Arteries: Mechanics, Mechanobiology, and the Need for a New Class of Models - Multiscale Modeling in Biomechanics and Mechanobiology - page 202

function and dysfunction. In this chapter, we will first review basic arterial structure
and prior constitutive approaches for describing observed biomechanical behaviors
of arteries. With this as a background, we will then briefly reviewmore detailed infor-
mation on arterial microstructure and, within the context of an illustrative example,
suggest that there is a need for a new class of multiscale biomechanical models
for arterial mechanics. The latter is motivated primarily by the recently discovered
importance of structural constituents within the wall that have not been included
explicitly in prior constitutive modeling. The general implications of such modeling
are much farther reaching, however.
9.2 Basic Arterial Structure
Arteries are generally classified as either elastic or muscular. Elastic arteries are
found closer to the heart and function to store elastic energy during systole that
can be used to work on the blood during diastole. This elastic response decreases
the work load on the heart, augments coronary perfusion by aiding retrograde flow
during diastole, and promotes a continuous flow in the microcirculation that aids in
gas and nutrient exchange. Muscular arteries are found near or in target organs and
tissues such as the heart, brain, kidneys, and skeletal muscle; via a strong smooth
muscle contractile response, they regulate local blood pressure and flow. Both types
of arteries consist of three layers: intima, media, and adventitia. The innermost layer,
or intima, consists primarily of a monolayer of endothelial cells that adhere to an
underlying basement membrane that consists largely of type IV collagen and laminin.
Albeit not structurally significant in young healthy individuals, this layer can thicken
in aging, particular diseases, and responses to injury primarily due to an accumulation
of synthetic cells and deposition of abundant collagen and proteoglycans. The middle
layer, or media, is the parenchymal layer of the arterial wall. It consists of abundant
smooth muscle cells as well as elastic fibers, fibrillar collagens, and proteoglycans.
In elastic arteries, the smooth muscle cells reside within concentric layers of elastic
fibers that are organized into fenestrated sheets, or laminae. In muscular arteries, the
smooth muscle cells reside within a more diffuse plexus of elastic fibers, collagen
fibers, and proteoglycans, typically with overall delimiting inner and outer elastic
laminae. The outermost layer, or adventitia, consists primarily of fibroblasts and
the plexus of collagen fibers that they synthesize and maintain along with admixed
elastic fibers and proteoglycans.
In summary, despite regional differences that serve local functionality, the arte-
rial wall consists primarily of three layers, three cell types (endothelial, smooth
muscle, and fibroblasts), and three primary types of structural constituents (elastic
fibers, fibrillar collagens, and proteoglycans)—see Wagenseil and Mecham [ 44 ]for
an excellent review. Of particular note, all three cell types are highly mechanosensi-
tive [ 5 , 6 , 32 ], hence understanding arterial mechanics is important for two reasons.
First, arteries are subjected to continuous loading, including pulsatile blood pres-
sure, and their structural integrity is fundamental to their function as conduits for