Aerospace physiology

Last updated

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.

Contents

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 m above sea level, barometric pressure is 523 mmHg, and at 15.240 m, 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. [1] Aviation physiology can also include the effect in humans and animals exposed for long periods of time inside pressurized cabins. [2]

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. [3] 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 m), and decreases more at higher altitudes. [4]

g-forces

g-forces are mostly experienced by the body during flight, especially high speed flight and space travel. This includes positive g-force, negative g-force and zero g-force, caused by simple acceleration, deceleration and centripetal acceleration. When an airplane turns, centripetal acceleration is determined by ƒ=mv2/r. This indicates that if speed increases, centripetal acceleration force also increases in proportion to the square of the speed. [5]

When an aviator is submitted to positive g-force in acceleration, the blood will move to the inferior part of the body, meaning that if the g-force is elevated, all the blood pressure in veins will increase. This means less blood reaches the heart, affecting its ability to function, with decreased circulation. [6]

The effects for negative g-force can be more dangerous producing hyperemia and also psychotic episodes. In space, G forces are almost zero, which is called microgravity, meaning that the person is floating in the interior of the vessel. This happens because the gravity acts on the spaceship and in the body equally, both are pulled with the same forces of acceleration and also in the same direction. [7]

Hypoxia (medical)

General effects

Hypoxia occurs when the bloodstream lacks oxygen. In an aerospace environment, this occurs because there is little or no oxygen. The work capacity of the body is reduced, decreasing the movement of all muscles (skeletal and cardiac muscles). The decrease in work capacity is related to the decrease of the oxygen of transportation velocity. [8] Some acute effects from hypoxia include: dizziness, laxity, mental fatigue, muscle fatigue and euphoria. These effects will affect a non-acclimated person starting in an altitude of 3650 meters above sea level. These effects will increase and can result in cramps or convulsions at an altitude of 5500 meters and will end in an altitude at 7000 meters with a coma. [8]

Mountaineering disease

One type of hypoxia related syndrome is mountaineering disease. A non-acclimated person that stays for a significant amount of time at a high altitude can develop high erythrocytes and hematocrit. Pulmonary arterial pressure will increase even if the person is acclimated, presenting dilatation of the right side of the heart. Peripheral arterial pressure is decreased, leading to congestive cardiac insufficiency, and death if exposure is long enough. [9] These effects are produced by a decrease of erythrocytes, which causes a significant increase of viscosity in blood. This causes diminished blood flow in tissues, so oxygen distribution decreases. The vasoconstriction of the pulmonary arterioles is caused by hypoxia in the right portion of the heart. Arteriole spasms include the major part of the blood flow through the pulmonary vessels, producing a short circuit in the blood flow giving less oxygen in blood. The person will recover if there is an administration of oxygen or if s/he is taken to low altitudes. [10]

Mountaineering disease and pulmonary edema are most common in those who climb rapidly to a high altitude. This illness starts from a few hours up to two or three days after ascension to a high altitude. There exist two cases: acute cerebral edema and acute pulmonary edema. The first one is caused by the vasodilatation of the cerebral blood vessels produced by the hypoxia; the second one is caused by the vasoconstriction of the pulmonary arterioles, caused by the hypoxia. [9]

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. [3] A person that stays for a period of time at higher altitudes acclimates, producing fewer effects over the human body. [3] 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. [11]

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. [12]

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. [13]

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 receptors produced by the hypoxia after the kidneys have recover the alkalosis. [14]

Related Research Articles

<span class="mw-page-title-main">Hypoxia (medical)</span> Medical condition of lack of oxygen in the tissues

Hypoxia is a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. Hypoxia may be classified as either generalized, affecting the whole body, or local, affecting a region of the body. Although hypoxia is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise.

<span class="mw-page-title-main">Respiratory system</span> Biological system in animals and plants for gas exchange

The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs; in mammals and reptiles these are called alveoli, and in birds they are known as atria. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood. These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.

<span class="mw-page-title-main">Altitude sickness</span> Medical condition due to rapid exposure to low oxygen at high altitude

Altitude sickness, the mildest form being acute mountain sickness (AMS), is the harmful effect of high altitude, caused by rapid exposure to low amounts of oxygen at high elevation. People can respond to high altitude in different ways. Symptoms may include headaches, vomiting, tiredness, confusion, trouble sleeping, and dizziness. Acute mountain sickness can progress to high-altitude pulmonary edema (HAPE) with associated shortness of breath or high-altitude cerebral edema (HACE) with associated confusion. Chronic mountain sickness may occur after long-term exposure to high altitude.

