Plasma volume changes during rest and exercise in different postures in a hot humid environment

Area of Science:

• Exercise Physiology
• Environmental Physiology
• Human Physiology

Background:

• Understanding plasma volume shifts is crucial for thermoregulation and performance.
• Previous research indicates posture influences fluid balance during rest and exercise.
• The effects of posture on plasma volume in hot, humid environments require further investigation.

Purpose of the Study:

• To investigate the effects of different postures (upright, low-sit, supine) on plasma volume shifts.
• To examine plasma volume changes during rest and exercise in a hot, humid environment.
• To compare the magnitude of plasma volume reduction across postures and work intensities.

Main Methods:

• Five male subjects participated in the study.
• Subjects underwent resting and exercising periods (45 min each) in upright, low-sit, and supine postures.
• Exercise involved work rates of 360 or 540 kpm/min in a hot, humid environment (49.5°C, 28.9 Torr).

Main Results:

• At rest, plasma volume decreased by 17.3% (upright), 9.0% (low-sit), and 2.2% (supine).
• During exercise, plasma volume reductions were 2.7% (upright), 7.1% (low-sit), and 11% (supine).
• Total plasma volume reduction was greatest in the upright posture (20%) and least in the supine (13.3%). Work intensity did not significantly affect plasma volume shifts.

Conclusions:

• Posture significantly influences plasma volume regulation during rest and exercise in the heat.
• The observed plasma volume shifts are qualitatively similar to those in cooler environments.
• These findings have implications for understanding physiological responses to heat stress and exercise in varying body positions.