W5 - Microgravity & G-Forces Flashcards by Jessica Snelling

What is effect of Earth’s gravitational field on blood in body?

Pooling of blood in legs
Decreases venous return
Decreases cardiac output

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What reflex ensures MAP is re-established when going from lying to standing position?

Baroreceptor reflex

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What is Newton’s Law of Motion?

Any movement causes a force to be exerted upon the body

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What forces act on pilots in aerial displays/combats?

Transient forces up to 8G, can last several seconds

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What forces act on people in fairground rides?

Rarely more than 2-3G but can be experienced for minutes and act in all directions

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When is microgravity or weightlessness experienced?

When body is falling freely

Occurs in manned satellites

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What is formula for pressure of column of fluid?

P = h x p x C

h: distance measured in direction of acceleration force
p: density of fluid
C: acceleration force acting upon column

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What is relationship of pressure to acceleration?

Pressure gradient varies in size and direction in accordance with acceleration forces acting upon body

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What is relationship of pressure to body orientation and acceleration?

Pressure gradient varies with orientation of body with respect to direction of acceleration

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How do you study increased G-forces?

Subject sits in huge centrifuge

Force exerted determined from radius of turning circle and speed of rotation

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Why is brain function less susceptible to effects of acceleration than vision?

Perfusion of brain vascular bed is better preserved than that to eyes

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Describe role of skull and CSF in preserving brain function during acceleration.

Brain surrounded by indistensible skull, centre filled with CSF

Hydrostatic pressure of CSF changes equal to that in BV so little change in BV diameter

Eyes not protected by this mechanism

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Describe role of arteriovenous pressure difference in preserving brain function during acceleration.

Arteriovenous pressure difference is preserved as venous pressure falls as much as arterial pressure.

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What happens to blood flow when forces > 5G?

Blood flow drops dramatically

Pressure produced by heart has been largely dissipated by time base of skull is reached so vessels collapsed

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Describe role of hyperaemia in acceleration.

If brain perfusion does decrease, autoregulation of vascular tone occurs = hyperaemia

Blood flow to brain is maintained although deterioration in mental performance can occur

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What is the major stimuli to baroreceptor reflex activity?

Blood pressure changes monitored by carotid sinuses

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What symptoms constitute a grey out?

Peripheral vision is impaired, perception of colour and detail by fovea deteriorates

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When does a grey out occur?

If subject is suddenly accelerated in headwards direction with a force of 4G

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What happens to CNS when 4.5 - 5G experienced?

4.5-5G : vision is lost, still conscious = blackout

> 5G: unconsciousness

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What causes changes to CNS at 4.5-5G?

Inadequate blood flow to eyes, brain

Fall in intraluminal pressure of BV of eyes

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Why is peripheral vision impaired in a grey out?

Arteries in eye radiate outwards from optic nerve so vessels supplying retinal periphery are longer

Length is associated with resistance, so intraluminal pressures are lower so peripheral vessels are more susceptible to collapse

Pressure across peripheral vessel wall falls below critical closing pressure = retinal blood supply to fail at periphery

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What is the immediate consequence of headwards acceleration?

Decrease in cardiac output

Acceleration causes internal organs of body to move away from head which reduces intrathoracic pressure

Increased pooling of blood in dependent parts of body = decreases venous return

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How does acceleration affect breathing movements?

Headwards acceleration, movement of internal organs facilitates inspiration by pulling down diaphragm but hinders expiration

Acceleration away from head produces opposite effect, inspiration becomes difficult

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How does acceleration affect respiratory gas exchange?

Acceleration in headwards direction accentuates gradients of blood perfusion and alveolar ventilation.
Apex, base function poorly wrt gas exchange.
Alveolar deadspace at apex & venous shunts at base cause hypoxaemia and allow ischaemia to develop

Acceleration in opposite direction, reverses gradients

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Describe apical alveoli during headward acceleration.

Apical alveoli will be almost fully distended throughout respiratory cycle but receive very little ventilation, zero perfusion so ventilation/perfusion ratio is abnormally high

Describe basal parts of lung during headward acceleration

Receive large amounts of blood (possibility of pulmonary oedema) but ventilation is not proportional so lowered ventilation/perfusion ratio

What causes the major problems with acceleration?

Redistribution of blood flow in response to pressure changes

What body position gives greater tolerance to G-forces?

When in supine position as thickness of body is less than height

What methods prevent the fall in CO during acceleration but only last a few seconds?

