Low partial pressure of oxygen. Partial pressure of oxygen in the blood

The partial pressure or tension of carbon dioxide (pCO2) is the pressure of CO2 in a gas mixture in equilibrium with arterial blood plasma at a temperature of 38°C. The indicator is a criterion for the concentration of carbon dioxide in the blood.

The change in pCO2 plays a leading role in respiratory disorders of the acid-base state (respiratory acidosis and respiratory alkalosis)

In respiratory acidosis, pCO2 increases due to a violation of lung ventilation, which causes the accumulation of carbonic acid,

In respiratory alkalosis, pCO2 decreases as a result of hyperventilation of the lungs, which leads to increased excretion of carbon dioxide from the body and alkalization of the blood.

With non-respiratory (metabolic) azidoses / alkalosis, the pCO2 indicator does not change.
If there are such shifts in pH and the pCO2 index is not normal, then there are secondary (or compensatory) changes.
When clinically evaluating a shift in pCO2, it is important to establish whether the changes are causal or compensatory!

Thus, an increase in pCO2 occurs with respiratory acidosis and compensated metabolic alkalosis, and a decrease occurs with respiratory alkalosis and compensation of metabolic acidosis.

Fluctuations in the value of pCO2 in pathological conditions are in the range from 10 to 130 mm Hg.

With respiratory disorders, the direction of the shift in the blood pH value is opposite to the pCO2 shift, with metabolic disorders, the shifts are unidirectional.

Bicarbonate ion concentration

The concentration of bicarbonates (HCO3- ions) in the blood plasma is the third main indicator of the acid-base state.

In practice, there are indicators of actual (true) bicarbonates and standard bicarbonates.

Actual bicarbonate (AB, AB) is the concentration of HCO3– ions in the test blood at 38°C and actual pH and pCO2 values.

Standard bicarbonates (SB, SB) is the concentration of HCO3– ions in the test blood when it is brought to standard conditions: full blood oxygen saturation, equilibrated at 38°C with a gas mixture in which pCO2 is 40 mmHg.

In healthy people, the concentration of topical and standard bicarbonates is almost the same.

The diagnostic value of the concentration of bicarbonates in the blood is, first of all, in determining the nature of the violations of the acid-base state (metabolic or respiratory).

The indicator primarily changes with metabolic disorders:

With metabolic acidosis, the HCO3– index decreases, because. spent on the neutralization of acidic substances (buffer system)

With metabolic alkalosis - increased

Since carbonic acid dissociates very poorly and its accumulation in the blood has practically no effect on the concentration of HCO3–, the change in bicarbonates in primary respiratory disorders is small.

When metabolic alkalosis is compensated, bicarbonates accumulate due to a decrease in respiration, and when metabolic acidosis is compensated, as a result of increased renal reabsorption.

Buffer Base Concentration

Another indicator characterizing the state of the acid-base state is the concentration of buffer bases (buffer bases, BB), reflecting the sum of all anions in whole blood, mainly bicarbonate and chlorine anions, other anions include protein ions, sulfates, phosphates, lactate, ketone body, etc.

This parameter is almost independent of the change partial pressure carbon dioxide in the blood, but reflects the production of acids by tissues and partly the function of the kidneys.

By the value of the buffer bases, one can judge the shifts in the acid-base state associated with an increase or decrease in the content of non-volatile acids in the blood (that is, all but carbonic acid).

In practice, the parameter used for the concentration of buffer bases is the parameter "residual anions" or "undetectable anions" or "anion mismatch" or "anion difference".

The use of the anion difference index is based on the postulate of electrical neutrality, i.e. the number of negative (anions) and positive (cations) in the blood plasma should be the same.
If we experimentally determine the amount of Na+, K+, Cl–, HCO3– ions most represented in blood plasma, then the difference between cations and anions is approximately 12 mmol/l.

An increase in the anion gap indicates the accumulation of unmeasured anions (lactate, ketone bodies) or cations, which is specified by the clinical picture or by history.

Indicators of total buffer bases and anion gap are especially informative in case of metabolic shifts in the acid-base state, while in case of respiratory disorders, its fluctuations are insignificant.

