Microgravity - Extreme Physiology

Microgravity

Gravity is the attraction of 2 masses - dependent on mass and distance

When an object is only subject to gravity it is in freefall - appears weightless

Space stations are in earth's orbit, effectively in free fall around earth

Objects accelerate due to gravity at the same rate irrespective of their mass

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Neurovestibular disturbances

Space motion sickness - 1-2 days after arriving in space

Nausea/vomiting caused by visual and neurovestibular mismatch

Can impair function during important decision making e.g. landing

Countered by:

Time - wears off naturally

Treatment - anti nausea medication

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Psychological effects

Unique environment creates psychological and social stresses

e.g. sleep deprivation due to acoustic noise and disrupted circadian cycles

Countered by:

1. Rigoruous selection procedures

2. Sleeping medication

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Immune dysregulation

Lymphocyte activity reduced 

Decreased natural killer cells and activation of T cells

Impaired cell mediated immunity - may be associated with cell cytoskeletal function which are activated by mechanical forces

High emotional and physical stress levels may also supress the immune system

Also microgravity promotes reactivation of latent herpes virus

Countered by:

1. Nutritional supplements

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Fluid Redistributions

Redistribution of fluid towards head and torse - 10% less in legs

Distention of baroceptors reduces renin-angiotensin and causes release of ANP 

This leads to increased salt and water excretion and reduction in plasma volume

In first 24hr of spaceflight - 17% reduction in plasma volume

Haemocrit rises which causes a decrease in erythropoietin (so total RBC mass is decreased)

Overall blood volume decreased by about 10%

Post flight - aerobic capacity decreased due to reduced stroke volume, and the restoration of blood can make astronauts light headed and prone to fainting

Countered by:

1. Exercise during microgravity

2. Negative pressure suits on re-entry to prevent blood pooling in head

3. Isotonic fluids to bulk out plasma volume and positive pressure for lower body

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Muscle effects

Loss of muscle mass which causes loss of muscle strength

Effects are similar to exercise detraining and enforced bed rest

Caused by absence of gravitational loading on bones and muscles

Protein synthesis is decreased by 15% and degradation is increased

Reduction of fibre size by 20-50% but not fibre number

Stress and sub-optimal nutrition may play role

Loss in muscle strength is greater than loss in muscle mass, suggesting other factors are also altered - fibre recruitment, muscle damage, efficiency

Countered by:

1. Exercise - 2hr day using elastic

2. Dietary supplements - amino acids

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Bone effects

Microgavity reduces bone density - similar way to muscle

Higher ambient CO2 may cause respiratory acidosis which is countered by phosphate and bicarbonate from bone, however also leads to loss of Ca

During first few days in space - urinary and faecal Ca increases 60-70% and is maintained in space

Less Ca reabsorption occurs - low light levels may result in lack of vitamin D

Elevated Ca excretion leads to increased risk of kidney stones

Loss of bone is about 1-2% per month - too much may prevent repair on earth (oesteoporosis)

Countered by:

1. Dietry supplements

2. Resistance exercise

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