Transient Valsalva manoeuvres Shouting Positive pressure breathing

What are Valsalva manoeuvres?

Forced expiration against closed glottis

What artificial aid was developed to combat fall in venous return?

Antigravity suit

How does antigravity suit work?

Based on principle that immersing person in water counters gravitational effect by producing equal hydrostatic gradient outside body Prevent venous pooling but also limit the descent of diaphragm Keeps heart nearer to head so decreasing amount of pressure required to drive blood up to brain

What countermeasures can be taken to reduce visual disturbances?

Goggles than enable extra-ocular pressure and pressure surrounding retinal vessels to be changed Decreases in extra-ocular pressure act to promote retinal blood flow and combat high G forces in headwards direction

How do you mimic effects of weightlessness?

Constant bed rest and water immersion techniques Volunteers tilted at 6 degrees to horizontal

Why 6 degrees to horizontal when mimicking weightlessness?

Position carotid sinuses to be level with aortic valve Reduces pressure difference = 0 Mimics effect of microgravity more accurately

How much fluid is redistributed during weightlessness?

2L of body fluids away from dependent parts of body and towards head, neck, thorax

What is physical symptoms of weightlessness in the 3 days?

Head, neck feel bloated Neck veins engorged Legs become thinner

What causes the weightlessness physical symptoms in the 3 days?

Diuresis takes place iniated by incr in pressure in volume receptors in atria, great veins Decr in ADH, incr ANP Diuresis corrects excess fluid in thoracic veins and so reduces increased CO and load on heart due to excess venous return

What is effect on RBC/WBC count from weightlessness?

Drop in RBC, WBC counts Production falls by more than decrease in plasma volume Bone marrow activity almost ceases

What is initial CVS change from weightlessness?

Increase in cardiac output due to lack of venous pooling

What is impact of correction on CVS during weightlessness?

After correction, CO and HR fall | Adjustment to CVS to microgravity environment

What causes adjustment of CVS to microgravity environment?

Decreased sympathetic tone Increased parasympathetic tone Reflects the amount of work that must be performed by heart has decreased

What happens to CVS after longer periods of weightlessness (>50d)?

Degree of ventricular atrophy occurs

What symptoms do astronauts experience the first few days in space?

Giddiness, disorientation, nausea, vomiting

What causes astronaut symptoms in first few days in space?

Imbalance between different neural inputs to the brain Skin, muscle, joint receptors (postural forces) and otolith receptors change considerably

What input reflects angular acceleration?

Input from semicircular canals

Which inputs relfect the postural forces acting on the body?

Inputs from skin, muscle, joint receptors

Which input signals the orientation of the head in a gravitational field?

Otolith receptors

What are the inputs that help with spatial orientation and balance?

Angular acceleration: semicircular canals Linear acceleration and gravity: otolith organs Proprioceptive system: limb positions Tactile system: pressure on body

What happens to muscles in astronauts in space?

Movement is limited by lack of cabin space Less need for tonic activity in antigravity extensor muscle Muscular atrophy occurs

What in the urine indicates muscular atrophy is occuring?

Increased excretion of urea in urine

What reflexes are exaggerated in continued weightlessness?

Postural reflexes

What happens to bones when body is in microgravity?

Demineralisation of bones occurs at unchanged rate throughout period

How do you measure loss of bone material?

Measured directly from densometric studies of individual bones Indirectly from rates of urinary excretion of hydroxyproline & calcium May increase incidence of renal calculi

Which bones are most affected by demineralisation in microgravity?

The bones that normally bear weight of body

What is rate of mineral loss in bones in microgravity?

0.5% of total per month Danger of fractures when return to normal 1G environment 2 y after being in space

What is the main issue of humans adjusting to microgravity environment?

Being able to withstand gravitiational field on return to earth

What actions do astronauts do after return to earth?

Drink copiusly Kidneys conserve salt Number of circulating reticulocytes increases as anaemia is corrected

How many months after return to Earth does it take astronauts to recover?

5 months Osteoporosis is never fully corrected

What countermeasures can be used to minimise side effects of microgravity?

Suction device = blood pooling in legs Constant load suits Bicycle ergometer/springs/isometric contractions Exercise apparatus adapted so muscles pull against each other

What countermeasures used just before astronaut return reduce CV deconditioning ?

Wear antigravity suits | Have body fluid volume deliberately expanded by drinking saline

What is issue with countermeasures of antigravity?

Effect is not continuous Gravity exerts effect continuously Duplicate this by creation of artifical gravity by imparting spin to spacecraft