Aviation and Space Physiology (1)

by Patience Joseph-Mamman

The field or discipline called Physiology is a life science that deals basically with study of functions in living things, whether plants or animals. Physiology in a general sense is simply a branch of life science that focuses majorly on understanding how a living entity functions.

Space, High altitude and Aviation physiology is a field of physiology that deals with how the body works or adjusts when exposed to high altitudes. One key thing to note is as you ascend high up in the sky, the atmospheric pressure drops. At about 16,000 feet above sea level, the oxygen saturation in the blood drops to about half its original value at sea level.

The reason behind this is phenomenon is explained thus: The percentage of oxygen at sea level and at high altitudes is the same, but what causes the drop is the reduction in the atmospheric pressure. At high altitudes, the air molecules are more dispersed than compressed as we see at sea levels. This dispersed nature of air molecules makes it difficult to get in the amount of oxygen adequate enough for respiration to take place successfully.

To cope with this, there are certain adjustments the body makes:

  1. At high altitudes, breathing rate (hyperventilation) increases to get in more oxygen into the lungs.
  2. Increased heart beat to help push out more blood to help circulate oxygen to the tissues to avoid tissue death.
  3. After an individual has begun to acclimatize there is increased production of red blood cells.

Acute Effects of Hypoxia

Hypoxia refers to a condition of low oxygen delivery to tissues. It relates to space and aviation physiology in the sense that as you go higher in altitude, there is a drop in the atmospheric pressure. This in turn makes less oxygen available for tissue functions which are what precipitates the bodily malfunctions experienced at high altitudes.

When a person who has never risen to a high altitude whether by flying or climbing is suddenly exposed to such the body reacts in a certain way due to the sudden exposure to low oxygen levels and this in turn produces the following effects;

  • Beginning at a height of about 12000 feet drowsiness, lassitude, mental fatigue, headache and nausea begins to occur.
  • At about 18, 000 feet above sea level the person begins to experience minor seizures
  • At above 23, 000 feet above sea level in the unacclimatized person, coma results and death if nothing is done to return back to lower altitudes.

This is the gradual adjustment the body makes to cope with the low oxygen partial pressure common at such heights. Such adjustments include:

  • Increased pulmonary ventilation: On exposure to low partial pressure of oxygen, some receptors known as chemoreceptors are activated to cause increased ventilation of the alveoli through increased breathing rate. This mechanism helps increase the rate of ventilation to about 5 times the normal rate at the end of 2-5days of being at high altitudes.
  • Increased red blood cells production and hemoglobin concentration: Low partial pressure of oxygen has been spotted as one of the key stimulants in the production of red blood cells. As the individual acclimatizes, his hemoglobin level rises from a normal value of about 15g/dl to about 20g/dl. Increased blood volume by about 20-30% results in a total increase of about 50% in hemoglobin level.
  • Increased oxygen diffusing capacity: The normal diffusing capacity for oxygen across the pulmonary membrane is about 21ml/mmHg/min. This capacity can increase about 3 fold when the body is exposed to high altitudes. The increased blood volume to the capillaries causes an increase in surface area where gas exchange can take place.
  • Increased Capillarity: Another adaptive feature that the body develops as it tries to cope with the change in altitude is the increase in number of capillaries in all body tissues. This is so that more oxygen can be delivered to the cells.
  • Cellular Acclimatization: At high altitudes, the body also witnesses an increased production of mitochondria and oxidative enzymes in the cells. This is to help maximum utilization of the oxygen delivered to the cells for optimum functioning.  

 Disorders associated with High Altitudes

  1. Acute Mountain Sickness: This is the common manifestation and disorders that take place when the body is exposed to high altitudes without acclimatization. There are 3 forms of this: mild, moderate and severe
    1. Mild
      Occurs at ascension of about 10, 000 feet or more.
      Symptoms include headache, nausea, weakness, decreased appetite.
      Onset is between 12-14 hours of reaching high altitudes.
      Symptoms disappear within 3days.

The best cure is rest and allowing the body to acclimatize or descend to a lower altitude.

  • Moderate: Here in addition to the symptoms in mild mountain sickness, there is ataxia (loss of muscle coordination) leading to the ‘drunk man walk’ sign.
    • If this occurs it is advisable to descend to lower altitudes.
    • In 1-3 days the symptoms tend to resolve.
  • Severe: There is severe ataxia.
    • Inability to think clearly.
    • Fluid builds up in the lungs.
    • In such conditions the victim is immediately transported to very low altitudes preferably below 4000 ft.

High Altitude Pulmonary Edema.
This is caused by excessive fluid build up in the lungs. For some the lack of oxygen at high altitudes causes vasoconstriction which elevates the blood pressure and leakage of fluids into the tissues and gas symptoms include persistent cough, feeling of tightness in the chest, discomfort when trying to lie flat on the ground, gurgling sound when breathing. Remedy is immediate descent to seek medical attention.

High Altitude Cerebral Edema.
This occurs when the effect of high altitude and low air pressure causes fluid to leak out of the capillaries that surround the brain. This fluid then collects within the brain substances. Symptoms of this include inability to think clearly, and severe ataxia(uncoordinated movements). This situation is always treated as an emergency by immediately taking the victim to a low altitude to commence immediate treatments.

Some Preventive measures to minimize the onset of Acute Mountain Sickness.

  1. If you begin to have symptoms do not proceed. Remain where you are or descend.
  2. Drink plenty of water as dehydration is common.
  3. Pain relief drugs and diuretics can also help ease some symptoms.

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