The understanding of cardiovascular effects of spaceflight has evolved
throughout the course of the American manned spaceflight program. Ori
ginally descriptive in nature, the present understanding is based on e
mpiric measurements of vascular volume, cardiac output, vascular refle
xes, and peripheral and central autonomic control, More detailed under
standing of cardiovascular effects has allowed us to separate those sy
mptoms from symptoms caused by musculoskeletal or neurovestibular abno
rmalities.