26
Dynamics
of
the
Vascular
System
where h is the height in terms of the mercury column,
p
is the density
of
mercury,
or
13.6
gm/cm3, and
g
is the gravitational acceleration. Hence
100
mmHg, or
10
cmHg, is equivalent to
P
=
100
mmHg
=
10
x
13.6
x
980
=
133,280
dynes/cm2 or about
1.33
x
1
O5
dynes/cm*.
(2.2.8)
Strain in the longitudinal direction,
or
along the length
of
the blood
vessel is expressed
as
the ratio of extension per unit length, or the ratio
of
the amount stretched longitudinally to the length
of
the original vessel
segment,
A1
fI
&
=-
(2.2.9)
Strain in the radial direction, or perpendicular to the vessel segment
length, is the fraction
of
distention
of
the vessel lumen radius or
diameter. It is given by
:
Ar
r
&
=-
r
200
-
(2.2.10)
Fig.
2.2.4:
Ultrasonic dimension gages recorded diameter of the aorta, together with
aortic blood pressure. Calculation of radial strain can be obtained from the fractional
change in diameter,
AD/D.
