Arterial growth and remodelling is driven by hemodynamics - Modeling of Physiological Flows - page 185

This preferential orientation is expected, in part, for the primary roles of the contrac-
tion of vascular smooth muscle are to modify the distensibility of large arteries or
to regulate the luminal diameter in medium and small arteries. Finally, the adven-
titia consists primarily of a dense network of type I collagen fibres with admixed
elastin, nerves, fibroblasts and vasa vasorum . The adventitial collagen fibres tend
to have an axial orientation in most arteries and they are slightly undulated in the
basal state. Although the adventitia comprises only 50 % of the arte-

rial wall in elastic and muscular constituents, respectively, it is thought to limit acute
overdistension in all vessels. That is, the collagenous adventitia may serve primarily
as a protective sheath, similar to the epicardium of the heart. Nonetheless, the pres-
ence of nerves within the adventitia also allows innervation of the outer media via
the diffusion of neurotransmitters. The fibroblasts are responsible for regulating the
connective tissue, particularly the type I collagen. In many vessels, the adventitia is
contiguous with perivascular tissue, which often provides additional structural sup-
port. Exceptions, again, are the cerebral arteries, which are sometimes sorrounded
mainly by cerebro-spinal fluid.

∼ 10 % and ∼

7.1.2 Arterial functions and the roles of hemodynamics
The aforementioned classification of arteries as elastic or muscular actually arises
more from considerations of function than structure. To appreciate this, consider the
following. The heart is a pulsatile pump that ejects blood into the vasculature only
during its contractile phase (i.e., systole). Whereas contraction of the heart results in
forward flow of most of the ejected blood, some of the blood is “stored” in the large
arteries as they distend under high pressure. When the blood pressure drops during
diastole, these distended vessels recoil elastically and thereby augment the flow of
blood by supplying a second pressure pulse. Moreover, augmentation of blood flow
via the recoil of large arteries helps convert it from a pulsatile to a nearly steady
flow in the capillaries, which in turn aids the requisite transport processes. It is the
large amount of intramural elastin and type III collagen in the media that gives rise
to this important “elastic behaviour” of the large arteries, and hence the terminol-
ogy. In contrast, the smaller arteries and arterioles play a key role in regulating local
blood flow via vasoconstriction or vasodilation, that is, by muscular activity. Note,
therefore, that arterial smooth muscle is partially contracted in its homeostatic or
basal state - this is referred to as basal tone. Further contraction or alternatively re-
laxation of the abundant smooth muscle in these vessels thus results in a decreased
or increased lumen, which in turn controls the resistance to the blood flow. It is in
this way that blood can be routed to regions having an increased need for oxygen
or nutrient exchanges (e.g., skeletal muscles during exercise or digestive system fol-
lowing eating) or away from regions where there is an injury to the vasculature (e.g.,
to minimize bleeding). Likewise, because of their ability to control blood flow, mus-
cular arteries and arterioles play a key role in thermal regulation within the body by
routing blood toward or away from the skin. Muscular arteries are thus those vessels
that perform their primary function via smooth muscle activity.