**6. The effects of space and its physiology on the human body**

Astronauts are healthy individuals who will pass many courses to be well trained and finally highly selected. They should encounter different physiology until accommodation to pathophysiological alterations as soon as possible. Space medicine has some similarities (hypoxia, dysbarism, thermal support, acceleration, and response to high altitudes) with both aviation medicine and diving medicine [9]. These branches of medicine are taught separately, but there are aerospace medicine residency programs in a few countries (aviation medicine plus space medicine). There is only one combined aerospace and subaquatic medicine residency course and faculty in Iran.

Living and working on the Earth differ with space as mentioned earlier. Standing or sitting is the ideal position for doing the tasks and responsibilities. Gravity direction is perpendicular to the surface of our planet, so the body fluids are pulled down. The same occurs to our bones and muscles. The following paragraphs show the alterations of the human body during spaceflights briefly.

Some changes due to of microgravity exposure would become evident after a few days (i.e., motion sickness), some after weeks (i.e., cardiovascular deconditioning), and some even after months (i.e., osteoporosis and muscular atrophy) [9].

#### **6.1 Immune system**

Immune problems that have been identified and reported during spaceflights are:


There are more reports about immune system alterations [25, 26, 29], but some authors have not reported significant changes [30].
