causing more vibration and increased mechanical stress on the artery walls. In aerodynamics, devices such as turbulators are added to prevent this phenomenon.
Other geometries can also produce blood flow separation.
Here we see a bifurcation in the artery again disrupting the blood flow, causing vortex shedding and hence vibrations in the artery wall which in turn lead to mechanical stress on the local tissue.
“At the outer wall of the aortic isthmus, pulse pressure after reflection is in resonance with the systolic pressure drop. Amplitude of stress and frequency of oscillation in the boundary layer increases. Flow separates. Frequency dispersion destroys the flow cell aggregates, increasing blood entropy, whereas at the vessel wall, denudes endothelial sheet.” – M. Beraia
Resonant vibration can destroy a bridge – so how do we prevent it?
- Heart Rate Variability (HRV) (HeartMath) has been correlated with low risk of heart disease and we can now see why; resonance depends upon a steady rhythm to build up vibrations and a pulse rate that varies from beat to beat will not allow violent oscillations to form.
- Even mild physical activity (non-exercise movement) will alter the heart rate and hence cause altered resonant wave systems in the aortic chamber.
- High pulse pressure leads to higher wave pressure and exacerbates the problem
- Arterial wall stiffness is another known risk factor and makes things worse by increasing the energy of the reflected waves
Frequency dispersion of the surface wave on the vessel wall – Primary reason for atherosclerosis M. Beraia
Electromagnetic blood flow in human body and initial factors of atherosclerosis M. Beraia
Arterial pulse impact on blood flow M. Beraia
Blood flow and the localization of atherosclerotic plaques. D E McMillan
The role of fluid mechanics in the localization and detection of atherosclerosis. Giddens, Zarins, Glagov.
Pulsatile flow and atherosclerosis in the human carotid bifurcation. Positive correlation between plaque location and low oscillating shear-stress. Ku, Giddens, Zarins, Glagov.
Heart rate variability and progression of coronary atherosclerosis Huikuri, Jokinen et al
Haemodynamics of atherosclerosis: a matter of higher hydrostatic pressure or lower shear stress? – Xinggang Wang, and Junbo Ge.