Vessel wall elasticity

Elastin is typically considered as an amorphous protein consisting of random chain sequences connected by a helical regions. The elastin content varies in elastic fibers such as those found in skin. Elastic fibers are termed oxytalan fibers in the upper dermal layer of skin and they are termed elaunin fibers in the deeper dermis where their elastin content is higher. In vessel wall elastic fibers have recently been differentiated based on histological staining patterns suggesting that differences in mechanical properties of different vessel walls may in part be due to differences in elastin... 

Blood vessels have elastic walls that expand or contract due to changes in pressure or the passage of corpuscles. How should Equations (14.11) and (14.12) be modified to reflect this behavior ... 

The normal arterial wall consists of the intima, media, and adventitia, as illustrated in Fig. 4—3A. The endothelium is located in the intima and consists of a layer of endothelial cells that line the lumen of the artery and form a selective barrier between the vessel wall and blood contents. The internal elastic lamina separates the intima and media, where vascular smooth muscle cells are found. The vascular adventitia comprises the artery s outer layer. Atherosclerotic lesions form in the subendothelial space between the endothelial cells and internal elastic lamina. 

Um Elastic strain energy in vessel walls V Volume of gas... 

Fig. 5.2 Immunofluorescent demonstration of smooth muscle actin (FITC, green channel) in the blood vessel wall of the human kidney. Red autofluorescence of erythrocytes, elastic lamellae and kidney tubules was captured with a filter exciting the autofluorescence in red spectrum under a longer exposure than with the filter exciting specific fluorescence in the green spectrum. Nuclei are counterstained with DAPI (blue channel)...

The gas molecules fly about and among each other, at every possible velocity, and bombard both the vessel walls and collide (elastically) with each other. This motion of the gas molecules is described numerically with the assistance of the kinetic theory of gases. A molecule s average number of collisions over a given period of time, the so-called collision index z, and the mean path distance which each gas molecuie covers between two collisions with other molecules, the so-called mean free path length X, are described as shown below as a function of the mean molecule velocity c the molecule diameter 2r and the particle number density molecules n - as a very good approximation ... 

Elastic fibers form the network in skin and cardiovascular tissue (elastic arteries) that is associated with elastic recovery. Historically the recovery of skin and vessel wall on removal of mechanical loads at low strains has been attributed to elastic fibers. Elastic fibers are composed of a core of elastin surrounded by microfibrils 10 to 15 nm in diameter composed of a family of glycoproteins recently termed fibrillins. Fibrillins are a family of extracellular matrix glycoproteins (MW about 350,000) containing a large number of cysteine residues (cysteine residues form disulfide crosslinks). Several members of the family have been described. The common molecular features include N and C terminal ends with 47 tandemly repeated epi-... 

Determination of Elastic and Viscous Properties of Vessel Wall... 

Elastic and viscous stress-strain curves have been measured for human aorta as well as other arteries. The curves are all similar in that the stress is much lower than that for skin (see Figure 7.11). The lower stress values are consistent with a different network structure of vessel wall compared to skin, which is reflected by the smooth muscle content of aortic tissue. Smooth muscle is absent from skin. The curves for different vessels have similar shapes, however, on careful review the curves have much lower values for the high strain moduli. This relates to the differences in the structure of the media from each of these vessels and potential crosslinking differences. 

In vivo measurements suggest, at least in vessel wall and perhaps tendon, that under normal conditions ECMs operate in the beginning of the high strain region of the stress-strain curve, therefore we can use the stress obtained in this manner to estimate the unloaded stress on a tissue. The unloaded elastic stress can then be corrected for the applied external elastic loading and the cellular stress can be estimated knowing the tissue... 

Silver FH, Horvath I, Foran DJ. Viscoelasticity of the vessel wall The role of collagen and elastic fibers. Crit Rev BME. 2001 29 279. 

Extracellular matrices (ECM) are the primary structural materials found in connective tissue in vertebrates that serve to maintain tissue shape (skin), aid in locomotion (bone), transmit and absorb mechanical loads (tendon and ligament), prevent premature mechanical failure (tendon, ligament, skin, and blood vessel wall), partition cells and tissues into functional units (fascia), act as scaffolds that define tissue and organ architecture (organ parenchyma), act as storage devices for elastic energy (tendon and blood vessel wall), and as the substrate for cell adhesion, growth, and differentiation of a variety of cell types. 

Other examples of tissues exhibiting active and passive tension include cornea, cardiovascular tissue, and cartilage. When a corneal transplant is trephined from a cadaver eye, the corneal material to be transplanted shrinks from about 8.5 mm in diameter to about 8.0 mm as a result of unloading of the passive and active tensions that exist. In the cardiovascular system passive and active stresses along the longitudinal and transverse directions of the vessel wall provide in situ strains that are as high as 50% in the carotid artery. Thus the elastic arteries contract when they are removed from the cardiovascular system. 

The dimensioning of vessel walls is generally controlled by the static failure mode "gross plastic deformation". Elastic stresses have to be kept below the limits according to the applicable stress category. 

Youngs modulus of elasticity Dimensionless initial fragment acceleration PeAR/Mde = PeR/mae for vessel completely shattered into many small fragments Acceleration due to gravity Vessel wall thickness... 

However, this reaction is generally accompanied by fibrosis and increased elasticity of the vessel wall. Portal hypertension leads to the dilation and reopening of veins which connect the portal vein system to the superior or inferior vena cava. Hence collateral circulation develops in the region of the oesophagus and gastric fundus as well as in the intestinal tract, retroperito-neum, lungs, spleen and kidneys, and at the anterior abdominal wall. (s. p. 253) (s. tab. 14.10) (s. figs. 7.5 14.11, 14.12 16.9)... 

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