**3.1 Effects of microgravity in the cardiovascular system**

The changes in the cardiovascular system begin solely with the fluid shift associated with microgravity, followed by the decreased circulatory blood volume, cardiac size, and aerobic capacity, and the most prominent symptom, postflight orthostatic intolerance. These symptoms are generically known as "cardiovascular deconditioning" [13–17].

When the spacecraft reaches low Earth orbit (LEO), body fluids move from the lower body to the thorax, which is associated with the increase in the intraocular pressure and morphological alterations in the central nervous system, demonstrated by changes in the magnetic resonance imaging (MRI).

As a result of fluid shift, the leg volume decreases and the face becomes puffy. The leg volume decreases by 1 L, whereas subcutaneous tissue at the forehead thickens by as much as 7% compared with in the preflight supine position. The pulmonary capillary blood volume increases by approximately 25%, and intraocular pressure can nearly double. Fluid shift increases the cardiac volume and stroke volume at the beginning of the spaceflight (first 24 to 48 hours), but over time, the heart rate, stroke volume, and cardiac output stabilize to the preflight sitting level. The arterial blood pressure slightly decreases compared with the preflight level. Compared with "space motion sickness," cardiovascular and fluid balance adaptation is gradual. The symptoms appear in 3–5 days and disappear after 1–2 weeks, causing facial edema, nasal stiffness, heavy headedness, papilledema, or jugular vein dilatation. These symptoms upon exposure to microgravity disappear at most 2 weeks after the reduction in circulatory plasma volume [13–17].

The cardiovascular changes in actual spaceflight differ from those in stimulations such as head-down bedrest or dry immersion. First, the volume of fluid shift is much larger than the orthostatic change from the supine to upright positions. The fluid volume loss during simulated microgravity (e.g., head-down bedrest or dry immersion) is less than 50% of that observed in actual spaceflight. Second, the central venous pressure measured during spaceflight does not increase as much as in head-down bedrest. Third, the diuresis caused during simulated microgravity is to a lower degree.

Then what is the cause of reduced blood volume after adaptation to microgravity? Diedrich et al. [18] explained the reduced blood volume in space as (1) a negative balance 2010 of decreased fluid intake and smaller reduction of urine output; (2) fast fluid shifts from the intravascular to interstitial spaces as a result of lower transmural pressure after reduced compression of all tissues by gravitational forces, especially of the thorax cage; and (3) fluid shifts from intravascular to muscle interstitial spaces because of lower muscle tone required to maintain body posture, and the attenuated diuresis during space flight is due to increased retention after stress-mediated sympathetic activation during the initial phase of space flight.
