Mechanisms of Impaired Brain Blood Flow With Aging

Inclusion Criteria: * 18-29 or 55-79 years old Exclusion Criteria: * Pregnancy or breastfeeding; * Blood chemistries indicative of abnormal renal, liver, thyroid and adrenal function (i.e., outside of normal reference range); estimated glomerular filtration rate using the MDRD prediction equation must be \>60 ml/min/1.73 m2; * Abnormal blood chemistry that is clinically relevant or any blood chemistry marker that is +/-2.5x the upper or lower limit; * Lack of a suitable temporal window for cerebrovascular assessments; * Current smoking; * Chronic clinical diseases (e.g., coronary artery, peripheral artery, or cerebrovascular diseases, diabetes, chronic kidney disease, COPD); * Major psychiatric disorder (e.g. Alzheimer's disease or other form of dementia, schizophrenia, bipolar disorder, major depression within past two years); * Neurological or autoimmune conditions affecting cognition (e.g. Parkinson's disease, epilepsy, multiple sclerosis, head trauma with loss of consciousness greater than 30 min, large vessel infarct); * Current medication use likely to affect CNS functions (e.g. long active benzodiazepines); * Having past or present alcohol dependence or abuse, as defined by the American Psychiatry Association, Diagnostic and Statistical Manual of Mental Disorders; * Body mass index (BMI) \>40 kg/m2 (FMD measurements can be inaccurate in severely obese patients).