<span class="mw-page-title-main">Lung volumes</span> Volume of air in the lungs

Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle.

<span class="mw-page-title-main">Pulmonary edema</span> Fluid accumulation in the tissue and air spaces of the lungs

Pulmonary edema, also known as pulmonary congestion, is excessive liquid accumulation in the tissue and air spaces of the lungs. It leads to impaired gas exchange and may cause hypoxemia and respiratory failure. It is due to either failure of the left ventricle of the heart to remove oxygenated blood adequately from the pulmonary circulation, or an injury to the lung tissue directly or blood vessels of the lung.

The control of ventilation is the physiological mechanisms involved in the control of breathing, which is the movement of air into and out of the lungs. Ventilation facilitates respiration. Respiration refers to the utilization of oxygen and balancing of carbon dioxide by the body as a whole, or by individual cells in cellular respiration.

<span class="mw-page-title-main">Carotid body</span>

The carotid body is a small cluster of chemoreceptor cells, and supporting sustentacular cells. The carotid body is located in the adventitia, in the bifurcation (fork) of the common carotid artery, which runs along both sides of the neck.

<span class="mw-page-title-main">Death zone</span> Mountaineering term

In mountaineering, the death zone refers to altitudes above a certain point where the pressure of oxygen is insufficient to sustain human life for an extended time span. This point is generally tagged as 8,000 m. The concept was conceived in 1953 by Edouard Wyss-Dunant, a Swiss doctor, who called it the lethal zone. All 14 peaks above 8000 m in the death zone are located in the Himalaya and Karakoram of Asia.

<span class="mw-page-title-main">Generalized hypoxia</span> Medical condition of oxygen deprivation

Generalized hypoxia is a medical condition in which the tissues of the body are deprived of the necessary levels of oxygen due to an insufficient supply of oxygen, which may be due to the composition or pressure of the breathing gas, decreased lung ventilation, or respiratory disease, any of which may cause a lower than normal oxygen content in the arterial blood, and consequently a reduced supply of oxygen to all tissues perfused by the arterial blood. This usage is in contradistinction to localized hypoxia, in which only an associated group of tissues, usually with a common blood supply, are affected, usually due to an insufficient or reduced blood supply to those tissues. Genralized hypoxia is also used as a synonym for hypoxic hypoxia This is not to be confused with hypoxemia, which refers to low levels of oxygen in the blood, although the two conditions often occur simultaneously, since a decrease in blood oxygen typically corresponds to a decrease in oxygen in the surrounding tissue. However, hypoxia may be present without hypoxemia, and vice versa, as in the case of infarction. Several other classes of medical hypoxia exist.

<span class="mw-page-title-main">Altitude training</span>

Altitude training is the practice by some endurance athletes of training for several weeks at high altitude, preferably over 2,400 metres (8,000 ft) above sea level, though more commonly at intermediate altitudes due to the shortage of suitable high-altitude locations. At intermediate altitudes, the air still contains approximately 20.9% oxygen, but the barometric pressure and thus the partial pressure of oxygen is reduced.

<span class="mw-page-title-main">Respiratory acidosis</span> Medical condition

Respiratory acidosis is a state in which decreased ventilation (hypoventilation) increases the concentration of carbon dioxide in the blood and decreases the blood's pH.

Hypoxic pulmonary vasoconstriction (HPV), also known as the Euler-Liljestrand mechanism, is a physiological phenomenon in which small pulmonary arteries constrict in the presence of alveolar hypoxia. By redirecting blood flow from poorly-ventilated lung regions to well-ventilated lung regions, HPV is thought to be the primary mechanism underlying ventilation/perfusion matching.

<span class="mw-page-title-main">Hypoxemia</span> Abnormally low level of oxygen in the blood

Hypoxemia is an abnormally low level of oxygen in the blood. More specifically, it is oxygen deficiency in arterial blood. Hypoxemia has many causes, and often causes hypoxia as the blood is not supplying enough oxygen to the tissues of the body.

<span class="mw-page-title-main">Armstrong limit</span> Altitude above which water boils at human body temperature

The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds. On Earth, the limit is around 18–19 km above sea level, above which atmospheric air pressure drops below 0.0618 atm. The U.S. Standard Atmospheric model sets the Armstrong pressure at an altitude of 63,000 feet (19,202 m).