Excess buffer bases

Base excess (BE, IO) - the difference between the actual and due values of buffer bases.
By value, the indicator can be positive (excess of bases) or negative (deficit of bases, excess of acids).

The indicator of diagnostic value is higher than the concentrations of topical and standard bicarbonates. Base excess reflects shifts in the number of bases in blood buffer systems, while actual bicarbonate only reflects concentration.

The greatest changes in the indicator are observed in metabolic disorders: in acidosis, a lack of blood bases is detected (deficit of bases, negative values), in alkalosis, an excess of bases (positive values).
Deficiency limit compatible with life, 30 mmol/l.

With respiratory shifts, the indicator changes slightly.

The pH value forms the activity of cells

Acid-base balance is a state that is provided by physiological and physico-chemical processes that make up a functionally unified system for stabilizing the concentration of H + ions.
The normal concentration of H+ ions is about 40 nmol/l, which is 106 times less than the concentration of many other substances (glucose, lipids, minerals).

H+ ion concentration fluctuations compatible with life range from 16-160 nmol/l.

Since metabolic reactions are often associated with the oxidation and reduction of molecules, these reactions necessarily involve compounds that act as an acceptor or donor of hydrogen ions. The participation of other compounds is reduced to ensuring the constancy of the concentration of hydrogen ions in biological fluids.

The stability of the intracellular concentration of H + is necessary for:

Optimal activity of enzymes in membranes, cytoplasm and intracellular organelles

Formation of the electrochemical gradient of the mitochondrial membrane at the proper level and sufficient production of ATP in the cell.

Shifts in the concentration of H+ ions lead to changes in the activity of intracellular enzymes, even within the limits of physiological values.
For example, gluconeogenesis enzymes in the liver are more active when the cytoplasm is acidified, which is important during starvation or muscle exercise, glycolysis enzymes are more active at normal pH.

The stability of the extracellular concentration of H+ ions provides:

Optimal functional activity of blood plasma proteins and intercellular space (enzymes, transport proteins),

Solubility of inorganic and organic molecules,

Nonspecific protection of the skin epithelium,

Negative charge on the outer surface of the erythrocyte membrane.

When the concentration of H+ ions in the blood changes, the compensatory activity of two major body systems is activated:

1. Chemical compensation system

The action of extracellular and intracellular buffer systems,

Intensity of intracellular formation of H+ and HCO3– ions.

2. Physiological compensation system

Pulmonary ventilation and CO2 removal,

Renal excretion of H+ ions (acidogenesis, ammoniumgenesis), reabsorption and synthesis of HCO3–.

With a decrease in barometric pressure, the partial pressure of the main gases that make up the atmosphere also decreases. The quantitative composition of the air mixture in the troposphere remains virtually unchanged. So atmospheric air under normal conditions (at sea level) contains 21% oxygen, 78% nitrogen, 0.03% carbon dioxide, and almost % is inert gases: helium, xenon, argon, etc.

Partial pressure(lat. partialis - partial, from lat. pars - part) - the pressure of a single component of the gas mixture. The total pressure of a gas mixture is the sum of the partial pressures of its components.

The partial pressure of a gas in atmospheric air is determined by the formula:

Ph is the barometric pressure at the actual altitude.

A decisive role in maintaining human life is played by gas exchange between the body and external environment. Gas exchange is carried out due to respiration and blood circulation: oxygen continuously enters the body, and carbon dioxide and other metabolic products are released from the body. In order for this process not to be disturbed, it is necessary to support partial pressure of oxygen in the inhaled air at a level close to the earth.

Partial pressure of oxygen (O 2) in air is called the part of the total air pressure attributable to O 2.

So, at sea level (Н=0m), in accordance with (1.1), the partial pressure of oxygen will be:

where αO 2 \u003d 21% is the gas content in atmospheric air in%;

P h \u003d 0 - barometric pressure at sea level

With an increase in altitude, the total pressure of gases decreases, but the partial pressure of such constituents as carbon dioxide and water vapor in the alveolar air remains practically unchanged.

And equal, at a human body temperature of 37 0 C approximately:

· for water vapor РН 2 О=47mm Hg;

· for carbon dioxide РСО 2 =40 mm Hg.

This significantly changes the rate of oxygen pressure drop in the alveolar air.

