The goal of these studies is to assess the use of phonocardiograhy to monitor blood volume. We will evaluate phonocardiographic parameters in normal subjects in increasing levels of lower body negative pressure (LBNP) and lower body positive pressure (LBPP).

Inclusion Criteria:

• At least 18 years old
• Good health
• Free of systemic diseases
• No contraindications to LBNP/LBPP exposure
• Normal resting systolic blood pressure (SBP, 100-140 mm Hg) and diastolic blood pressure (DBP, 60-90 mm Hg)

Exclusion Criteria:

• Pregnancy
• Prior history of cardiovascular disease