  Human spaceflight has been fascinating man for centuries, representing the
intangible need to explore the unknown, challenge new frontiers, advance
technology and push scientific boundaries further. A key area of importance is
cardiovascular deconditioning, that is, the collection of hemodynamic changes -
from blood volume shift and reduction to altered cardiac function - induced by
sustained presence in microgravity. A thorough grasp of the 0G adjustment point
per se is important from a physiological viewpoint and fundamental for
astronauts' safety and physical capability on long spaceflights. However,
hemodynamic details of cardiovascular deconditioning are incomplete,
inconsistent and poorly measured to date; thus a computational approach can be
quite valuable. We present a validated 1D-0D multiscale model to study the
cardiovascular response to long-term 0G spaceflight in comparison to the 1G
supine reference condition. Cardiac work, oxygen consumption and contractility
indexes, as well as central mean and pulse pressures were reduced, augmenting
the cardiac deconditioning scenario. Exercise tolerance of a spaceflight
traveler was found to be comparable to an untrained person with a sedentary
lifestyle. At the capillary-venous level significant waveform alterations were
observed which can modify the regular perfusion and average nutrient supply at
the cellular level. The present study suggests special attention should be paid
to future long spaceflights which demand prompt physical capacity at the time
of restoration of partial gravity (e.g., Moon/Mars landing). Since spaceflight
deconditioning has features similar to accelerated aging understanding
deconditioning mechanisms in microgravity are also relevant to the
understanding of aging physiology on Earth.