Atmospheric pressure and air temperature at heights

according to international standard

Table 1.4

No. p / p	Height, m	Barometric pressure, mm Hg	Air temperature, 0 С
1.
2.		715,98	11,75
3.		674,01	8,5
4.		634,13	5,25
5.		596,17
6.		560,07	-1,25
7.		525,8	-4,5
8.		493,12	-7,15
9.		462,21	-11,0
10.		432,86	-14,25
11.		405,04	-17,5
12.		378,68	-20,5
13.		353,73	-24,0
14.		330,12	-27,25
15.		307,81	-30,5
16.		286,74	-33,75
17.		266,08	-37,0
18.		248,09	-40,25
19.		230,42	-43,5
20.		213,76	-46,75
21.		198,14	-50,0
22.		183,38	-50,25
23.		169,58	-56,5
24.		156,71	-56,5
25.		144,82	-56,5
26.		133,83	-56,5
27.		123,68	-56,5
28.		114,30	-56,5
29.		105,63	-56,5
30.		97,61	-56,5
31.		90,21	-56,5
32.		83,86	-56,5

Alveolar air- a mixture of gases (mainly oxygen, carbon dioxide, nitrogen and water vapor) contained in the pulmonary alveoli, directly involved in gas exchange with blood. The supply of oxygen to the blood flowing through the pulmonary capillaries and the removal of carbon dioxide from it, as well as the regulation of respiration, depend on the composition maintained in healthy animals and humans within certain narrow limits due to ventilation of the lungs (in humans, it normally contains 14-15% oxygen and 5-5.5% carbon dioxide). With a lack of oxygen in the inhaled air and some disease states, changes in the composition occur, which can lead to hypoxia.

The meaning of breath

Breathing is vital required process constant exchange of gases between the body and its external environment. In the process of breathing, a person absorbs oxygen from the environment and releases carbon dioxide.

Almost all complex reactions the transformation of substances in the body are with the obligatory participation of oxygen. Without oxygen, metabolism is impossible, and a constant supply of oxygen is necessary to preserve life. As a result of metabolism, carbon dioxide is formed in cells and tissues, which must be removed from the body. The accumulation of a significant amount of carbon dioxide inside the body is dangerous. Carbon dioxide is carried by the blood to the respiratory organs and exhaled. Oxygen entering the respiratory organs during inhalation diffuses into the blood and is delivered by the blood to organs and tissues.

There are no reserves of oxygen in the human and animal body, and therefore its continuous supply to the body is a vital necessity. If a person, in necessary cases, can live without food for more than a month, without water for up to 10 days, then in the absence of oxygen, irreversible changes occur within 5-7 minutes.

Composition of inhaled, exhaled and alveolar air

By alternately inhaling and exhaling, a person ventilates the lungs, maintaining a relatively constant gas composition in the pulmonary vesicles (alveoli). A person breathes atmospheric air with a high oxygen content (20.9%) and a low carbon dioxide content (0.03%), and exhales air in which oxygen is 16.3%, carbon dioxide is 4% (Table 8).

The composition of alveolar air is significantly different from the composition of atmospheric, inhaled air. It has less oxygen (14.2%) and a large amount of carbon dioxide (5.2%).

Nitrogen and inert gases, which are part of the air, do not take part in respiration, and their content in inhaled, exhaled and alveolar air is almost the same.

Why is there more oxygen in exhaled air than in alveolar air? This is explained by the fact that during exhalation, the air that is in the respiratory organs, in the airways, is mixed with the alveolar air.

Partial pressure and tension of gases

In the lungs, oxygen from the alveolar air passes into the blood, and carbon dioxide from the blood enters the lungs. The transition of gases from air to liquid and from liquid to air occurs due to the difference in the partial pressure of these gases in air and liquid. Partial pressure is the part of the total pressure that falls on the proportion of a given gas in a gas mixture. The higher the percentage of gas in the mixture, the correspondingly higher its partial pressure. Atmospheric air, as you know, is a mixture of gases. Atmospheric air pressure 760 mm Hg. Art. The partial pressure of oxygen in atmospheric air is 20.94% of 760 mm, i.e. 159 mm; nitrogen - 79.03% of 760 mm, i.e. about 600 mm; there is little carbon dioxide in the atmospheric air - 0.03%, therefore its partial pressure is 0.03% of 760 mm - 0.2 mm Hg. Art.

