The tissue of cardiovascular

TISSUES OF THE VASCULAR WALL:
The endothelium: is a specialized epithelium that acts as a semipermeable barrier between two internal compartments: the blood plasma and the interstitial tissue fluid. Vascular endothelial cells are squamous, polygonal, and elongated with the long axis in the direction of blood flow. Endothelium with its basal lamina is highly differentiated
to mediate and actively monitor the bidirectional exchange of molecules by simple and active diffusion, receptormediated endocytosis, transcytosis.
Smooth muscle fibers: occur in the walls of all vessels larger than capillaries and are arranged helically in layers. In arterioles and small arteries, the smooth muscle cells are connected by many more gap junctions and permit vasoconstriction and vasodilation which are of key importance in regulating the overall blood pressure.
Connective tissue :components are present in vascular walls in variable amounts and proportions based on local functional requirements. Collagen fibers are found in the subendothelial layer, between the smooth muscle layers, and in the outer covering. Elastic fibers provide the resiliency required for the vascular wall to expand under pressure. Elastin is a major component in large arteries where it forms parallel lamellae, regularly distributed between the muscle layers. Variations in the amount and composition of ground substance components such as proteoglycans and hyaluronate also contribute to the physical and metabolic properties of the wall in different vessels, especially affecting their permeability.
The walls of all blood vessels larger than the microvasculature
have many components in common and similar organization. Branching of the vessels helps produce reductions in their size which are accompanied by gradual changes in the composition of the vascular wall. Transitions such as those from “small
arteries” to “arterioles” are not clear-cut. However, all of these
larger vessels have walls with three concentric layers, or tunics
(L. tunica, coat.
? The innermost tunica intima consists of the endothelium and
a thin subendothelial layer of loose connective tissue sometimes containing smooth muscle fibers. In arteries and large veins, the intima
includes a prominent limiting layer, the internal elastic lamina, composed of elastin, with holes allowing better diffusion of substances from blood deeper into the wall.
? The tunica media, the middle layer, consists chiefly of concentric layers of helically arranged smooth muscle cells. Interposed among the