High Altitude and Space Physiology Notes

High Altitude and Space Physiology

High Altitude

Any altitude above 8000 ft from mean sea level is called high altitude. People can ascend up to this level without any adverse effects.

Table of Contents

The different altitudes are given in Table.
At high altitudes, the barometric pressure is low.
However, the amount of oxygen available in the atmosphere is the same as it is at sea level.
Due to low barometric pressure, the partial pressure of gases, particularly oxygen proportionally decreases.
It leads to hypoxia.

Different attitudes:

High Atitude And Physiology Barometric Pressure

The carbon dioxide in high altitudes is very much negligible and it does not create any problem.

Barometric Pressure And Partial Pressure Of Oxygen At Different Altitudes

The barometric pressure decreases at different altitudes and, accordingly, the partial pressure of oxygen also decreases leading to various effects on the body.
Barometric pressure and partial pressure of oxygen at different altitudes and their common effects on the body are given in Table.

Changes In The Body At High Altitude

When a person is exposed to high altitude, particularly by rapid ascent, the various systems in the body cannot cope with the lowered oxygen tension and, the effects of hypoxia start.

Barometric pressure, partial pressure of oxygen, and common effects at different attitudes:

High Atitude And Physiology Different Altitude

Besides, hypoxia, some other factors are also responsible for the changes in the functions of the body at high altitudes.

Various factors affecting the physiological functions at high altitudes are:

Hypoxia
Expansion of gases
Fall in atmospheric temperature
Light rays.

Effects Of Expansion Of Gases On The Body:

The volume of gases increases when the barometric pressure is reduced. So at high altitudes, due to the decreased barometric pressure, the volume of all gases increases in atmospheric air as well as in the body.
At sea level with an atmospheric pressure of 760 mm Hg, if the volume of gas is one liter, at the height of 18,000 feet (where atmospheric pressure is 379 mm Hg), it becomes 2 liters.
And, it becomes 3 liters, at the height of 30,000 feet (where atmospheric pressure is 226 mm Hg).
The expansion of gases in the gastrointestinal tract causes painful distention of the stomach and intestine.
It is minimized by supporting the abdomen with a belt or by evacuation of the gases. The expansion of gases also destroys the alveoli.
During very rapid ascent from sea level to over 30,000 feet height, the gases evolve as bubbles, particularly the nitrogen, resulting in decompression sickness.

Effects Of Reduced Atmospheric Temperature:

The environmental temperature falls gradually at high altitudes.
The temperature decreases to about 0°C at the height of 10,000 feet.
It becomes -22°C at the height of 20,000 feet.
And, at the altitude of 40,000 feet, the temperature falls to – 44°C.
Injury due to cold or frostbite occurs if the body is not adequately protected by warm clothing.

Effects Of Light Rays:

Skin becomes susceptible to injury due to many harmful rays like ultraviolet rays of sunlight.
Moreover, the sunrays reflected by the snow might injure the retina of the eye if it is not protected with suitable tinted glasses.
The severity of all these effects depends upon the speed at which one ascends in high altitude. The effects are comparatively milder or moderate in slow ascent and severe in rapid ascent.

Read And Learn More: Medical Physiology Notes

Mountain Sickness

Mountain Sickness Definition:

Mountain sickness is a condition characterized by adverse effects of hypoxia at high altitudes.
It is commonly developed in persons going to high altitudes for the first time. It occurs within a day in these persons before they get acclimatized to the altitude.

Mountain Sickness Symptoms:

In mountain sickness, the symptoms occur mostly in the digestive system, cardiovascular system, respiratory system, and nervous system.
The symptoms of mountain sickness are:

Mountain Sickness Digestive System:

Loss of appetite, nausea, and vomiting occur because of the expansion of gases in the gastrointestinal tract.

Cardiovascular System: Heart rate increases.

Mountain Sickness Respiratory System:

Pulmonary blood pressure increases due to increased blood flow.
Blood flow increases because of hypoxia-induced vasodilatation.
Increased pulmonary blood pressure results in pulmonary edema which causes breathlessness.

Mountain Sickness Nervous System:

The symptoms of the nervous system are headache, depression, disorientation, irritability, lack of sleep, weakness, and fatigue.
These symptoms are developed because of cerebral edema.
Sudden exposure to hypoxia in high altitudes causes vasodilatation in the brain.
The autoregulation mechanism of cerebral blood flow fails to cope with hypoxia.
It leads to increased capillary pressure and leakage of fluid from capillaries into the brain tissues.

Treatment:

The symptoms of mountain sickness disappear by breathing oxygen.

Acclimatization

Acclimatization Definition:

Acclimatization refers to the adaptations or adjustments by the body at high altitudes.
While staying at high altitudes for several days to several weeks, a person slowly gets adapted or adjusted to the low oxygen tension so that, hypoxia causes lesser and lesser effects on the body.
It enables the person to ascend further.

