Aviation Physiology

Introduction

Aviation physiology studies the effects of high altitudes on the body during aviation-related journeys. Specifically, it largely deals with the fluctuations in levels of oxygen and pressure during the flight. The body reacts differently at higher altitudes, which can lead to increased cardiac output and the production of erythrocytes. These changes result in the fact that the body wastes more energy, and it causes muscle fatigue. The barometric pressure in space or in the sky differs significantly from that on the ground. Its standard value at sea level is 760 mmHg, while three meters above sea level, the barometric pressure stands at 523mmHg (Brown, 2016). At 15.240 meters above sea level, it reaches 87mmHg. The values indicated above show that as barometric pressure decreases, there is a corresponding decrease in the partial pressure. This pressure represents about 20% of the total barometric pressure (Gradwell & Rainford, 2016). The alveolar partial pressure of oxygen at sea level is 104mmHg (Brown, 2016; Gradwell & Rainford, 2016). This pressure decreases depending on how an individual has acclimatized to different altitudes. In acclimatized persons, it decreases to about 52mmHg, while in non-acclimatized individuals, it decreases to 40mmHg (Brown, 2016; Gradwell & Rainford, 2016). Apart from the changes in barometric pressure, hypoxia, or a decrease in oxygen levels, is the other crucial issue associated with changes in altitude. Hypoxia is caused by a decrease in oxygen, as the body lacks the sufficient barometric pressure. There are different forms of hypoxia that occur during the flight, including hypoxic hypoxia, hyperemic hypoxia, stagnant hypoxia, and histotoxic hypoxia. This condition has a negative effect on the body, considering the important role that oxygen plays in its functioning. Changes in oxygen levels lead to hypoxia, which is bound to have negative effects on the body’s respiratory system and on the whole body in general during the flights.

The Respiratory System

Respiratory system of a human is composed of several body organs which help to inhale oxygen and exhale carbon dioxide. Lungs are considered the chief organs of the breathing system, since they are responsible for the exchange of gases. Oxygen gas is important for the body, as it stimulates aerobic processes which result in the production of more energy in different cells (Gunga, Ahlefeld, & Coriolano, 2016). Carbon dioxide gas acts as a buffer in the cells, though it is not required in the amounts equal to those of the oxygen and thus is expelled from the respiratory system. After the exchange of different gases, red blood cells start carrying oxygen to other parts of the body, where it maintains the majority of metabolic processes. A decrease in the level of oxygen levels in the body is fatal, since brain cells start dying if they lack oxygen for more than four minutes (Gunga et al., 2016). It ultimately leads to brain damage and subsequent death. Therefore, the role of oxygen is not restricted to the respiratory system, and it is relevant for all other systems within the body.

The respiratory system comprises several parts, such as nose, windpipe or trachea, bronchi, bronchioles, and alveolar. Oxygen goes through the nose and passes through the sinuses that regulate the humidity and temperature of the air. Afterward, it goes to trachea, where the air is filtered, and then moves to bronchi — the tubes which carry the air to each lung. These tubes are lined with cilia and mucus, and germs, dust, as well as other foreign substances present in the air are trapped in this region. Mucus is expelled together with the aforementioned substances when the individuals cough or spit (Gunga et al., 2016). The bronchial tubes lead the air directly to the different lobes of the lung. The left and the right lung have two and three lobes respectively. The left lung is always smaller, since it is closer to the heart. These lobes contain small sacs, referred to as alveoli, where the exchange of carbon dioxide and oxygen occurs. The walls of alveoli are thin and consist of a layer of epithelial cells, defined as pulmonary capillaries. Blood moves through these capillaries to collect or provide different gases to other parts of the body. The final constituent of the respiratory system is the diaphragm, which is dome-shaped and found at the bottom of the lungs. It controls the breathing process and separates the abdominal cavity from the chest cavity. When the air is taken in, it flattens and moves in order to provide more space for the lungs.

Atmosphere and Gas Laws

The atmosphere is composed of a mixture of several different gases. Oxygen remains an essential gas, as it supports life and equals to about one fifth of the total gas present in the atmosphere. It is colorless and tasteless. Other gases in the atmosphere include carbon dioxide, nitrogen, and inert gases. There are several different gas laws applicable to the aviation physiology (Brown, 2016; Iqbal, 2015). The gas laws outline the relationship between various variables, including temperature, pressure, and volume of various gases. Boyle’s law states that the volume of gas is inversely proportional to the pressure when the temperature is constant (Brown, 2016; Iqbal, 2015). This law indicates that gases expand as altitude increases due to the less pressure in the atmosphere. It also means that gases contract when altitude decreases.

