Brain Blood Vessel Responses to Changes in Blood Flow

Inclusion Criteria: * Between the ages of 55-69 years * Have a BMI less than or equal to 34.5 kg/m2 * Nonsmoker * are less than 72 in (182.88 cm) height * Females must be postmenopausal Exclusion Criteria: * Diagnosis of Mild Cognitive Impairment (MCI) or Alzheimer's disease (AD) * History or evidence of hepatic disease, hematological disease, or peripheral vascular disease; severe kidney injury requiring hemodialysis * History of cardiovascular disease including: severe congestive heart failure, coronary artery disease, ischemic heart disease (stents, coronary artery bypass grafts) and tachycardia * Uncontrolled hypertension * History of clinically significant ischemic or hemorrhagic stroke, or significant cerebrovascular disease * Severe untreated obstructive sleep apnea * History of diabetes with HbA1c greater than 9.5% * Major neurologic disorders other than dementia (e.g., MS, ALS, brain surgery, etc.) * Current or recent (\<1 year) major psychiatric condition (Axis I) or addictive disorders * Significant surgical history * Other significant medical conditions at investigators' discretion * Contraindications to MRI * Prescribed medications that interfere with prazosin * Lactose allergy * lower body will not fit in lower body negative pressure (LBNP) chamber (regardless of overall height and BMI).