Passive Heat Therapy and Age-associated Physiological Function

Inclusion Criteria: * Ability to provide informed consent. * Aged 55-79 years. Women must be postmenopausal and not taking hormone replacement therapy within the past year. * Sedentary to recreationally active: participating in vigorous aerobic exercise \<4x/wk for ≥ 3 yrs. * Able to refrain from the use of dietary supplements, anti-inflammatory medications, and prescription medications for 24-48h prior to experimental testing. * Generally healthy (no overt clinical disease), as determined by medical history, physical examination, blood and urine chemistries. * Ankle-brachial blood pressure index \>0.7 * Total cholesterol \<240 mg/dl (\<6.2 mmol/L) * Fasting plasma glucose \<126 mg/dl (\<7.0 mmol/L) * Systolic blood pressure \< 160 mmHg and \>100 mmHg * Glomerular filtration rate \> 60 ml/min/1.73 m2 * Mini mental state exam score \> 20 points * Body mass index (BMI) \<30 kg/m2 * Weight stable in the prior 3 months (≤ 2 kg weight change) and willing to remain weight stable over the course of the study * Free from alcohol dependence or abuse, as defined by the American Psychiatry Association, Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). Exclusion Criteria: * Current use of certain prescription medications that may interfere with thermoregulation and/or blood pressure control during heat stress. These include anti-hypertensives (in particular beta-blockers), nitrates and nitrites (e.g. nitroglycerin), PDE5 inhibitors (e.g. Viagra), anticholinergics (e.g. amitriptyline), and alpha-blockers (e.g. Flomax). * Orthostatic hypotension * Current or past smoking within the last 5 years * Recent major change in health status within previous 6 months (i.e. surgery, significant infection or illness). * Chronic clinical diseases (e.g., inflammatory bowel diseases/irritable bowel syndrome, coronary artery/peripheral artery/cerebrovascular diseases, diabetes, chronic kidney disease requiring dialysis, neurological disorders or diseases that may affect motor/cognitive functions \[multiple sclerosis, Parkinson's disease, polio, Alzheimer's disease, dementia or other brain diseases of aging\]). * History of heat-related illness * Open wounds or skin lesions. History of skin-related conditions or sensitivities to prolonged water immersion or exposure to pool chemicals. * Blood donation within the past 2 months * Inability to tolerate blood draws, intravenous catheters, and/or fingertip blood sampling, including past fainting in response to blood sampling. * Rectal, anal, vaginal, or prostate surgery within the last 6 months, current or recent (within the last 6 months) hemorrhoids, or related concerns regarding use of rectal thermistor