Changes During Acclimatization:

The various changes during acclimatization help the body cope with the adverse effects of hypoxia at high altitudes.
Following changes occur in the body during acclimatization:

Changes in Blood:

During acclimatization, the RBC count increases, and pecked cell volume rises from the normal value of 45% to above 59%.
The hemoglobin content in the blood rises from 15-20 g%. So, the oxygen-carrying capacity of the fated is increased.
Thus, more oxygen can be carried to tissues in spite of hypoxia. The increase in packed cell volume and hemoglobin content is due to erythropoietin actions.
The increase in RBC count, packed cell volume, and hemoglobin content is due to erythropoietin that is released from the juxtaglomerular apparatus of the kidney.
Erythropoietin stimulates the red bone marrow. So, the RBC count increases. Thus, the oxygen-carrying capacity of blood is maintained by an increase in RBC count and hemoglobin content in the blood.

Changes in Cardiovascular System:

The overall activity of the cardiovascular system is increased at high altitudes. There is an increase in the rate and force of contraction of the heart and cardiac output.
Hypoxia-induced vasodilatation increases the vascularity in the body. So, blood flow to vital organs such as the heart, brain, muscles, etc. increases.

Respiratory System :

Pulmonary ventilation:

Pulmonary ventilation increases by up to 65%.

This is the immediate compensation for hypoxia in high altitudes and, this alone helps the person to ascend several thousand feet.
The increase in pulmonary ventilation is due to the stimulation of chemoreceptors.
The chemoreceptors give a response to changes in the chemical constituents of blood.
The chemoreceptors send impulses to the respiratory centers, which in turn increase the rate and force of respiration.
These receptors are situated at different places.
Peripheral chemo-receptors play an important role in increasing ventilation during hypoxia.

Pulmonary hypertension:

Due to increased cardiac output, the pulmonary blood flow also increases. It leads to pulmonary hypertension.
Pulmonary hypertension is very common even in persons acclimatized to high altitude.
In some of these persons, pulmonary hypertension is associated with right ventricular hypertrophy.

Diffusing capacity of gases:

Due to the increased pulmonary blood flow and increased ventilation, the diffusing capacity of the gases increases in the alveoli.
It enables more diffusion of oxygen in the blood.

Changes in Tissues:

Both in human beings and animals residing at high altitudes permanently, the cellular oxidative enzymes involved in metabolic reactions are more than in the inhabitants at sea level.
Even, when a sea level inhabitant stays at a high altitude for a certain period, the amount of oxidative enzymes is not increased.
So, the elevation in the number of oxidative enzymes occurs only in fully acclimatized persons.
An increase in the number of mitochondria is observed in these persons.

Aviation Physiology

Aviation physiology is the study of the physiological responses of the body in the aviation environment.
Flying exerts great effects on the body through accelerative forces and gravitational forces which are developed during flight maneuvering.
The pilots and crew members of aircraft are trained to overcome the effects of these forces.

Accelerative Force:

Acceleration means the change in velocity. Flying straight in a horizontal plane with constant velocity has minimum effects on the body.
However, changes in velocity produce severe physiological effects. Accelerative forces are developed in the flight during linear, radial, or centripetal and angular acceleration.

Gravitational Force:

The gravitational force (G force) is the major factor that develops accelerative force.
G force and the direction in which the body receives the force are responsible for physiological changes in the body during acceleration.
G is considered the force or “pull” of gravity upon the body. On the earth, this pull is responsible for body weight.
The force of gravity while sitting, standing, or lying position is considered to be equal to body weight and it is referred to as 1G. The G unit increases in acceleration.
If we say that the G unit increases to 5G during acceleration, it means that the force of gravity on the body at that moment is equal to five times the body weight.
While traveling in an airplane, elevator, or car, when there is a sudden change in speed or direction, passengers are thrown or centrifuged in the opposite direction, it is because of a change in the G unit.
G unit may increase or decrease, the increase in G unit is called positive G, and the decrease in G unit is called negative G.
Positive G occurs white Increasing the speed (acceleration). Negative G occurs while decreasing the speed (slowing down – deceleration).
G unit is altered during the change in directions also.
While Hying both positive G and negative G cause physiological changes in the body.

Effects Of Gravitational Forces On The Body:

Effects of Positive G:

The major effects of positive G during acceleration are on blood circulation.

When the G unit increases to about 4 to 5G, the blood is pushed toward the lower parts of the body including the abdomen.
So the cardiac output decreases resulting in reduced blood supply to the brain and eyes.
Decreased blood flow in turn decreases the oxygen supply (hypoxia) to the head and leads to the following disturbances:

Grayout:

Grayout is the graying of vision that occurs when blood flow to the eyes starts diminishing.
It occurs because the retina is more sensitive to hypoxia than the brain.
Though physical impairment does not occur, gray out is considered a warning for decreased blood flow to the head.

Blackout:

It is the complete loss of vision that occurs when retinal function is affected by hypoxia.
The consciousness and muscular activities are still retained.
But it indicates the risk of loss of consciousness.

Loss of consciousness:

When the force increases beyond 5G, hypoxia reaches the critical level causing loss of consciousness.
It may be associated with convulsions.
The unconscious state may last for about 15 seconds.
After recovery from unconsciousness, the person needs another 10-15 minutes for orientation.
If the affected person happens to be a lone pilot, then he will lose control of his aircraft.

