Principles of Gravitational Biology

Gravity influences the design of terrestrial organisms, an observation that was originally made by Galileo in 1638. He recognized that large animals have relatively thicker weight-supporting bones than smaller animals. As animals increase in body size there is an increase in relative skeletal size, a property known as the scale effect (Table 1-03). These differences are due to gravitational influence, since marine mammals have lesser skeletal size than terrestrial mammals with a comparable body size (Smith 1975). Consequently, large animals are more dramatically affected by gravity than small animals. Smaller organisms, such as insects, are more affected by such forces as surface tension, whereas viscosity, Brownian movement, and other intermolecular phenomena principally affect microorganisms like bacteria.

Animal

Body

Skeleton

Mouse

0.02

8

Dog

5

13-14

Man

75

17-18

Table 1-03. Increase in body mass (in kg) and relative skeletal mass (in % of body mass).

Table 1-03. Increase in body mass (in kg) and relative skeletal mass (in % of body mass).

However, it is controversial as to whether living systems perceive gravity at cellular levels, hence a direct effect, or if the effect of gravity translates to physiologically important phenomena at multicellular or tissue levels, i.e., an indirect effect. In fact, at the cellular level, structures remain generally insensitive to forces on the order of Earth's gravity. For example, to selectively remove the organelles of animal cells, a centrifuge must rotate at speeds generating centrifugal forces in the order of 1,000 g. To separate large molecules, such as proteins, forces in the order or 100,000 g are required. By the same principle, the tolerance to high acceleration is inversely correlated to body size. Small animals are intrinsically more tolerant to high g than large animals (Figure 1-08). The tolerance limits are also affected by posture, which results in great part from the structure of the vascular apparatus, especially a column of blood. A significant fraction of the total blood flow (defined as the cardiac output) is directed to the brain and is necessary for the brain to function. The pressure drop in a blood column is proportional to the height of the column, the fluid density, and the acceleration. For prolonged acceleration exposures, body fluid shifts become relatively important and tend to dominate the deleterious effects of acceleration.

Figure 1-08. Body size is inversely related to the peak acceleration that the animals can sustained before serious injury occur, Other studies have found that body weight is also inversely related to the threshold g-value at which animals are resistant to a prolonged acceleration. By comparison, small, young plants can easily withstand 10 minutes at 30-40 g without noticeable structural change and can even endure several hundred g without evident structural breakage (Smith 1975).

The development of terrestrial organisms resulted in an increased susceptibility to gravity. Life originated in unicellular organisms in an aquatic environment wherein gravity had little direct effects. Certain animals became terrestrial at a relatively small size, after an intermediate step of developing increased body mass and a skeleton. In becoming terrestrial and increasing their size, such animals had to adapt and conform to the more stressful requirements imposed by gravity-induced loads. It is perhaps significant in this regard that the largest terrestrial animals became extinct. Or that the largest animals that ever lived, the blue whales, returned to an aquatic existence, where gravitational influence is greatly reduced (Smith 1975).

We know that organic form and metabolism are adapted to body size, but the relative importance of gravity and genetic factors in such adaptations is not fully understood. By the use of sensitive methods it should now be feasible to study the role of gravity in the manifestation of scale effects in animals, and also in plants, removed from the gravitational stimulus.

A significant challenge is that Space Biology grew so fast and in such a sporadic fashion that basic whole-organism biology has barely been studied in microgravity. Richard Wassersug (2001) advocated the fundamental importance of studying "Integrative Biology" in space, i.e., not just fly invito experiments, but whole organism. The whole is more than the sum of the parts. Knowing how cells perform in a culture in space may reveal little about how whole organisms will respond in microgravity. Evidence today suggests that cells and tissues studied individually in space may not reflect how whole organisms respond in the same environment. Ultimately, in the crucial area of astronaut health, it is the whole organism that most assuredly counts most. Also, the public likes whole organisms and can identify with and understand biology at that level more so than most any experiment in cell biology. In fact,

Muscle Microgravity

the more carefully scientists observe the behavior of the developing system, the more they see evidence of altered behavior. These alterations are organized, adaptive responses to microgravity. Research on board the International Space Station must incorporate the expertise of behavioral scientists to attain accurate, meaningful results.

Another issue is that animals were often flown in confined systems to prevent them from endangering themselves during launch and reentry or damaging sensors or instrumentation during the flight. If the animals cannot float freely within their habitats while in the microgravity environment, then their responses, particularly those related to muscle atrophy, bone loss, and spatial orientation, cannot be directly compared to those in astronauts. Before the ISS, no spacecraft, Mir included, has had the space or equipment onboard for anything but simple studies of highly confined vertebrates. It is now time to move from studies using passive (restrained) exposure to studies allowing active (free-floating) exposure to microgravity.

A comprehensive understanding of microgravity effects will require analyses of both the whole organism as well as some of its component cells, tissues or organs. In fact, the first microgravity effects to be recognized in whole organisms may have been preceded by prior effects, more subtle, which were not detected. Most organisms contain effective homeostatic mechanisms for masking environmental challenges such as changes in the amplitude of the gravitational vector.

One thing is sure: a variety of organisms have flown during spaceflight and most of them have survived in the space environment. After extended stays in space, can those organisms grow, develop, and reproduce normally? Such questions are much more difficult than questions of survival because they require sophisticated scientific experimentation in order to understand just how the living organisms are affected by the conditions of spaceflight, including microgravity.

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