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Aerospace physiology

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Aerospace Physiology is the study of the effects of high altitudes on the body, such as different pressures and levels of oxygen. At different altitudes the body may react in different ways, provoking more cardiac output, and producing more erythrocytes. These changes cause more energy waste in the body, causing muscle fatigue, but this varies depending on the level of the altitude.

Effects of altitude

The physics that affect the body in the sky or in space are different from the ground. For example, barometric pressure is different at different heights. At sea level barometric pressure is 760 mmHg; at 3.048 mts above sea level, barometric pressure is 523 mmHg, and at 15.240 mts, the barometric pressure is 87 mmHg. As the barometric pressure decreases, atmospheric partial pressure decreases also. This pressure is always below 20% of the total barometric pressure. At sea level, alveolar partial pressure of oxygen is 104 mmHg, reaching 6000 meters above the sea level. This pressure will decrease up to 40 mmHg in a non-acclimated person, but in an acclimated person, it will decrease as much as 52 mmHg. This is because alveolar ventilation will increase more in the acclimated person. Aviation physiology can also include the effect in humans and animals exposed for long periods of time inside pressurized cabins

The other main issue with altitude is hypoxia, caused by both the lack of barometric pressure and the decrease in oxygen as the body rises. With exposure at higher altitudes, alveolar carbon dioxide partial pressure (PCO2) decreases from 40 mmHg (sea level) to lower levels. With a person acclimated to sea level, ventilation increases about five times and the carbon dioxide partial pressure decreases up to 6 mmHg. In an altitude of 3040 meters, arterial saturation of oxygen elevates to 90%, but over this altitude arterial saturation of oxygen decreases rapidly as much as 70% (6000 mts), and decreases more at higher altitudes.

Adaptation to low oxygen environments

Hypoxia is the principal stimulus that increases the number of erythrocytes, increasing the hematocrit from 40 up to 60%, with an increase of the hemoglobin concentration in blood from 15 g/dl up to 20-21 g/dl. Also the blood volume increases 20% producing an increase of the corporal hemoglobin up 15% or more. A person that stays for a period of time at higher altitudes acclimates, producing fewer effects over the human body. There are several mechanisms that help with acclimation, which are an increase of pulmonary ventilation, higher erythrocytes levels, increase of the pulmonary diffusion capacity and increase of the vascularization of the peripheral tissues.

Arterial chemical receptors are stimulated by exposure to a low partial pressure and hence increase alveolar ventilation, up to a maximum of 1.65 times. Almost immediately, compensation for the higher altitude begins with an increase of pulmonary ventilation eliminating a large amount CO2. Carbon dioxide partial pressure reduces and corporal fluids pH increase. These actions inhibit the respiratory center of the encephalic trunk, but later this inhibition disappears and the respiratory center responds to the stimulation of the peripheral chemical receptors because of the hypoxia increasing ventilation up to six times.

Cardiac output increases up to 30% after a person rises to a high altitude, but it will decrease back to normal levels, depending on the increase of the hematocrit. The quantity of oxygen that goes to the peripheral tissues its relatively normal. Also a disease called “angiogenia” appears.

The kidneys respond to low carbon dioxide partial pressure by decreasing the secretion of hydrogen ions, and increasing the excretion of bicarbonate. This respiratory alkalosis reduces the concentration of HCO3 and return plasma pH to normal levels. The respiratory center responds to the stimulation of the peripheral chemical receptros produced by the hypoxia after the kidneys have recover the alkalosis.