Mechanical Properties Of Vessels Research Articles

Conceptualizing Conduction as a Pliant Vasomotor response: Impact of Ca2+ fluxes and Ca2+ Sensitization.

Coordinating blood flow to active tissue requires vasomotor responses to conduct among resistance arteries. Vasomotor spread is governed by the electrical and mechanical properties of vessels; the latter being linked to the sigmoid relations between membrane potential (VM), [Ca2+], and smooth muscle contractility. Proteins guiding electrical-to-tone translation are subject to regulation; thus, vasomotor conduction could be viewed as "pliant" to the current regulatory state. Using simple in silico approaches, we explored vasomotor pliancy and how the regulation of contractility impacts conduction along a skeletal muscle artery and a branching cerebrovascular network. Initial simulations revealed how limited electromechanical linearity affects the translation of electrical spread into arterial tone. Subtle changes to the VM-[Ca2+] or [Ca2+]-diameter relationship, akin to regulatory alterations in Ca2+ influx and Ca2+ sensitivity, modified the distance and amplitude of the conducted vasomotor response. Simultaneous changes to both relationships, consistent with agonist stimulation, augmented conduction although the effect varied with stimulus strength and polarity (depolarization vs hyperpolarization). Final simulations using our cerebrovascular network revealed how localized changes to the VM-[Ca2+] or [Ca2+]-diameter relationships could regionally shape conduction without interfering with the electrical spread. We conclude that regulatory changes to key effector proteins (e.g. L-type Ca2+ channels, myosin light chain phosphatase), integral to voltage translation, not only impact conducted vasomotor tone but likely blood flow delivery to active tissues.

UNIAXIAL AND BIAXIAL MECHANICAL PROPERTIES OF THE HUMAN SAPHENOUS VEIN

Cardiovascular diseases are one of the major causes of death in the world and are closely related to blood dynamics and wall mechanical properties of vessels. This makes the study of mechanical properties of arteries and veins essential. In this regard, study on common vessels used in bypass grafting operations is of special importance due to the frequency of this surgery. Human saphenous vein is one of the vessels used for bypass surgeries. The objective of the present study is to characterize the behavior of human saphenous vein using uniaxial and biaxial planar tests. Forty human saphenous samples were obtained after coronary artery bypass surgery (CABG). The planar tensile tests were performed on the tissue specimens by applying loads along two directions. These tests provided the force-displacement curves. The stress-strain curves from uniaxial tests were modeled with a mathematical function and the Young modulus was obtained in both longitudinal and circumferential directions. Measured data of uniaxial tests were then fitted into a hyperelastic four-parameter Fung-type model and also an isotropic Mooney–Rivlin model. The biaxial stress-stretch curves were fitted to a hyperelastic anisotropic four-parameter Fung-type model and a five-parameter Mooney–Rivlin model. The specimens showed some degrees of anisotropy. In both uniaxial and biaxial tests, specimens showed stiffer behaviour in longitudinal as opposed to circumferential directions. The stretch ratio in the circumferential direction was much higher than in the longitudinal orientation.

APPLICATION OF DOPPLER OPTICAL COHERENCE TOMOGRAPHY IN RHEOLOGICAL STUDIES: BLOOD FLOW AND VESSELS MECHANICAL PROPERTIES EVALUATION

Doppler Optical Coherence Tomography (DOCT) is a noninvasive optical diagnostic technique, which is well suited for the quantitative mapping of microflow velocity profiles and the analysis of flow-vessel interactions. The noninvasive imaging and quantitative analysis of blood flow in the complex-structured vascular bed is required in many biomedical applications, including those where the determination of mechanical properties of vessels or the knowledge of the mechanic interactions between the flow and the housing medium plays a key role. The change of microvessel wall elasticity could be a potential indicator of cardiovascular disease at the very early stage, whilst monitoring the blood flow dynamics and associated temporal and spatial variations in vessel's wall shear stress could help predicting the possible rupture of atherosclerotic plaques. The results of feasibility studies of application of DOCT for the evaluation of mechanical properties of elastic vessel model are presented. The technique has also been applied for imaging of sub-cranial rat blood flow in vivo.

Differences in Transmural Pressure and Axial Loading Ex Vivo Affect Arterial Remodeling and Material Properties

Arterial axial strains, present in the in vivo environment, often become reduced due to either bypass grafting or the normal aging process. Since the prevalence of hypertension increases with aging, arteries are often exposed to both decreased axial stretch and increased transmural pressure. The combined effects of these mechanical stimuli on the mechanical properties of vessels have not previously been determined. Porcine carotid arteries were cultured for 9 days at normal and reduced axial stretch ratios in the presence of normotensive and hypertensive transmural pressures using ex vivo perfusion techniques. Measurements of the amount of axial stress were obtained through longitudinal tension tests while inflation-deflation test results were used to determine circumferential stresses and incremental moduli. Macroscopic changes in artery geometry and zero-stress state opening angles were measured. Arteries cultured ex vivo remodeled in response to the mechanical environment, resulting in changes in arterial dimensions of up to approximately 25% and changes in zero-stress opening angles of up to approximately 55 degrees . While pressure primarily affected circumferential remodeling and axial stretch primarily affected axial remodeling, there were clear examples of interactions between these mechanical stimuli. Culture with hypertensive pressure, especially when coupled with reduced axial loading, resulted in a rightward shift in the pressure-diameter relationship relative to arteries cultured with normotensive pressure. The observed differences in the pressure-diameter curves for cultured arteries were due to changes in artery geometry and, in some cases, changes in the arteries' intrinsic mechanical properties. Relative to freshly isolated arteries, arteries cultured under mechanical conditions similar to in vivo conditions were stiffer, suggesting that aspects of the ex vivo culture other than the mechanical environment also influenced changes in the arteries' mechanical properties. These results confirm the well-known importance of transmural pressure with regard to arterial wall mechanics while highlighting additional roles for axial stretch in determining mechanical behavior.

