Upon arrival for each visit, participants will be told which study session they will be participating in that day. Food and water consumption as well as fluid loading will be timed throughout the day during other testing described below. Participants will be asked to empty their bladder, and body weight will be measured using a standard calibrated medical scale. Then, an intravenous (IV) catheter (a small plastic tube) will be inserted into a vein in the participant's arm. This IV catheter will remain in their arm for the duration of the study day. This is the same method that is used in a doctor's office or hospital. All blood samples will be obtained using the IV catheter in the arm.

Next, a test called Carbon Monoxide Re-Breathing (CORB) will be done to measure plasma volume, which is the liquid part of blood. A clip will be placed on the participant's nose so that they are breathing only through their mouth. Participants will breathe through a mouthpiece connected to a rebreathing circuit containing 100% oxygen, which is scrubbed by soda lime to remove carbon dioxide. After \~5 minutes, the first blood sample (\~10 Milliliters \[mL\]) will be collected from an arm vein using the IV catheter. Thereafter, a small amount of carbon monoxide will be added into the breathing circuit, and the participant will breathe this mixture of carbon monoxide and oxygen for \~10 min. Then, a second \~10 mL blood sample will be obtained. Analysis of the blood samples (i.e., hematocrit, hemoglobin, and carboxyhemoglobin) will be used to calculate red blood cell mass, plasma volume, and total blood volume. The total amount of carbon monoxide will be determined based on the participant's body weight (1 mL/kg body mass for men; 0.8 mL/kg body mass for women), and this dose should result in no symptoms. Mild symptoms of carbon monoxide toxicity may occur at carboxyhemoglobin levels \>10% but given the dose of carbon monoxide administered in this study, reaching this level is unlikely.

Participants will be seated for the duration of the study, except for short bathroom breaks allowed periodically during the study session. The volume of urine and the time of collection will be measured and recorded throughout the day. Participants will be provided food during the morning as well as water to drink during the first 5 hours of each visit. Meals will be like those typically consumed by astronauts on the International Space Station. During 3 of the visits, participants will be given additional fluids to drink on a set schedule (Fluid Loading). Participants will not be allowed to consume any fluids or food during the last 3.5 hours of the visit. Participants will be asked about the presence of gastrointestinal symptoms periodically throughout each study session. Additionally, we ask that participants report to the PI/study lead if any off-nominal gastrointestinal symptoms occur in the day of the study visit after testing is completed.

During the last 3.5 hours of each visit, 5 additional blood samples will be obtained (one each hour), used to calculate the change in plasma volume at each time point. Participant blood pressure and heart rate will be measured periodically throughout the day. (IRB protocol section 4.0)