The Distensibility of the Systemic Resistance Blood Vessels

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Abstract
Summary.: Tlie distensibility of the systematic resistance blood vessels has been quantitatively measured at known levels of a steady vascular tone. Their distensibility is, from a liaemodynamc point of view. considerable except at more intense vasoconstriction where the lnmen‐to‐wall ratio is decreased enough to minimize the dietending effect of the intravascular pressure.Often, however, vascular distension is more or less balanced by adaptive changes of smooth muscle tone. Therefore. a standard pressure‐flow relationship cannot be given. as this relationship varies, not only with the region studied, but also with the level of vascular tone and reactivity.Generally, a ‘critical closing pressure’, correlated with the prevailing tone, can only be recorded if the normally occurring, adaptive changes of smooth muscle tone are eliminated. The basis for this closing is discussed at length. Reasons are presented that it might simply be an extension of the well‐known intermittent closings of the praecapillary sphincter regions to include all parallel channels at this level. It is suggested that the complete closing of this section of the vascular tree is a consequence of a pronounced distensibility of the thin intima and muscle layers of these small vessels, where an outer limit for distension is set by a surrounding network of collagen fibrils.