·The microcirculation is relatively inaccessible to direct visualization and investigation.The retina offers a readily accessible site to noninvasively evaluate the microcirculation.Recently,reliable methods of quantifying retinal microvascular changes from retinal photographs have been developed.In particular,quantitative measurement of retinal vascular caliber has greatly increased knowledge of the clinical significance and influence of systemic factors on the retinal arteriolar and venular caliber.Recent studies have reported that the AV,RV and AVR may mirror preclinical structural changes in the subclinical cerebrovascular,cardiovascular and metabolic outcomes.