Vessel Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 349

Fig. 7.1 Wall tunicae. The
media (intermediate layer) is
bounded by the elasticae
interna and externa from the
intima (inner layer) and
adventitia (outer layer).

ENDOTHELIUM

INTIMA

MEDIA

ADVENTITIA

Veins that are the major blood storage compartments, accommodate volume changes
by shrinking to possibly reach a collapse state.
Wall expansion and relaxation depend on the rheology of the vessel wall, and thus
on its composition and structure. In particular, distension and subsequent recoil are
achieved by the architecture and interactions between coupled elastin and collagen
fibers. In addition to passive caliber changes, the vessel wall undergoes active
variations due to vasomotor tone. Vascular smooth muscle activity fits the blood
flow to the tissue requirements.
Structural components exist in every kind of blood vessel, except capillaries,
although the element amount and structure vary between vessel types. Endothelial
cells with a hemodynamic stress-dependent shape (50-100 m), line

the blood-wall interface of the whole vasculature (Chap. 9 ). Smooth myocytes are
responsible for vessel tone and hence lumen size (Chap. 8 ). They thus influence the
vascular impedance.
Specific cells of connective tissue (Vol. 1 - Chap. 1. Cells and Tissues) include
fibroblasts (FB; Vol. 1 - Chap. 2. Cells of the Blood Circulation)) that produce
ground matrix and fibers in growing tissues and fibrocytes (FC). The ground
matrix fills the space between cells and fibers. It is composed of interstitial fluid
that contains electrolytes and plasma molecules, proteoglycans, and glycoproteins.
Elastin forms low-stiffness elastic fibers, taut in the unstretched configuration
that can stretch up to 60% with elastic behavior. Collagen makes high-stiffness
fibers that are tortuous in the unstretched configuration and are responsible for
structural integrity. Thereby, elastin provides vessel distensibility and collagen
tensile strength. 4
The wall structure is characterized by a tunica set that comprises intima, media,
and external adventitia in large arteries and veins (Fig. 7.1 ). Tunica number and
structure vary according to vessel type and size.

× 10 × 0.5-2

4 Wall distensibility increases when tension rises. Elastin is involved at low stresses and collagen is
progressively recruited when tension increases.