Fluid and protein shifts after postural changes in humans

Area of Science:

• Physiology
• Biomedical Engineering

Background:

• Understanding fluid shifts and their impact on blood composition is vital for physiological studies.
• Previous methods for measuring blood volume changes have limitations in dynamic assessment.

Purpose of the Study:

• To introduce and validate a new mass density detection method for measuring blood density (BD), plasma density (PD), and erythrocyte density (ED).
• To investigate dynamic changes in blood and plasma volume in response to different head-up tilt (HUT) postures in euhydrated humans.

Main Methods:

• Utilized a novel mass density detection technique to measure BD, PD, and ED.
• Subjects underwent various head-up tilt (HUT) procedures, including stepwise and repeated tilting.
• Analyzed fluid shifts and calculated plasma volume (PV) changes based on density measurements.

Main Results:

• Erythrocyte density (ED) remained stable across all HUT positions.
• Changes in PD and BD correlated with hemodilution and hemoconcentration.
• The mass density of shifted fluid (FD) indicated a protein concentration of approximately 30 g/l.
• Stepwise HUT to 90 degrees caused an 18% mean PV loss; repeated HUT to 70 degrees resulted in a 14% PV loss.

Conclusions:

• Erythrocyte volume is constant during postural changes in euhydrated individuals.
• Postural changes lead to a net loss or gain of intravascular protein, with shifted fluid protein concentration similar to whole-body lymph.
• Microsample densitometry is a precise method for assessing rapid blood volume dynamics in humans.