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Blood volume redistribution during hypovolemia
Andrew P Blaber1, Helmut Hinghofer-Szalkay, Nandu Goswami
1School of Kinesiology, Simon Fraser University, Burnaby, B.C., Canada.
Aviation, Space, and Environmental Medicine
|January 12, 2013
Summary
Splanchnic blood volume redistribution, not leg vasoconstriction, is key for maintaining blood pressure during central hypovolemia. This study reveals how the body adapts to reduced blood volume to prevent fainting.
Area of Science:
- Cardiovascular Physiology
- Human Physiology
- Hemodynamics
Background:
- Progressive central hypovolemia is induced by graded lower body negative pressure (LBNP).
- Understanding the compensatory mechanisms for blood pressure maintenance during hypovolemia is crucial.
Purpose of the Study:
- To investigate the roles of splanchnic volume redistribution and lower limb vasoconstriction in maintaining blood pressure during LBNP-induced central hypovolemia.
- To test the hypothesis that splanchnic blood volume loss buffers decreases in thoracic blood volume.
Main Methods:
- 15 healthy subjects (8 men, 7 women) underwent LBNP at -10, -20, -30, and -40 mmHg.
- Segmental impedance analysis measured central and splanchnic volume changes.
- Near-infrared spectroscopy (NIRS) assessed calf venous volume and vasoconstrictor tone.
Main Results:
- LBNP induced significant lower limb venous pooling (57% increase in deoxygenated calf blood) and decreased cardiac output (28%).
- Splanchnic impedance increased by 13.1%, indicating substantial splanchnic blood volume redistribution (over 50% to the thoracic compartment).
- While heart rate increased in all subjects, only men showed significant leg vasoconstriction; mean arterial blood pressure was maintained without presyncope.
Conclusions:
- Splanchnic blood volume redistribution is a primary mechanism for blood pressure regulation during central hypovolemia.
- Lower limb vasoconstriction plays a lesser role compared to splanchnic redistribution in this context.
- These findings highlight the importance of splanchnic circulation in cardiovascular homeostasis under stress.
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