For gases dissolved in a liquid, the term "voltage" is used, corresponding to the term "partial pressure" used for free gases. Gas tension is expressed in the same units as pressure (in mmHg). If the partial pressure of the gas in the environment is higher than the voltage of that gas in the liquid, then the gas dissolves in the liquid.

The partial pressure of oxygen in the alveolar air is 100-105 mm Hg. Art., and in the blood flowing to the lungs, the oxygen tension is on average 60 mm Hg. Art., therefore, in the lungs, oxygen from the alveolar air passes into the blood.

The movement of gases occurs according to the laws of diffusion, according to which a gas propagates from an environment with a high partial pressure to an environment with a lower pressure.

Gas exchange in the lungs

The transition in the lungs of oxygen from the alveolar air into the blood and the flow of carbon dioxide from the blood into the lungs obey the laws described above.

Thanks to the work of the great Russian physiologist Ivan Mikhailovich Sechenov, it became possible to study the gas composition of the blood and the conditions of gas exchange in the lungs and tissues.

Gas exchange in the lungs takes place between the alveolar air and blood by diffusion. The alveoli of the lungs are surrounded by a dense network of capillaries. The walls of the alveoli and capillaries are very thin, which contributes to the penetration of gases from the lungs into the blood and vice versa. Gas exchange depends on the size of the surface through which the diffusion of gases is carried out, and the difference in the partial pressure (voltage) of the diffusing gases. With a deep breath, the alveoli stretch, and their surface reaches 100-105 m 2. The surface of the capillaries in the lungs is also large. There is a sufficient difference between the partial pressure of gases in the alveolar air and the tension of these gases in the venous blood (Table 9).

From table 9 it follows that the difference between the tension of gases in the venous blood and their partial pressure in the alveolar air is 110 - 40 = 70 mm Hg for oxygen. Art., and for carbon dioxide 47 - 40 = 7 mm Hg. Art.

Empirically, it was possible to establish that with a difference in oxygen tension of 1 mm Hg. Art. in an adult at rest, 25-60 ml of oxygen can enter the blood in 1 minute. A person at rest needs about 25-30 ml of oxygen per minute. Therefore, the oxygen pressure difference of 70 mm Hg. st, sufficient to provide the body with oxygen at different conditions his activities: during physical work, sports exercises, etc.

The diffusion rate of carbon dioxide from the blood is 25 times greater than that of oxygen, therefore, with a pressure difference of 7 mm Hg. Art., carbon dioxide has time to stand out from the blood.

Carrying gases in the blood

Blood carries oxygen and carbon dioxide. In the blood, as in any liquid, gases can be in two states: physically dissolved and chemically bound. Both oxygen and carbon dioxide dissolve in very small amounts in the blood plasma. Most of the oxygen and carbon dioxide is transported in chemically bound form.

The main carrier of oxygen is hemoglobin in the blood. 1 g of hemoglobin binds 1.34 ml of oxygen. Hemoglobin has the ability to combine with oxygen to form oxyhemoglobin. The higher the partial pressure of oxygen, the more oxyhemoglobin is formed. In the alveolar air, the partial pressure of oxygen is 100-110 mm Hg. Art. Under these conditions, 97% of the hemoglobin in the blood binds to oxygen. The blood carries oxygen to the tissues in the form of oxyhemoglobin. Here, the partial pressure of oxygen is low, and oxyhemoglobin - a fragile compound - releases oxygen, which is used by tissues. The binding of oxygen by hemoglobin is also affected by the tension of carbon dioxide. Carbon dioxide reduces the ability of hemoglobin to bind oxygen and promotes the dissociation of oxyhemoglobin. An increase in temperature also reduces the ability of hemoglobin to bind oxygen. It is known that the temperature in the tissues is higher than in the lungs. All these conditions help the dissociation of oxyhemoglobin, as a result of which the blood releases the oxygen released from the chemical compound into the tissue fluid.