Calculations of plastic collapse load of pressure vessel using FEA

This paper proposes a theoretical method using finite element analysis (FEA) to calculate the plastic collapse loads of pressure vessels under internal pressure, and compares the analytical methods according to three criteria stated in the ASME Boiler Pressure Vessel Code. First, a finite element technique using the arc-length algorithm and the restart analysis is developed to conduct the plastic collapse analysis of vessels, which includes the material and geometry non-linear properties of vessels. Second, as the mechanical properties of vessels are assumed to be elastic-perfectly plastic, the limit load analysis is performed by employing the Newton-Raphson algorithm, while the limit pressure of vessels is obtained by the twice-elastic-slope method and the tangent intersection method respectively to avoid excessive deformation. Finally, the elastic stress analysis under working pressure is conducted and the stress strength of vessels is checked by sorting the stress results. The results are compared with those obtained by experiments and other existing models. This work provides a reference for the selection of the failure criteria and the calculation of the plastic collapse load.

Пептидоглікан змінює механічні властивості стінок судин еластичного типу

Пептидоглікан змінює механічні властивості стінок судин еластичного типу

Complex structure of the common carotid artery of sheep

Mechanical properties of blood vessels are dictated by the vessel wall structure. In many large conduit vessels the tunica media is a sheath of circular musculature and the tunica adventitia a layer of fibrous connective tissue with limited longitudinal extensibility. In contrast, the carotid artery of the sheep displays in each tunica a more complex architecture of muscle and connective tissue. Vessels collected from ewes were measured and processed for light microscopy and for transmission electron microscopy. Layers of histologically different materials are found within the tunica intima, media and adventitia. (1) The tunica media is made of circumferentially arranged muscle cells markedly different at different depths. In the innermost third of the media, muscle cells are small and with irregular profiles, the cells are widely separated, and the extracellular material is abundant and composed mainly of elastic fibres. In the outermost third, muscle cells are larger and with more regular profiles, the cells are relatively close to each other and the extracellular material is sparse and consists mainly of collagen fibrils. (2) A small number of fibroblasts is found in all parts of the media amongst the preponderant muscle cells. (3) The intima contains fibroblast-like cells and longitudinally arranged muscle cells. (4) The adventitia contains a thick layer of collagen and elastic fibres; external to this, it displays a conspicuous musculature, made of large bundles of longitudinal muscle. The carotid artery of the sheep presents in all three coats of its wall features which are at variance from those in the better known vessels of small laboratory animals. The presence of many layers of material within the wall, the heterogeneity of the tissues found, and the occurrence of an extensive longitudinal musculature, have important effects on the mechanical properties of the vessel.

Umbilical and placental vessels: Modifications of their mechanical properties in preeclampsia

Our objective in this study was to assess the basic mechanical properties of umbilical and chorionic vessels of placentas delivered after both normal and preeclamptic pregnancies. Placentas were selected when the parturient women were admitted to the delivery room. Normal pregnancy (n = 17) was characterized by delivery at term (38 to 40 weeks) after uncomplicated pregnancy without any medication. Preeclamptic pregnancy (n = 7) was characterized by delivery after 28 weeks of pregnancy (28 to 39 weeks) after sustained hypertension and proteinuria. Arteries and veins from the umbilical cord and chorionic plate were prepared in rings for in vitro study in tissue baths. Passive and active (on stimulation by potassium chloride or serotonin) mechanical properties of these vessels were studied. In vessels from normal pregnancy, responsiveness, but not sensitivity, was increased with increasing passive tension on vessels until optimal tension was reached. The passive stretch-tension curve was shifted downward in umbilical veins and upward in umbilical arteries and chorionic veins obtained from preeclamptic mothers in comparison with normal parturient women. In the absence of passive tension, contractions in response to potassium chloride were produced in all umbilical veins and some chorionic veins from preeclampsia but not from normal pregnancy. Developed wall tension curves in chorionic vessels from preeclampsia were shifted upward. In umbilical veins and arteries and in chorionic veins, the optimal passive tension was lower in tissues from preeclampsia than in tissues from normal pregnancy. Our results indicate that both passive and active mechanical properties of umbilical vessels are modified after pregnancy complicated by preeclampsia.

Collapse and viscoelasticity of diseased human arteries

A laboratory study of the hydrostatic collapse of diseased tibial arteries demonstrated hysteresis in the pressure-flow behaviour which resembled that seen in the stress-strain relations of the arterial tissue. The pressures at which the vessels collapsed were found to be considerably lower than expected on the basis of theoretical elastic models. Also, the pressures at which the vessels reopened were consistently lower than the pressures at which they collapsed. These findings were explained on the basis of viscoelasticity. The difference between collapse and opening pressure may provide insight into the mechanical properties of vessels, and a clue to errors in non-invasive measurements of blood pressure which depend upon collapse of arteries.