Biomedical Engineering Reference
In-Depth Information
in the regulation of vascular tone, and various modelling strategies that incorporate
different components of the arterial wall are presented together with computational
results.
1.1 Arterial Structure
The arterial wall consists of three tunicæ, or layers: the intima, the media and the
adventitia, which are, respectively, an inner, middle and outer sheath common to
most blood vessels. The intima is made up of a single layer of endothelial cells
(ECs), the endothelium, supported by an internal elastic lamina that separates the
intima from the media. Described as an active metabolic and endocrine organ, the
endothelium forms a selective permeable membrane, mediating many aspects of
blood-tissue exchange. The media lies between the internal and the external elastic
laminæ, and is composed of smooth muscle cells (SMCs), embedded in a matrix of
elastin and collagen fibres, the amount of elastic tissue varying depending on the size
of the vessel. In arterioles, it consists largely of smooth muscle (SM) arranged in
lamellæ, wrapped around the vessel. Changes in their contractile tension cause the
vessel to dilate or constrict, thereby regulating vessel diameter and blood perfusion.
Myoendothelial gap junctions provide a communication pathway between the intima
and the media. The adventitia, or tunica externa, is a layer of connective tissue, that
often contains sympathetic fibre terminals and, in large arteries and veins, the vasa
vasorum.
1.2 Arterial Function, Transport and Wall Dynamics
Vascular health is characterised by the ability of a blood vessel to adapt to the vari-
able requirements of its local environment. Atherosclerosis, coronary and carotid
artery disease are all associated with a progressive impairment of this vascular reac-
tivity, or endothelial dysfunction [
1
-
3
], a consequence of which may be the long
term dysregulation of the mechanisms within the SM associated with vascular tone.
Vascular tone, i.e. the tension exerted by the vascular SM, is an essential prerequisite
for vasodilation. In skeletal muscle, for example, blood flow can be increased up to
20-fold by vasodilation, to meet local demand of exercise [
4
]. Vascular arteriolar tone
also acts at a local level to regulate capillary recruitment and pressure, enhancing per-
fusion, nutrient and water exchange. At the systemic level, arterial and central venous
pressures are modulated by the continuous adaptation of resistance vessel and periph-
eral vein tone, respectively. In concert with extrinsic mechanisms, vascular tone is
controlled by endothelial factors, vasoactive metabolites and extracellular autocrine
and/or paracrine signallingmolecules. Vascular blood flow regulation involves a hier-
archy of control processes. The endothelium exerts its influence primarily at a local
or middle level, by modulating the myogenic response. It acts as an active interface,
