Microgravity

Gravitational law

• Every particle in matter in the universe attracts every other particle with a force directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance separating them

Equation for force of gravity

What is the force of gravity between the ISS and Earth

8.7 m/s²

• around 89% of that arising from the effect of gravity at the earth’s surface

What underlies apparent weightlessness? (is it weightlessness?)

1. ISS in orbital velocity

2. accelerate toward single point at Earth’s centre

3. but ISS moves enough tangentially that you are not falling

4. outward centrifugal force ‘balances’ the downward gravitational force

5. so orbit is a state of continuous free-fall

6. THEREFORE do not perceive the external force

7. so feel weightless

8. this is MICROGRAVITY (not weightless ness or zero-G)

Apparent weight equation

• feeling we experience when a lift cable snaps and we accelerate to the ground

  • no apparent weight because individual and floor of the lift accelerate at same rate

  • so no opposing force generated by the floor

Can gravitational force ever reach 0?

• no

• there is always matter so there is always an attractive force

What gravity is zero-gravity

1 millionth of that on Earth

When is true weightlessness experienced + example

If the gravitation influences of cosmic bodies are balanced:

Example:

• Positition between the Earth and moon where grav pull of each is in equilibrium

  • Lagrangian point

Strategies used by NASA and ESA to stimulate microgravity and its physiological consequences

1. Head down tilt bedrest (HDBR)

   • bed tilted 6 degree for 30 days to a year

   • replicate headward fluid shift and disuse of load-bearing tissues

   • combined with other stressors (0.5% CO2) to mimic ISS

2. Hindlimb suspension in rodents

   • stimulate unloading of muscle and bone

3. Water immersion

   • neutral buoyancy facility

   • train for extra-vehicular activity (space walks)

4. Parabolic flight

   • vomit comet

   • 22 seconds of weightlessness

   • test equipment

5. Concordia station antarctica

   • extreme isolation

   • for psychological testing

Atomospheric conditions on ISS

Similar to on Earth

• 101.3kPa barometric pressure

• 21% O2

How is O2 made

• by electrolysis

• using solar power

How is water made?

• recycled

How is water recovered and CH4 made

1. CO2 scrub from atmosphere

2. reacted with H2

3. recover water and mades CH4

   1. CH4 vented overboard

What do astroanuts breathe when on space walk and advantage

• 100% O2 at 1/3 of atm

• allows to extend their time outside the spacecraft

• reduced pressure prevents O2 toxicity

Disadvantage of the reduced pressure

Decompression sickness

Henry’s law

• amount of any given gas that is dissolved in a liquid at a given temperature is a function of the partial pressure of that gas in contact with the liquid

Why get decompression sickness in EMU and not ISS

ISS

• dissolved gases in blood and tissues are proportional to the partial pressures of those gases in the lungs at 1 atm

• So N2 is in equilibirium→ sets the amount of gas dissolved in the tissues and plasm

EMU

• marked N2 gradient between tissues, plasma and lungs

• THEREFORE: nitrogen rapidly come out of solution in response to reduction in its partial pressure

Where is N2 normally accumulated

• body tissues (fat) where N2 solubility is higher

Why does equilibrium between tissues and environment slow

• blood supply to adipose is poor

• blood carries very little N2

What happens when decompression is rapid

1. N2 bubbles form in the blood

2. Small→ removed by circulation in the lungs

3. Large→ painful and lethal

   • N2 embolism in the joints ‘bends’

4. bubbles result in neurological disorders

5. result in death if they occlude coronary vessels/ cerebral vessels

Solution to this issues in astronauts?

1. denitrogenate bodies prior to donning EMU

   • minimum 12 hour staged decompression of spacecraft

2. 100 mints of preoxygenation (breathing 100% O2) at lower pressure to replace N2 with O2

What four major stresses happen when experience weighltessness

1. altered hydrostatic pressure gradients

2. unloading/disuse of load-bearing tissues

3. altered vestibular function

4. altered sensory and balance information

1. Altered hydrostatic pressure gradients: What happens when arrive in microgravity First few minutes

1. Within minuts: 2 litres of body fluid redistributed towards the head and chest

2. puffy face

3. engorged neck and facial veins

4. feeling of bulging eyes

5. fluid accumulation in sinuses (lose sense of taste and smell)

6. leg volume decreases by 10% → bird legs

In first 48 hours

1. 17% loss of plasma volume

2. decreased transmural pressure and lower muscle tone

   • → short-term shift in fluid from plasma to interstitial/intracellular compartments

3. Headward fluid shift→ distends baroreceptors of central vasculature (henry-Gauer reflex)

   1. Suppresses RAAS and increases ANP and suppress ADH:

      1. increase diuresis

      2. decreased thrist

4. fluid loss→ increases haematocrit

5. erythopoeiss is suppressed to stabilise equilibirium

6. BUT many returning show depressed haematocrits space anaemia

What is the most recent study explanation of space anaemia

1. Actually no sustained suppression of EPO

2. increase haemolysis (blood ferritin, transferrin and free iron and CO clearance) elevated in spaceflight

