Vascular Biology, Structure and Function
21
There are several branching junctions before the pulse reaches the
vascular beds. In relation to this, the number of generations of blood
vessels are given by Green (1950) and lberall(l967).
Experimental data by Li (1987) gives typical values of internal
diameters in
a
20
kg
dog: ascending aorta, 15 mm; abdominal aorta,
8
mm; femoral artery, 3 mm; small artery,
0.
1 mm. These values reveal
an appreciable "geometric taper" in the aorta from the root to the aorto-
iliac junction. Together with branching, it contributes to the "geometric
nonuniformity, observed throughout the arterial system.
The term "geometrical taper" is appropriate when applied to
a
single
continuous conduit, such as the aorta. The area change
of
the aortic cross
section is close to an exponential form and can be expressed as:
A(z)
=
A(0)e-k'r
(2.2.1)
where:
z
=
distance in the longitudinal axial direction along the vessel
r
=
vessel lumen radius in cm
k
=
taper factor, dimensionless
A(0)
=
the cross-sectional area at the entrance of the vessel in cm2
A(z)
=
the cross-sectional at distance
z
along the vessel in cm2
The vessel area is calculated, assuming a circular cross-section,
A=m2
(2.2.2)
The taper factor
k,
can be readily obtained as
(2.2.3)
Taper factor,
k,
for the aorta has been reported to be in the range of
0.0314-0.0367 for 20-30 Kg dogs (Li, 2000).
Geometric taper factor
can change substantially during varied vasoactive conditions and in
disease conditions. When vasoactive drugs are administered which have
