Cardiovascular regulatory response to lower body negative pressure following blood volume loss.

An attempt is made to explain the cardiovascular regulatory responses to lower body negative pressure (LBNP) stress, both in the absence of and following blood or plasma volume loss, the latter being factors regularly observed with short- or long-term recumbency or weightlessness and associated with resulting cardiovascular deconditioning. Analytical expressions are derived for the responses of mean venous pressure and blood volume pooled in the lower body due to LBNP. An analysis is presented for determining the HR change due to LBNP stress following blood volume loss. It is concluded that the reduced orthostatic tolerance following long-term space flight or recumbency can be mainly attributed to blood volume loss, and that the associated cardiovascular responses characterizing this orthostatic intolerance is elicited by the associated central venous pressure response.