580 
HEMODYNAMICS 
stifFer in the longitudinal direction than in the 
circumferential direction; and (4) the CA is 
stiffer in the circumferential direction than in 
the longitudinal direction. 
The values of elastic moduli for the MDTA 
were found to decrease when the vessel segment 
was removed from the animal and restudied in 
vitro. The longitudinal modulus decreased by 
32% (p <.01), while E, decreased by 20% 
(p <.05) and E# by 9% (0.5 <p <0.1). Thus, 
the in vivo relation between E# and E^ is just 
the opposite of that indicated in Table I. The 
32 % decrease in the value of Ez can be explained 
by the removal of the in vivo longitudinal teth- 
ering of the vessel segment. Using data from a 
previous study,' the contribution of vascular 
tethering to the in vivo longitudinal modulus is 
calculated to be 40 % . 
Figure 3 shows the relation of the in vivo, 
static moduli and the extension ratios Ke and K. 
That is, the moduli are plotted against the mean 
values of three groups of Xo (upper panels) and 
against the mean values of three groups of K 
(lower panels). It is seen that (1) the average 
Figure 3. — Incremental elastic moduli and extension ratios. Ee = 
circumferential modulus; = longitudinal modulus; Er = radial 
modulus; Xe = circumferential extension ratio; Xi = longitudinal 
extension ratio ; horizontal bars indicate the standard error of the 
mean for each of the groups. In the upper panels, the moduli are 
plotted vs. the mean \e for each of three groups; the corresponding 
values of X. (from left to right) are 1.45 ± .04 SE, 1.56 ± .02, 
and 1.51 ± .02. In the lower panels, the moduli are plotted vs X, ; 
the corresponding values of Xe for each of these groups (left to 
right) are 1.46 ± .02, 1.58 ± .02 and 1.48 ± .02. Adapted from 
Patel, D. J., Janicki, J. S., and Carew, T. E. Static anisotropic 
elastic properties of the aorta in living dogs. Circ. Res. 25:765, 
1969. 