Fracture of bones:

When the force increases to about 20G, the bones, particularly the spine, become susceptible to fracture even during sitting posture.

Effects of Negative G:

The negative G develops while flying downwards (inverted flying). It causes the following disturbances;

Hyperemia:

When the force decreases to – 4 to – 6 G, hyperemia (abnormal increase in blood flow) occurs in the head because the blood is pushed towards the head.
Sometimes the blood accumulates in the head resulting in brain edema.
There is congestion, flushing of the face, and mild headache.
Negative G at this level is tolerable and the effects are only momentary.
The brain also can withstand hyperemia in such conditions.

Red-out and headache:

Red-out is the blurring of vision and sudden reddening of the visual field caused by the engorgement of blood vessels in the head.
When the negative G reaches to about – 15-20, there is dilatation and congestion of blood vessels in the head and eyes resulting in red-out and headache.
The blood vessels in the brain may not be affected much because of CSF.
When blood accumulates in the brain, there is simultaneous pooling of CSF in the cranium.
And the high pressure exerted by CSF acts as a cushion (buffer) and protects the blood vessels of the brain.

Loss of consciousness:

But high negative G affects the body by other means.
It increases the pressure in the blood vessels of the chest and neck.
It causes bradycardia or irregular heartbeat which adds to the stagnation of blood in the head.
All these factors ultimately lead to unconsciousness.

Prevention Of Effects Of G Forces On The Body:

The body can be protected from the effects of G forces particularly the positive G by the following methods:

By Using Abdominal Belts:

The pooling of blood in the abdominal blood vessels is prevented by using the abdominal belt and leaning forward while sitting in the aircraft.
This procedure postpones gray out or blackout.

By Using Anti-G-suit:

The anti-G-suit exerts positive pressure on the lower limbs and abdomen and prevents the pooling of blood in the lower part of the body.

Space physiology

Space physiology is the study of physiological responses of the body in space and space crafts.
The major differences between the environments of Earth and space are atmosphere, radiation, and gravity.
These three factors are the characteristics of the space environment that challenge human survival.
The atmospheric factors include atmospheric pressure, temperature, humidity, and gas composition.
The spacecraft or the space lab is provided with a stable and sophisticated environmental control system that maintains all the atmospheric factors close to Earth’s environment.
The astronauts also wear launch and entry suits (LES). LES is a pressurized suit that protects the body from the space environment.
Another factor that affects the body in space is weightlessness. Weightlessness is because of the absence of gravity (microgravity).

Effects Of Travel By Spacecraft:

While traveling by spacecraft, the astronauts experience some intense symptoms only during blast-off due to acceleration and during landing because of deceleration.
Otherwise, the accelerative forces are the least while traveling in a spacecraft since the spacecraft cannot make rapid changes in speed or direction like an aircraft.
Most of the physiological changes occur due to weightlessness in space travel.
The bodily changes in the astronauts are responsible for the adaptation of their bodies to the space environment.
During the adaptation process, the body establishes the changes required for space conditions. Further problems develop only when they return to Earth.
And they require a long-time to readapt to Earth’s environment and establish the changes for normal conditions on Earth.
The effects of weightlessness in spacecraft are:

Effects on Cardiovascular Systems and Kidneys:

The cardiovascular changes are due to the fluid shift.

Due to the absence of gravity, the blood moves from the lower part to the upper part of the body (upper trunk and head).
It causes enlargement of the heart to cope with increased blood flow. In addition, there is an accumulation of other body fluids in the upper part.
Now the compensatory mechanism in the body interprets the increase in blood and other fluids as a serious threat and starts correcting it by excreting large amounts of fluid through the kidneys. It causes a decrease in blood volume.
And the heart need not pump blood against gravity in space. So, the initially enlarged heart starts shrinking slowly and becomes small.
So, during the initial fluid shift, the astronauts experience dizziness or a feeling of fainting.
Along with water, kidneys excrete electrolytes also. Because of this osmolarity of the body fluids is not altered.
So the thirst center is not stimulated because of this and the astronauts do not feel thirsty during space travel.

Effects on Blood:

The plasma volume decreases due to excretion of fluid through urine. RBC count also decreases and it is called space anemia.

Effects on Musculoskeletal System:

Because of microgravity in space, the muscles need not support the body against gravity.
The astronauts move by floating instead of using their legs.
This leads to a decrease in muscle mass and muscle strength.
The endurance of the muscles also decreases.
The bones become weak. Osteoclastic activity increases during space travel.
The calcium removed from bone is excreted through urine.

Effects on Immune System:

Space travel causes suppression of the immune system in the body.

Space Motion Sickness:

After obtaining weightlessness, some astronauts develop space motion sickness.

It is characterized by nausea, vomiting, headache, and malaise (a generalized feeling of discomfort or lack of well-being or illness that is associated with the sensation of exhaustion).
It persists for two or three days and then disappears.
It is thought that motion sickness occurs due to abnormal stimulation of the vestibular apparatus and fluid shift.

High Altitude and Space Physiology Notes