The ability of hemoglobin to bind oxygen is vital to the body. Sometimes people die from a lack of oxygen in the body, surrounded by the cleanest air. This can happen to a person who finds himself in a low pressure environment (at high altitudes), where the rarefied atmosphere has a very low partial pressure of oxygen. On April 15, 1875, the Zenith balloon, carrying three aeronauts, reached a height of 8000 m. When the balloon landed, only one person survived. The cause of death was a sharp decrease in the partial pressure of oxygen at high altitude. At high altitudes (7-8 km), arterial blood in its gas composition approaches venous blood; all tissues of the body begin to experience an acute lack of oxygen, which leads to serious consequences. Climbing above 5000 m usually requires the use of special oxygen devices.

With special training, the body can adapt to the reduced oxygen content in the atmospheric air. In a trained person, breathing deepens, the number of erythrocytes in the blood increases due to their increased formation in the hematopoietic organs and from the blood depot. In addition, heart contractions increase, which leads to an increase in the minute volume of blood.

Pressure chambers are widely used for training.

Carbon dioxide is carried in the blood in the form of chemical compounds - sodium and potassium bicarbonates. The binding of carbon dioxide and its release by the blood depend on its tension in the tissues and blood.

In addition, blood hemoglobin is involved in the transfer of carbon dioxide. In tissue capillaries, hemoglobin enters into a chemical combination with carbon dioxide. In the lungs, this compound breaks down with the release of carbon dioxide. About 25-30% of the carbon dioxide released in the lungs is carried by hemoglobin.

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The gases that make up breathing air affect the human body depending on the value of their partial (partial) pressure:

where Pg is the partial pressure of the gas, kgf / cm², mm Hg. st or kPa;

Pa - absolute air pressure, kgf/cm², mm Hg. Art. or kPa.

Example 1.2. Atmospheric air contains 78% nitrogen by volume. 21% oxygen and 0.03% carbon dioxide. Determine the partial pressure of these gases at the surface and at a depth of 40 m. Take the atmospheric air pressure equal to 1 kgf / cm².

Solution: 1) absolute pressure of compressed air at a depth of 40 m according to (1.2)

2) partial pressure of nitrogen according to (1.3) on the surface
at a depth of 40 m
3) partial pressure of oxygen on the surface
at a depth of 40 m
4) partial pressure of carbon dioxide on the surface
at a depth of 40 m
Consequently, the partial pressure of the gases that make up the breathing air at a depth of 40 m increased by 5 times.

Example 1.3. Using the data of Example 1.2, determine what percentage of gases should be at a depth of 40 m so that their partial pressure corresponds to normal conditions on the surface.

Solution: 1) the nitrogen content in the air at a depth of 40 m, corresponding to the partial pressure on the surface, according to (1.3)

2) oxygen content under the same conditions

3) carbon dioxide content under the same conditions

Consequently, physiological action on the body of the gases that make up the breathing air at a depth of 40 m will be the same as on the surface, provided that their percentage decreases by 5 times.

Nitrogen air begins to have a toxic effect almost at a partial pressure of 5.5 kgf / cm² (550 kPa). Since atmospheric air contains approximately 78% nitrogen, according to (1.3), the indicated partial pressure of nitrogen corresponds to an absolute air pressure of 7 kgf / cm² (immersion depth - 60 m). At this depth, the swimmer becomes agitated, working capacity and attentiveness decrease, orientation becomes difficult, sometimes dizziness is observed. At great depths (80 ... 100 m), visual and auditory hallucinations often develop. Practically at depths of 80 ... 90 m, the swimmer becomes disabled, and descent to these depths while breathing air is possible only for a short time.

Oxygen in high concentrations, even under conditions of atmospheric pressure, it has a toxic effect on the body. So, at a partial pressure of oxygen of 1 kgf / cm² (breathing with pure oxygen in atmospheric conditions), inflammation develops in the lungs after 72 hours of breathing. At a partial pressure of oxygen of more than 3 kgf / cm², after 15 ... 30 minutes, convulsions occur and the person loses consciousness. Factors predisposing to the occurrence of oxygen poisoning: the content of carbon dioxide impurities in the inhaled air, strenuous physical work, hypothermia or overheating.