The second and most applied gas law related to aviation is Dalton’s law of partial pressure. It states that “total pressure of a gas mixture is equal to the sum of all the gases in the mixture”(Brown, 2016; Gradwell & Rainford, 2016). In essence, this law implies that if the oxygen amounts to 21% of the gases present in the atmosphere, then it accounts for 21% of total atmospheric pressure (Brown, 2016; Gradwell & Rainford, 2016). Ideally, it means that the fractional pressure occupied by each gas becomes its partial pressure. It means that at sea level, the partial pressure of the gas like oxygen is expected to be 160mmHg, which represents 21% of the total atmospheric pressure that stands at 760mmHg (Brown, 2016; Gradwell & Rainford, 2016). A decrease in the atmospheric pressure in cases where there is an increase in altitude means that the partial pressure of the oxygen will also decrease, though it will still retain 21% of its value (Brown, 2016; Gradwell & Rainford, 2016). The transfer of the oxygen to the bloodstream from the lungs depends on the established pressure gradient. This pressure ensures that oxygen molecule moves across the membrane effectively. If the pressure gradient is higher, the molecules move faster. When atmospheric pressure decreases due to an increase in altitude, it means that the partial pressure of oxygen will decrease.

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Hypoxia

Hypoxia occurs when there is an insufficient level of oxygen exchanged in the body due to an increase in the altitude. There are various forms of hypoxia that can occur during a flight, including hypoxic hypoxia, hyperemic hypoxia, stagnant hypoxia, and cytotoxic hypoxia. Hypoxic hypoxia occurs when there is a reduction in the partial pressure of oxygen due to an increase in altitude. It may also occur due to the hypoventilation or reduction in alveolar ventilation, because of alveolar-capillary diffusion block, or during ventilation perfusion, which commonly occurs in the individuals with defects associated with lung diseases (Asmaro, Mayall, & Ferguson, 2013; Holt, Luedtke, & Schindler, 2017). This form of hypoxia occurs when the sufficient level of air does not reach the lungs.

Hyperemic hypoxia is largely caused by the reduction in cells during the process of carrying the oxygen. In essence, it means that oxygen is available, yet its carrying capacity is low. It is commonly caused by anemia, which refers to a decline in the number of red blood cells in the body, by bleeding or hemorrhage, by carbon monoxide, which has high affinity with the red blood cells in regard to the oxygen, and lastly, by hemoglobin anomalies. In cytotoxic hypoxia, there is no pathologic condition that affects or interferes with the diffusion of oxygen and leads to the lack of its available amounts (Asmaro et al., 2013; Holt et al., 2017). This condition is largely centered around the release of oxygen from hemoglobin and is mainly caused by cyanide poisoning, alcohol, and drug ingestions. The last form of hypoxia, referred to as stagnant hypoxia, occurs when there is a decrease in the supply of the oxygen in all the tissues and body organs. It remains the most common form of hypoxia, associated with heart failure, heart attack, or cardiac arrest. In these cases, hypoxic hypoxia and stagnant hypoxia remain the two forms of hypoxia that can be associated with the respiratory system.

Hazards Linked with Hypoxia in Aviation

Hypoxia, a decrease of oxygen in the cells, results in the insufficient oxygen provision in the body cells. These cells maintain nearly all body operations and require the sufficient oxygen to ensure that they operate at the maximum levels when generating energy. A rise in the altitude decreases the partial pressure of oxygen in the airplanes, and finally results in the induced forms of hypoxia, with the most common one being hypoxic hypoxia. This condition is accompanied by some symptoms, such as euphoria, impaired judgment, increased response time to specific activities, dizziness, drowsiness, numbness, limp muscles, tingling of the fingers and toes, and cyanosis (Asmaro et al., 2013; Malle et al., 2013). The condition progresses in an individual until the debilitating effects gain force. In this case, the individuals fail to recognize their current setting.

