Vascular Endothelium - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 565

Table 9.29. Circumferential tension T θ per unit length estimated by the Laplace law at a first
approximation, assuming a membrane-like behavior ( T θ ∼ hc θ ∼ p i R ).

Vessel p i R T θ

(kPa) (mm) (Pa)

Aorta 13.3 10 133

Large arteries

Small arteries 11.3 2 22.6

Arterioles 8 0.25 2

Capillaries 4 0.01 0.04

Venules 2.7 0.25 0.7

Ve i n s 2 3 6

Vena cava 1.3 15 20

transmitted throughout the wall continuum, which reacts. Intramural stresses are
composed of axial, radial, and circumferential components 150 (Table 9.29 ).
Vascular endothelial cells synthesize vasodilators, such as nitric oxide, prostacy-
clin, hydrogen peroxide, carbon monoxide, and epoxyeicosatrienoic acids, among
others, as well as vasoconstrictors, such as endothelin-1, thromboxane-A2, and
prostaglandin-H2. During inflammation, tryptophan is catabolized into kynurenine
by indoleamine (2,3)-dioxygenase in some types of activated macrophages and
other immunoregulatory cells. 151

Kynurenine generates an endothelium-dependent

vascular relaxation [ 1047 ].
Three enzymes - manganese superoxide dismutase , cyclooxygenase -2, and nitric
oxide synthase - have anti-oxidant, antithrombotic, and anti-adhesive activities.
They react differently according to the type of mechanical stimulus [ 1048 ].
Mechanical removal of the endothelium alters the flow-dependent response, but
not reaction of mural cells to acetylcholine and noradrenaline [ 1049 ]. Mechanical
stress exerted on the wall by blood generates a basal tone in smooth myocytes in the
absence of neurogenic and hormonal influences. Local metabolism and appropriate
ion ([Ca 2 + ], [Na + ]) concentrations are required. The hemodynamic stresses act
on the smooth myocytes via direct stress transmission or compound release by
endothelial cells.

150 For a cat thoracic aorta at a pressure of 16 kPa, with an axial stretch ratio of

∼ 1 . 7andan

opening angle of 70 degrees in the unstressed state, intramural longitudinal and circumferential

tensile Cauchy stress and radial compressive stress at outer and inner walls have been found to be
95 and 71, 105 and 61, and

∼

16 and 0, respectively [ 1046 ].
151 Kynurenine is catabolized by kynureninase to anthranilic acid, by kynurenine-oxoglutarate
transaminase to kynurenic acid, and by kynurenine 3-hydroxylase to 3-hydroxykynurenine.

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