1.Human:

a.Bed rest (prolonged bed rest protocols and head-down tilt experiments, about 6°)

Head tilt is the most used space analog in scientific researches. The head is tilted down (−6° almost in all cases) during lying in a bed for different periods according to the study. There would be a cephalic fluid shift, immobilization, and isolation in this kind of modeling [16].


An airplane has alternative flies so that there would be three phases (normal gravity, hypergravity, and microgravity) in each period. Microgravity lasts about 20 seconds. Parabolic flight is the only simulation model that the person feels real microgravity. The airplane might fly until 30 periods maximally in a flight [18].

2.Computer simulation (especially in musculoskeletal biomechanics):

The data which are gathered or extracted from spaceflight and other experiments such as parabolic flights have been documented to validate the results of simulations [19].

	- a.Bone and muscle radiation
	- b.Detection of genetic effects on osteoporosis and muscle atrophy
	- c.To identify the process of bone fracture restoration in the space travel environment (i.e., weightlessness and immune system alterations)

The main reason for performing animal models and simulation is that spaceflights are rare and expensive [20].

Several studies have been performed using tail suspension model, hind-limb suspension model, and so on (especially in rodents such as rat and mouse) [21, 22].

4.Cell [23]:

a.Rotating wall vessel (a kind of two-dimensional clinostat)

b.Random positioning machine (a kind of three-dimensional clinostat)

c.Other types of clinostats

Scientists use ground-based models because of two reasons [24]:


Parabolic flight is such an analog. An airplane repeats a particular flight, so there would be a few seconds of free fall (weightlessness or no gravity) in each period. Importance of human and animal simulations is stated as [5]:


**69**

are:

*Vitamin D in Space*

and faculty in Iran.

**6.1 Immune system**

counted:

*DOI: http://dx.doi.org/10.5772/intechopen.82314*

vibration are also entangled.

Cell simulators are important regarding creating insight into the suitability of future space missions, but it can be argued that these devices may still not replace the experiences of space travel. In this regard, two reasons can be

1.Space travel has its complexities and complications since microgravity is not considered as the unique problem alone, but hypergravity, radiation, and

2.The rotation kinetics of this simulator may cause fluid movement and shake in the chamber, which adds extra force to the cells present in the medium.

Therefore, it is necessary to consider that the results of these devices and tech-

Astronauts are healthy individuals who will pass many courses to be well trained

niques should be confirmed with the experiences in real microgravity.

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

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

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

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),

Immune problems that have been identified and reported during spaceflights

show the alterations of the human body during spaceflights briefly.

• Alteration in adaptive immunity regulation

• Decrease in function of some leukocyte types

• Change in cytokine profiles

• More infectious disease incidence

and some even after months (i.e., osteoporosis and muscular atrophy) [9].

• Disordered interaction between adaptive and innate immunity

• Changes in the distribution of peripheral leukocytes

The reasons for not replacing cellular simulators instead of real flights are as the following [23]:

### *Vitamin D in Space DOI: http://dx.doi.org/10.5772/intechopen.82314*

*Fads and Facts about Vitamin D*

simulations [19].

4.Cell [23]:

human studies, such as:

flights are rare and expensive [20].

c.Other types of clinostats

• The cost would be much less.

• To study on high sample numbers

exercise, drugs, sleep, and others)

a.Bone and muscle radiation

20 seconds. Parabolic flight is the only simulation model that the person feels real microgravity. The airplane might fly until 30 periods maximally in a flight [18].

2.Computer simulation (especially in musculoskeletal biomechanics):

b.Detection of genetic effects on osteoporosis and muscle atrophy

c.To identify the process of bone fracture restoration in the space travel environment (i.e., weightlessness and immune system alterations)

The main reason for performing animal models and simulation is that space-

Several studies have been performed using tail suspension model, hind-limb suspension model, and so on (especially in rodents such as rat and mouse) [21, 22].

b.Random positioning machine (a kind of three-dimensional clinostat)

Parabolic flight is such an analog. An airplane repeats a particular flight, so there would be a few seconds of free fall (weightlessness or no gravity) in each period.

• Understanding human sexual differences (effectiveness of interaction)

• Reaction and interaction of different countermeasures (including nutrition,

The reasons for not replacing cellular simulators instead of real flights are as the

a.Rotating wall vessel (a kind of two-dimensional clinostat)

Scientists use ground-based models because of two reasons [24]:

• They have more control over the research situation.

Importance of human and animal simulations is stated as [5]:

The data which are gathered or extracted from spaceflight and other experiments such as parabolic flights have been documented to validate the results of

3.Animal: Particularly about the points that could not be implemented even by

**68**

following [23]:

Cell simulators are important regarding creating insight into the suitability of future space missions, but it can be argued that these devices may still not replace the experiences of space travel. In this regard, two reasons can be counted:


Therefore, it is necessary to consider that the results of these devices and techniques should be confirmed with the experiences in real microgravity.