The onset of hypoxia due to the increase in altitude is guided by Effective Performance Time (EPT) and Time of Useful Consciousness (TUC). EPT refers to the pilot’s ability to function regardless of their consciousness, while TUC refers to the ability to remain conscious even under high-pressure altitudes. There are several factors that predispose pilots to other forms of hypoxia and may have an effect in determining TUC and EPT levels. Unhealthy pilots display the aforementioned effects after short periods of the increasing altitude (Asmaro et al., 2013; Malle et al., 2013). The performance is most likely to deteriorate within fifteen minutes of being at 15,000 feet. The deteriorating conditions are rarely noted in healthy pilots below 12000 feet (Heled et al., 2012; Malle et al., 2013). However, between 12,000 feet and 15000 feet, the effects of hypoxia start appearing in most pilots (Brown, 2016; Gradwell & Rainford, 2016). Drowsiness, euphoria, belligerence, impaired judgment, and memory loss occur more frequently. If the mentioned effects are not moderated, then there are the increased incidences of flight accidents.

Sequential Phases of Hypoxia during the Flights

Hypoxia occurs in four different phases. The first phase is referred to as asymptomatic, or indifferent. In this case, people are not initially aware of the effects of hypoxia. Some common symptoms associated with this condition include loss of night and color vision. These changes commonly take place at the moderately mild altitudes, such as 4000 feet, and become more pronounced in pilots who travel at night. Arterial oxygen saturation is between 90 and 95% in such cases. The second phase is referred to as the compensatory phase. During it, the body has the ability to repel the effects of hypoxia by increasing the depth of ventilation as well as cardiac output. Arterial oxygen saturation, in this case, is between 80 and 90%. The third phase is referred to as the deterioration phase. In this condition, people are unable to compensate for their lack of oxygen, and adverse effects are displayed during this phase, as will be indicated in the diagram below. If corrective actions are not taken during this stage, there is the increased likelihood of the accident occurring. The last stage is considered to the critical one, which leads to death if the effects of hypoxia remain untreated. In most instances, people become mentally and physically incapacitated before becoming unconscious, and finally have convulsions before dying. It occurs when the arterial oxygen saturation is less than 70%

Implications on Flight in Case of Failure to Manage Hypoxia

As mentioned in the above cases, hypoxia occurs when there is the insufficient level of oxygen in the body. Oxygen plays a crucial role in ensuring that all activities within the body are maintained. There are several different methods that are commonly used to manage hypoxia during flights, including the use of pressurized oxygen for pilots who are in charge of the plane. Since the pilots bear the greatest risk, the failure to control or manage hypoxia results in aviation accidents. Once the saturation levels have reached the critical stage, the pilot is more likely to operate at the substandard levels, which makes the control of the airplane more difficult. There have been numerous cases that have been reported on hypoxia-related accidents. The examples include the case in 2005 involving the B733 plane, where 6 crew members and 115 passengers died due to the lack of pressurization, which is meant to control the effects of hypoxia. The pilot and co-pilot became incapacitated due to the effects of hypoxia, leaving the plane to be run on autopilot control and flight management control. The plane ran out of fuel and crashed. Another case involves RJIH plane, which was coming from the southwest of Stockholm. In this case, the flight crew failed to notice that the aircraft had not been pressurized before it took off. However, they initiated the deployment of the passenger masks throughout the flight, and as the result, the adverse effects, such as crash, were averted.

Conclusion

Hypoxia, or the insufficient oxygen level in the atmosphere, occurs when there is little oxygen available in the cells of the body. Oxygen is required by the body for maintaining the functioning of numerous organs, and even more so for producing the energy that is needed for all activities. An increase in altitude results in a change in the amount of oxygen present in the atmosphere. As a result, hypoxia is bound to be induced at high altitudes, unless planes are pressurized. There are four different forms of hypoxia — namely, hypoxic hypoxia, hyperemic hypoxia, stagnant hypoxia, and cytotoxic hypoxia. Hypoxic hypoxia is also referred to as altitude hypoxia and is associated with four different phases. These phases represent a decrease in oxygen saturation and result in an increase in adverse effects. The last phase, referred to as the critical phase, causes full incapacitation of an individual and may result in death. Hypoxia has been mentioned as one of the major causes of aviation-related accidents — therefore, there is a need to control the effects of hypoxia in all flights in order to reduce the number of accidents.