Effects of gravity on blood pressure

Under normal conditions, blood flows up through the legs and up to the head against gravitational hydrostatic pressure even when the body is upright (during orthostasis). This is largely due to an increase in total peripheral resistance caused by vasoconstriction. Contraction of leg muscles can supplement venous constriction and one-way valves ensure that blood does not flow backward through the veins.

In some cases, the control system malfunctions. Orthostatic hypotension or intolerance (low blood pressure when upright) is a persistent problem during re-entry after the microgravity environment of space. The lack of blood flow to the brain can impair function, and the mechanisms are unknown. Scientists are investigating the body's response to improve countermeasures with EU funding of the ARTHEROSPACE (Arthero-space project: A model based exploration of the regulatory mechanism of the microcirculation for the prevention of orthostatic intolerance) project.

One set of in vivo experiments induced orthostatic stress via a tilt table and with lower body negative pressure and studied interactions with the muscle pump effect, increased blood return via muscle constriction forcing venous constriction. The hemodynamic response to calf muscle contraction was similar with or without orthostatic stress.

A second set of in vivo experiments relied on ballistography, the non-invasive measurement of the mechanical recoil (an acceleration signal) of the body in response to cardiac activity. Subjects were assessed during static squats, dynamic squats and synchronised respiration. When combined with computational models, the data are expected to aid the use of ballistography in a clinical setting.

Team members also developed a 1D biomechanical model to study the role of gravity on venous blood pressure regulation. The model enables investigation of haemodynamics in three different configurations consisting of a single artery-vein without valves, the same with valves and a configuration including superficial veins. The model has demonstrated the importance of proximal venous valves and the superficial venous system for muscle pump effectiveness.

The ARTHEROSPACE research group is making important progress in untangling the mechanisms associated with orthostatic hypotension with important implications for the safe return of astronauts to Earth.