   1. indicative of a marked and sustained destruction of RBC

What cardiovascular changes happen

Reduction in blood volume which causes:

1. reduces work output of the heart

2. overall reduction in heart size (cardiac atrophy) due to reduced left ventricular volume and left ventriucalr end-diastolic volume

3. increase HR post-spaceflight

4. MAP reduced

Orthostatic intolerance post-spaceflight cauused by what

Postrual changes that require regulation of arterial blood pressure to maintain cerebral blood flow

1. reduction in arterial baroreflex

2. loss of plasma volume

Orthostatic intolerance post-spaceflight what it causes

can cause

1. causea

2. vomiting

3. light-headedness (presyncope)

4. fainting (syncope)

How to counteract the reduced plasma volume loss

1. lower body negative pressure

2. isotonic water loading immediately before return to 1G

What is Spaceflight Associated neuro-ocular syndrome (SANS)

• collection of morphological and functional changes to the eye

• 1/3 of astronauts experience

• leads to longer-term impairmnets when back on earth

  • some permanent loss of vision

Characteristics of SANS

• optic disc oedema

• globe flatterning

• increased choroidal folding in retina

What is the cause of SANS

1. hydrostatic gradient which causes

2. elevated intracranial pressure

In what way is the lung sensitive to gravity (1G)

influences the control of

• pulmonary perfusion

• alveolar ventilation

1. Gravity upon blood flow

2. gravitationl influences on lung compliance leading to preferential ventilation of the alveoli at the bas of the lungs

Wherefore what happens in a microgravity environemnt

• removes regional variations in blood flow

• BUT not eniterly uniform

Wht does this suggest

• there are also non-gravitational variables controlling regional differences in these parameters in lungs

Changes in lung volumes in response to microgravity

1. Functional Residual Capacity FRC)→ decreased by 15%

   • why: cranial shift of diaphragm and abdominal contents

   • outward movement of rib cage as weight of abdomen removed

   • upward movement f shoulder girdle follwoing exposure to microgravity

2. Residual volume→ decreased by 18%

   • Why: large apicobasal gradient in regional lung volume is abolished in microgravity

What happens to the hypoxic ventilatory response (HVR) during spaceflight?

Suppressed during spaceflight

Why does this happen?

Explained by HVR being different in standing and supine positions

1. increase in carotid level pressure of 10mmHg

2. reduces ventilatory response to breathing 10% O2 by 33%

3. blood pressure changes when move from standing to supine because of abolition of hydrostatic differences

4. so pressures in carotid region in supine subject are 15-20 mmHg higher

5. pooling blood in lower extremities

6. predicted to elevate pressure in carotid region

7. explains depressed response to isocapnic hypoxia

2. Reduced loading and disuse of weight bearing tissues: What happens when you remove the load from the muscles and bones

1% per month loss of weight bearing bone mass

though non-load bearing bones (arms) relatively protected

1. Accompanied by signficiant calcium balance

   • altered Caclium homeostasis with excess plasma calcium excreted in the urine and faeces

     • more excretion

     • can cause more kidney stones

2. reduction in formation of new bone

   • decline in osteroblast activity

   • incrased osteroclasts

What have bed rest studies shown

• accelerated bone resorption

• inhbition of bone formation

→ causes of loss of bone

Who is susepatble to bone remodelling?

• depends on the astronaut

• alterations in blood flow brought about by loss of hydrostatic pressures may play a crucial role?

How to counteract these changes: exercise?

• does not really work

What happens to stature?

Increases

• why: removal of continual compressive force

• expands vertebral column and significant increase in height

• increases in first 2 weeks and stabilises after

• noramlises again after return to Earth

Counteract loss of load solution

1. Short arm centrigfugation

• effective during bed rest experiment

• but massive equipment to take up space

2. Nutritional (vit D and Ca2+ supplements)

Bone remodelling also causes

• decreased muslce mass

• decreased strength

• decreased metabolsim

How does muscle mass decline

Muscle fibre corss sectional area declines

• Type I and type II fibres undergo decline in mean corss sectional area

• muscle atrophy

• EVIDENCE: needle biopsies taken before and after microgravity exposure

  • confirm atrophy

  • confirm mitochondrial morphology changes

• As cross sectional area is prop to force: have loss of strength

Metabolic changes: mitochondrial protein changes

• differentially expressed between samples from scott Kelly and his twin brother who did not go to space

• but both trained the same

• evidece of mitochondrial stress

Importance of HIT exercise

max oxygen consumption becomes suppressed unless moderarelty high intenesity exericse

• needed to counteract muscle disuse

• BUT important implications since aerobic exercie in microgravity taxes the already tight space allocations of current orbiting space craft

• plus equipment is massive to have in the ISS