Hypertension, or high blood pressure, is accompanied by numerous anatomical and functional changes in the cardiovascular system. Thickening of the walls of the aorta and ventricle, often seen in patients with (presumptive) high blood pressure, was first described in the early 1800's, decades before blood pressure could be measured. In spite of numerous experiments designed to investigate the role of modified vascular structure in the genesis of hypertension, it is not clear to what degree these changes result from high blood pressure, and to what extent they cause it. Strong theoretical, biophysical and bioengineering arguments can be made for an important role of alterations in vascular structure in providing ''feedforward'' control of blood pressure. High blood pressure causes thickening of the vascular and ventricular walls through hyperplasia (increased number of cells) and hypertrophy (increased size of cells); these processes exacerbate high blood pressure, forming a self-perpetuating cycle. The precise mechanisms underlying these phenomena are not known, but most likely include a variety of endogenous factors (within the body) that are known to enhance smooth muscle growth, including platelet-derived and fibroblast growth factors, interleukin-1, endothelin, norepinephrine, insulin, and others, as well as the mechanical forces acting on vessel walls. The degree to which these anatomical alterations can be reversed by antihypertensive medications is not fully known; however, it appears that angiotensin-converting enzyme (ACE) inhibitors do at least partially reverse the changes, and alpha-adrenergic blocking agents may also have such an effect. Thiazides and hydralazine-like vasodilators (drugs that relax vascular smooth muscle), on the other hand, do not appear to be effective in this regard. There are not enough data available to make even tentative conclusions concerning the effects of calcium channel blockers or beta adrenergic blocking agents. (Consumer Summary produced by Reliance Medical Information, Inc.)