Young's modulus is a constant of proportionality which is unique to each material (Table 1). Elastin fibers have a low value of Young's modulus, consistent with their very stretchable nature. Collagen, on the other hand, has a value of Young's modulus several orders of magnitude higher, indicating a far lower stretchability. Because endothelium is so thin, its contribution to the overall length-tension diagram is negligible. Vascular smooth muscle resists stretch to a variable degree, dependent on the contractile activity. Thus when the blood vessel wall is first stretched, it is the elastin fibers which contribute most to the tension.
The collagen fibers which are laid down in the wall with some slackness are initially not stretched; eventually they also come to be stretched and resist stretch greatly. It is the collagen which is responsible for the steeply ascending phase of the length-tension diagram. Because the collagen is laid down in an accordion fashion (i.e. pleats), normal wall stretch is permitted during systolic ejection, but a safety net is provided to prevent vessel bursting at very high transmural pressures.
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