Vascular Cell Physiology Under Shear Flow: Role of Cell Mechanics and Mechanotransduction - Mechanical and Chemical Signaling in Angiogenesis - page 121

Table 1 Size and function of the arteries found in the pulmonary circulation [ 11 ]

Artery Diameter Artery Type

Elastic artery [2 mm Elastic

Semi muscular artery 400 lm-2 mm Semi-Elastic

Muscular/microcirculation \400 lm Resistance

maintains flow during diastole. The storage capacity of the elastic vessels is given by
the distensibility, D ¼ dV = dp ; or the change in volume over the change in pressure.
According to the distensibility equation, the greater stroke volume, the greater
amount of blood to be stored in the elastic arteries, and thus greater rise in pulse
pressure. If the vessels were not compliant, pressure and flow pulsatility in
downstream arteries would become exceedingly high, exerting a detrimental
hemodynamic load on downstream arteries. In addition, if large arteries were not
distensible, the blood flow through tissue would only occur during systole, while
blood flow would cease during diastole [ 5 ]. Blood flow, Q, from the ventricle into
the arteries, can be calculated by taking into account the blood pressure in the
elastic artery, p, and the peripheral resistance of the small arteries, R. For an elastic
artery, the change in volume flow is proportional to the pressure, Q ¼ p = R : When
the ventricle ejects blood from the heart the proximal elastic arteries are first
distended until the pressure in these arteries rises higher than the pressure in the
arterioles, once a pressure gradient is established blood flows to other parts of the
arterial tree. The more compliant the elastic vessels, the longer the blood flow
wave takes to reach the periphery, while the less compliant or stiffer the elastic
arteries, the faster the velocity of the pressure pulse and blood flow propagation to
the microcirculation. Therefore, propagation of blood flow to the microcirculation
is driven by arterial compliance.
As the arterial lumen diameter decreases through the arterial tree, a muscular
smooth muscle cell layer replaces the elastic lamina, and elasticity decreases. In the
semi-muscular and muscular vessels of the microcirculation, pressure pulsations
become smaller until the pulsations dampen to a semi-steady flow in the capillaries.
The muscular arterioles located at the periphery, are the major resistance vessels
and are the main sites for arterial wave reflection [ 6 ]. As an artery reaches an organ,
it branches into many smaller-radius vessels; resistance in these smaller diameter
vessels will be relatively high due to friction from the vessel wall. The greater the
surface area of the vessel, the more friction, and thus more resistance, between the
blood and the vessel wall. Vessel resistance dampens the pulsatile flow by limiting
the amount of blood through the vessel, the greater the resistance the harder it is to
advance blood to the next arterial segment. Therefore, small changes in arterial
resistance have a large effect on the upstream arteries. For a small increase in
resistance, the pressure in the large arteries must increase to advance the flow to the
arterioles [ 7 ]. The arterioles contain very little elastin, yet have a thick layer of
smooth muscle cells (SMCs) that will contract the vessel, increasing the resistance
and decreasing the flow (vasoconstriction). Vasoconstriction and vasodilatation
(arterial expansion) occurs in the proximal as well as the peripheral arteries.