Aviation, High Altitude &

Space Physiology Chapter

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Aviation, High Altitude & Space Physiology

As human beings ascend to higher altitudes

during mountain climbing ,aviationand space crafts it is important to understand the a) Effects of low PO2 at high altitudesb) Effects of acceleratory forces during

aviationc) Effects of weightlessness in space

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Effects of high altitude on barometeric pressure and atmospheric PO2

Barometeric pressure mmHg

Atmospheric pressure mmHg

At sea level 760 159

At 10,000 feet 523 110

At 50,000 feet 87 18

Alveolar po2 at different levels

EFFECT OF CO2 AND WATER VAPORS ON ALVEOLAP PO2

even at high altitude CO2 is continuously expelled out and water vapor added to inspired air from respiratory sufaces.PO2 decreases due to dilution. Water vapor in alveoli is 47 mmHg as long as body temperature is kept constant

At sea level PO2 is 40 mmHg which is reduced to < 40In the acclimatized person it may decrease to 7 mmHg because of 5 fold increase in pulmonary ventilation

At top of mounteverest29,028 feet

At sea level barometric pressure is 760mmHgAt top of mounteverest 253 mmHgWater vapor pressure -47 mmHgPCO2 in acclimatized person 199mmHgIt is 1/5th of oxygen and 4/5th of N2

At alveolar PO2 35 mmHg only best acclimatized person can survive when breathing airAt sea level of alveolar PO2 is 104mmHgAt 20,000 feet it decreases to 40mmHg in un acclimatized and 53 mmHg in acclimatized person

Effect of breathing pure o2 and alveolar po2 at different altitudes

In unpressurized air plane when aviator breaths pure O2 at 39,000 feet alveolar PO2 is 139 mmHg instead of 18 mmHg when breathing pure air

Saturation of hemoglobin with O2 at different altitudes

When breathing air arterial blood Hb saturation is 90% at 10,000 feet and < 70% at 20.000 feetWhen breathing pure O2 Hb saturation with O2 is 90% at 39,000 feet and 50% at 47,000 feet

“The Ceiling” When Breathing Air & When Breathing Oxygen in an Unpressurized Air Plane

When breathing pure oxygen aviator has 50% saturation at 47,000 feet.When breathing air aviator as 50% oxygen saturation at 23,000 feet.Importance:

Unacclimatized person can remain concious until the arterial PO2 saturation falls < 50%

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Acute Effects of Hypoxia in an Unacclimatized Person When Breathing

Air Effects of hypoxia begin at 12,000 feet which include;

1. Drowsiness2. Lassitude3. Mental & muscle fatigue 4. Some times headache 5. Occasionally nausea & 6. Some times euphoria

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Cont:

7.More then 18,000 feet – twitchings or seizures.

8. More then 23,000 feet- end in coma following shortly by death .

9. One of the most important effects of hypoxia is decreased mental proficiency which decreases judgment, memory & performance of discrete motor movements e.g

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Cont:

if the Unacclimatized person stays at the height of 15,000 feet for 1 hour, his mental proficiency is decreased by 50% of normal & after 18 hours it is decreased to about 20% of normal.

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Acclimatization or adaptation to Low PO2 at high altitude

When a person stays at high altitude for days, weeks, or years becomes more & more acclimatized to low PO2, so it causes fewer deleterious effects on the body. And it becomes possible for the person to work harder without hypoxic effects or to ascend to still higher altitudes.

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Acclimatization occurs by following means:

By a great increase in pulmonary ventilation.Increase in number of RBCs & Hb concentration, increase in blood volume to 20%Increased diffusing capacity of the lungs.Increased vascularity of the peripheral tissues.Increased ability of the tissue cells to use oxygen despite low PO2. 16

Increased pulmonary ventilation:

Immediate exposure to low PO2 stimulates peripheral chemoreceptor's which increases alveolar ventilation to a maximum of 1.65 times normal. Then, if person remains at very high altitude for several days, the chemoreceptor’s increase pulmonary ventilation to about 5 times normal.

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Cont:

The immediate increase in pulmonary ventilation blows off large quantities of carbon dioxide which decreases PCO2 & increases pH of the body fluids causing respiratory alkalosis. These changes inhibit brain stem respiratory center & there by oppose the effect of low PO2 on peripheral chemoreceptor’s. But during next 2-5 days the inhibitory effect fades away. And increases the ventilation by 5 times the normal. 18

Cont:

The cause of this fading of inhibition is believed to be mainly a reduction of HCO3

- concentration in the CSF & brain tissues. This in turn decreases the pH in the fluids surrounding the chemosensitive neurons of the respiratory center, thus increasing stimulation of respiratory center. The HCO3

- ions are decreased by renal compensation to respiratory alkalosis.

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2.Increase in RBCs count & Hb concentration:

When a person remains exposed to low oxygen for weeks time then hematocrit is increased from 45-60% & Hb concentration is increased from 15g/dl to about 20g/dl. In addition, the blood volume is also increased by 20-30%.