<span class="mw-page-title-main">2,3-Bisphosphoglyceric acid</span> Chemical compound

2,3-Bisphosphoglyceric acid (2,3-BPG), also known as 2,3-diphosphoglyceric acid (2,3-DPG), is a three-carbon isomer of the glycolytic intermediate 1,3-bisphosphoglyceric acid (1,3-BPG).

A pulmonary shunt is the passage of deoxygenated blood from the right side of the heart to the left without participation in gas exchange in the pulmonary capillaries. It is a pathological condition that results when the alveoli of parts of the lungs are perfused with blood as normal, but ventilation fails to supply the perfused region. In other words, the ventilation/perfusion ratio of those areas is zero.

<span class="mw-page-title-main">Effects of high altitude on humans</span> Environmental effects on physiology

The effects of high altitude on humans are mostly the consequences of reduced partial pressure of oxygen in the atmosphere. The oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around 2,100 metres (6,900 ft) above sea level, the saturation of oxyhemoglobin begins to decrease rapidly. However, the human body has both short-term and long-term adaptations to altitude that allow it to partially compensate for the lack of oxygen. There is a limit to the level of adaptation; mountaineers refer to the altitudes above 8,000 metres (26,000 ft) as the death zone, where it is generally believed that no human body can acclimatize. At extreme altitudes, the ambient pressure can drop below the vapor pressure of water at body temperature, but at such altitudes even pure oxygen at ambient pressure cannot support human life, and a pressure suit is necessary. A rapid depressurisation to the low pressures of high altitudes can trigger altitude decompression sickness.

The Alveolar–arterial gradient, is a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. It is a useful parameter for narrowing the differential diagnosis of hypoxemia.

Hypoxic ventilatory response (HVR) is the increase in ventilation induced by hypoxia that allows the body to take in and transport lower concentrations of oxygen at higher rates. It is initially elevated in lowlanders who travel to high altitude, but reduces significantly over time as people acclimatize. In biological anthropology, HVR also refers to human adaptation to environmental stresses resulting from high altitude.

<span class="mw-page-title-main">Breathing</span> Process of moving air in and out of the lungs

Breathing is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen.

References

  1. ↑ GUYTON, A.C., HALL, J.E "Tratado De Fisiologia Médica" 10. Ed. Rj . Elsevier Saunders: 2011;527
  2. ↑ GUYTON, A.C., HALL, J.E "Tratado De Fisiologia Médica" 10. Ed. Rj . Elsevier Saunders: 2011; 527
  3. 1 2 3 GUYTON, A.C., HALL, J.E "Tratado De Fisiologia Médica" 10. Ed. Rj . Elsevier Saunders: 2011; 528
  4. ↑ GUYTON, A.C., HALL, J.E "Tratado De Fisiologia Médica" 10. Ed. Rj . Elsevier Saunders: 2011;528
  5. ↑ GUYTON, A.C., HALL, J.E "Tratado De Fisiologia Médica" 10. Ed. Rj . Elsevier Saunders: 2011;531
  6. ↑ BORON, Walter. Et al. "Medical Physiology" 3 ed. España. Elsevier Saunders: 2012; 220
  7. ↑ BORON, Walter. Et al. Medical Physiology 3 ed. España. Elsevier Saunders: 2012; 224
  8. 1 2 BEST &TAYLOR. Bases fisiológicas de la práctica médica. 11 ed. España. gElservier Saunders: 2006; 230
  9. 1 2 BEST &TAYLOR. Bases fisiológicas de la práctica médica. 11 ed. España. gElservier Saunders: 2006; 228
  10. ↑ DOUGLAS, C. R. Tratado De Fisiologia Aplicada As Ciencias Da Saude. 5 Ed. Sp. Robe Ed Belman Ed. Imp. Exp. 2002.
  11. ↑ BORON, Walter. Et al. Medical Physiology 3 ed. España. Elsevier Saunders: 2012;221
  12. ↑ BORON, Walter. Et al. Medical Physiology 3 ed. España. Elsevier Saunders: 2012; 223
  13. ↑ GUYTON, A.C., HALL, J.E "Tratado De Fisiologia Médica" 10. Ed. Rj . Elsevier Saunders: 2011; 530
  14. ↑ GUYTON, A.C., HALL, J.E "Tratado De Fisiologia Médica" 10. Ed. Rj . Elsevier Saunders: 2011; 532

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.