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When you have a lower blood volume, your blood vessels don't expand as much as they normally would – ultimately lowering blood pressure.

She was losing so much blood that she needed the equivalent of around three people's blood volume given back to her, says NHS Blood and Transplant.

“For athletes who don’t want to cheat, altitude training is one of the unique ways to enhance a training response and improve the blood volume and red cell mass in particular,” said Dr. Benjamin Levine, professor of internal medicine at UT Southwestern and director of Texas Health Dallas’ Institute for Exercise and Environmental Medicine.

Two main techniques are used: imaging photoplethysmography, detecting blood volume changes, and periodic movement extraction, measuring heart and respiratory rates from body movements; both have been validated in nonhuman primates but have certain limitations.

The researchers found that the most important factor in how the right ventricle responded to VAD implantation was how well the pulmonary vascular system -- the network of vessels that carries blood between the heart and lungs -- adapted to changes in blood volume and flow induced by the VAD.