Biomedical Engineering Reference
In-Depth Information
of the blood vessel wall and therefore is the only layer in contact with blood (under nor-
mal conditions). This layer is composed of endothelial cells and a small connective tissue
layer. The connective tissue within the tunica intima is mostly composed of elastic fibers
and helps secure the endothelial cells in place. In most arteries, this layer is not smooth,
due to the constant contraction and dilation (pressure pulse) and the elastic recoil of the
blood vessels. The tunica media is composed of smooth muscle cells organized in concen-
tric rings. These muscle cells are responsible for altering the blood vessel diameter in
response to neuronal input and local humoral control. The tunica media is typically the
thickest layer within the arterial wall (especially for large and medium-sized arteries),
allowing arteries to withstand the large pressure forces generated by the heart. The tunica
adventitia is composed of connective tissue, mainly collagen and elastin. In tissues, this
arterial wall layer is responsible for anchoring the blood vessel to adjacent tissue. The
tunica adventitia is normally nearly as thick as the tunica media layer.
Figure 5.1
illus-
trates a large artery (top panel) and a muscular artery (bottom panel).
The systemic circulation begins at the ascending aorta, immediately downstream of the
aortic valve (
Figure 5.2
). Within a few centimeters, the aorta makes a 180
turn (this is
called the aortic arch) that leads to the descending aorta. The aortic arch contains branches
for blood flow to the head (carotid arteries) and the arms (subclavian arteries). As the
aorta descends toward the pelvis, it continually tapers and directly branches to deliver
blood to many of the major abdominal organs (i.e., renal arteries, hepatic arteries, among
others). At the pelvic region, the aorta branches into the left and right iliac arteries, which
supply the legs with blood, as well as the sacral artery. The aorta can be described as
twisting (moves through different planes), tapering, and branching throughout its entire
length and therefore, the blood flow through the aorta is very complex. Furthermore,
blood velocity and pressure is normally the greatest in this vessel. However, nearly all of
the secondary arteries to the smallest arterioles and every artery between are fairly straight
(at least in two dimensions, radial and theta; it can vary in the z-dimension) and do not
taper significantly until there is a branching point. At every branching point, the diameter
of the branching vessel(s) (termed daughter vessel[s]) and the diameter of the main branch
are reduced. The reduction in blood vessel diameter is not constant (e.g., the vessel diame-
ter is not always halved at a branch point). For a comparison, the diameter of the ascend-
ing aorta is approximately 30 mm in humans. The diameter of the descending aorta,
immediately distal to the aortic arch, is approximately 27 mm. By the time the aorta has
reached the diaphragm, the diameter has reduced to approximately 20 mm. As a compari-
son, the iliac and carotid arteries have a diameter of approximately 10 mm. Therefore,
within a relatively short length (approximately 30 cm), the aorta experiences a reduction
in diameter of approximately one-third and therefore an approximate five-ninths reduc-
tion in the cross-sectional area. This is significant for the flow conditions throughout the
vessel. Again, the secondary arteries maintain a fairly constant diameter until another ves-
sel branches off from the original vessel.
After blood has passed through the arteries, it enters the smaller arterioles, which are
the last branches of the arterial system prior to blood entering the microcirculation. The
arterioles are also under high pressure (approximately 40 mmHg to 80 mmHg), to aid in
the rapid blood movement throughout the arterial system, and therefore, they are also
composed of a thick muscular layer. Not only do the muscles in the arterioles act to
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