In the space there is no gravity. In order to compare the blood pressure taken preflight and the one in the space, BP should be taken in a position where gravity is nullified. In lying position the gravitational force is nullified, so preflight BP measurement should be done in the lying down position. In supine position, legs are at the same level as the hea so that the gravitational forces that come into play in the upright position are diminished. When a person is lying on his back, the tricuspid valve is located at almost exactly 60% of the chest thickness in front of the back. This the zero pressure reference level. Ref: High-Yield Physiology, Pa 845, Volume 2008 By Ronald W. Dudek, Page 51; Essentials Of Medical Physiology By Khurana, Page 185.