The human blood circulation is a closed system of conduits that carries blood from the heart to all tissues, and from these tissues back to the heart. In this system, the aorta and other, smaller arteries distribute blood from the heart to the various parts of the body. A normal artery has a tube-like structure comprising of three distinctive layers: the intima, the media and the adventitia (figure 1). The intima the inner layer of the artery that is in contact with blood, consists of a monolayer of endothelial cells supported by an underlying connective tissue membrane. The media comprises of e.g. collagens and elastin, with a dense population of smooth muscle cells (SMCs) that are oriented perpendicular to the blood flow. The adventitia, the ou er layer of the artery, is a collagenous matrix containing fibroblasts, small blood vessels and nerves. Normal vascular endothelium maintains a delicate balance in the vascular system, between growth promotion and inhibition of the underlying SMCs and fibroblasts, vasoconstriction and vasodilatation, blood cell adherence, and anticoagulation and procoagulation. In this process, endothelial cells actively secrete various factors affecting platelet adhesion and aggregation (prostacyclin, PGI_2), blood coagulation (von Willebrand factor, vWF) and fibrinolysis (tissue plasminogen activator, tPA, and tissue plasminogen activator inhibitor, PAI). Contraction and relaxation of the SMCs in the adventitia regulates blood flow, whereas the fibroblasts in the adventitia add to the rigidity and shape of a blood vessel.
展开▼
