Gravitational stress and volume regulation

Area of Science:

• Human physiology
• Renal function
• Gravitational stress

Background:

• Ground-based experiments over 30 years have investigated human renal responses to altered gravitational stress.
• Simulated gravitational changes impact fluid and electrolyte balance.

Purpose of the Study:

• To investigate the effects of increased and decreased gravitational stress on human renal response.
• To understand the role of central blood volume in regulating renal function under varying gravity.

Main Methods:

• Simulated increased gravity using centrifugation, head-up tilt (HUT), and lower body negative pressure (LBNP).
• Simulated decreased gravity using water immersion (WI), head-down tilt (HDT), and lower body positive pressure (LBPP).
• Measured renal excretion of sodium and water, and plasma levels of hormones like AVP, PRA, PA, NE, and ANP.

Main Results:

• Increased gravity (+Gz) reduced renal sodium and water excretion, accompanied by hormonal changes (increased AVP, PRA, PA, NE; decreased ANP).
• Decreased gravity (approx. 0 Gz) via WI increased renal water and sodium excretion (100-400%) with opposite hormonal changes (decreased AVP, PRA, PA, NE; increased ANP).
• HDT also increased renal fluid and sodium excretion compared to seated controls, though results were less consistent than WI.

Conclusions:

• Changes in central blood volume are key determinants of renal functional changes under altered gravitational stress.
• Ground-based experimental models are valuable tools for studying volume regulation physiology and pathophysiology.
• Spaceflight data on renal function during weightlessness is inconsistent but suggests cephalad fluid shift and decreased total body fluid.