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3. Increase of diffusing capacity of lungs after

acclimatization

The normal diffusing capacity for oxygen through pulmonary membrane is about 21ml/mmHg/min & this is increased as much as 3 fold

1. during exercise. 2. at a high altitude.

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Cont:Partly it results from

• increased pulmonary capillary surface area along with increased pulmonary capillary blood volume.

• increase in alveolar surface area and lung air volume.

• increase in pulmonary arterial blood pressure through the increase no of capillaries– especially the upper parts of the lungs which are poorly perfused under usual conditions.

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4.Increased vascularity of the peripheral tissues:

Immediately cardiac output is increased by 30% & then over a period of weeks cardiac output decreases but hematocrit increases. Also new capillaries are developed in non-pulmonary tissues (angiogenesis).

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Increased ability of the tissues to use oxygen

despite low PO2:

This is achieved by increase in cell mitochondria & cellular oxidative enzyme systems in natives and acclimatized low landers. Natural acclimatization of native human beings living at high altitudes is better than low lenders who stay for many days weeks or years at high altitude. as shown in the following table.

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Cont:

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Acute mountain sickness at high altitude

A small number of people who ascend rapidly to high altitudes become acutely sick and can die if not given O2 or removed to low altitudeIt is characterized by acute cerebral edema and acute pulmonary edema

acute cerebral edema

Occurs as a result of cerebral vasodilatation due to hypoxiaDilatation of arterioles …..increased capillary pressure and permeabilityCharacterized by disorientation and other features of cerebral dysfunction

Acute pulmonary edema

Cause is still unknownSevere hypoxia as a result of pulmonary vasoconstriction also there are areas of increased pulmonary capillary permeability

PreventionSlow or stepwise ascentTaking complete rest on reaching desired heightMedicine…… acetazolamide

Chronic mountain sickness

Occasionally a person stays too long at altitude and develops chronic mountain sickness (over and excessive adaptation)Characterized by

Red cell mass and hematocrit becomes exceptionally high

Increased viscosity of blood …..decrease blood flow to tissues….Hypoxia

Cyanosis occurs

Pulmonary arterial pressure is more even more than that occurs

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Protection of the Body Against Acceleratory Forces

Specific procedures and apparatuses have been developed.If the aviator tightens his or her abdominal muscles & leans forwards to compress abdomen, so that some pooling of blood in large abdomen vessels can be prevented.Special “anti G suits” have be devised. Limit of safety < -10G.

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Effects Linear Acceleratory Forces on the Body (in space travel)

Blast off acceleration & landing deceleration:9G first stage booster.8G 2nd stage booster.Human body can not with stand this pressure in standing position, but in semirecling position. That is transverse to the axis of acceleration. That is why astronauts use recling seats.

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Deceleration

Problems also occur during deceleration when the space craft-enters the atmosphere.A person traveling at march-I (speed of sound) can be safely decelerated in a distance of about 0.12 miles.

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Cont:

A person traveling at march 100 would require a distance of 10.000miles for safe deceleration. Deceleration should be accomplished much slowly for high velocities than for low velocities.

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Cont:

Deceleratory forces associated with parachute jumps:When the parachuter jumps from the airplane velocity of deceleration is 0feet/sec. after 1second it is 32feet/sec, after 2second it is 64feet/sec & after 12seconds it is 175feet/second.Resistance of air and use of parachute decrease the velocity to 20feet/sec.

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Cont:

Deceleratory forces will cause fracture of pelvis, vertebrae or extended legs.

Trained parachuter hits the ground with knees bent but muscles tightened.

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Artificial Climate (in the space craft)

Because there is no atmosphere in outer space, an artificial atmosphere & climate must be produced in a space craft. It is important that oxygen concentration must be kept high & CO2 Kept low to prevent suffocation. Previously pure oxygen was used but now a days mixture of gases. This prevents likelihood of fire & explosion & development of patches of atelectasion in the lungs. 41

Cont:

For space travel lasting for several months:Recycling techniques are used.Physical process H2O → O2 Use of algae – biological process chlorophil present in algae under goes p photosynthesis to release O2

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Cont:

Weightlessness in space:A person in an orbiting satellite or nonpropelled space craft experiences weightlessness or a state of 0G force which is sometime called microgravity. The person is not drawn toward the bottom, sides or top of the space craft.

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Cont:

Physiological problems:Weightlessness→

1. Motion sicknesses during first few days of travel in the form of nausea & vomiting.

2. Translocation of fluids with body.3. Diminished physical activity.

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Cont:Effects of prolonged stay:

1. Decrease in blood volume.2. Decrease in red cell mass.3. Decrease in muscle strength & work capacity.4. Decrease in maximum CO.5. Loss of calcium & phosphate from the bones as well as loss of bone

mass. (at the rate of 1%/month)6. Impaired baroreceptor reflexes

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