With a low partial pressure of oxygen in the inhaled air (below 0.16 kgf / cm²), the blood flowing through the lungs is not completely saturated with oxygen, which leads to a decrease in efficiency, and in cases of acute oxygen starvation - to loss of consciousness.

Carbon dioxide. Maintaining normal levels of carbon dioxide in the body is regulated by the central nervous system, which is very sensitive to its concentration. An increased content of carbon dioxide in the body leads to poisoning, a lower one - to a decrease in the frequency of breathing and its stop (apnea). Under normal conditions, the partial pressure of carbon dioxide in atmospheric air is 0.0003 kgf / cm² (~ 30 Pa). If the partial pressure of carbon dioxide in the inhaled air rises more than 0.03 kgf / cm² (-3 kPa), the body will no longer be able to cope with the removal of this gas through increased breathing and blood circulation, and severe disorders may occur.

It should be borne in mind that, according to (1.3), a partial pressure of 0.03 kgf/cm² on the surface corresponds to a carbon dioxide concentration of 3%, and at a depth of 40 m (absolute pressure 5 kgf/cm²) - 0.6%. The increased content of carbon dioxide in the inhaled air enhances the toxic effect of nitrogen, which can already manifest itself at a depth of 45 m. That is why it is necessary to strictly monitor the content of carbon dioxide in the inhaled air.

Saturation of the body with gases. Staying under high pressure entails saturation of the body with gases that dissolve in tissues and organs. At atmospheric pressure on the surface in a human body weighing 70 kg, about 1 liter of nitrogen is dissolved. With increasing pressure, the ability of body tissues to dissolve gases increases in proportion to the absolute pressure of air. So, at a depth of 10 m (absolute air pressure for breathing 2 kgf / cm²), 2 liters of nitrogen can already be dissolved in the body, at a depth of 20 m (3 kgf / cm²) - 3 liters of nitrogen, etc.

The degree of saturation of the body with gases depends on their partial pressure, the time spent under pressure, as well as on the rate of blood flow and pulmonary ventilation.

During physical work, the frequency and depth of breathing, as well as the speed of blood flow, increase, therefore, the saturation of the body with gases is directly dependent on the intensity of the physical activity of a diver-submariner. With the same physical load, the rate of blood flow and pulmonary ventilation in a trained person increase to a lesser extent than in an untrained person, and the saturation of the body with gases will be different. Therefore, it is necessary to pay attention to increasing the level of physical fitness, stable functional state of the cardiovascular and respiratory systems.

A decrease in pressure (decompression) causes the body to become desaturated from indifferent gas (nitrogen). In this case, the excess of dissolved gas enters the bloodstream from the tissues and is carried by the bloodstream to the lungs, from where it is removed by diffusion into the lungs. environment. If the ascent is too fast, the gas dissolved in the tissues forms bubbles of various sizes. They can be carried by the blood stream throughout the body and cause blockage of blood vessels, which leads to decompression (caisson) sickness.

The gases formed in the intestines of a diver-submariner during his stay under pressure expand during ascent, which can lead to pain in the abdomen (flatulence). Therefore, it is necessary to ascend from depth to the surface slowly, and in case of a long stay at depth - with stops in accordance with the decompression tables (Appendix 11.8).

The main air parameters that determine the physiological state of a person are:

absolute pressure;

percentage of oxygen;

temperature;

relative humidity;

harmful impurities.

Of all the listed air parameters, the absolute pressure and the percentage of oxygen are of decisive importance for a person. Absolute pressure determines the partial pressure of oxygen.

The partial pressure of any gas in a gas mixture is the fraction of the total pressure of the gas mixture attributable to that gas in proportion to its percentage.

So for the partial pressure of oxygen we have

where
− percentage of oxygen in the air (
);

R H − air pressure at altitude H;

− partial pressure of water vapor in the lungs (backpressure for breathing
).

The partial pressure of oxygen is of particular importance for the physiological state of a person, since it determines the process of gas exchange in the body.

Oxygen, like any gas, tends to move from a space in which its partial pressure is greater to a space with a lower pressure. Consequently, the process of saturation of the body with oxygen occurs only when the partial pressure of oxygen in the lungs (in the alveolar air) is greater than the partial pressure of oxygen in the blood flowing to the alveoli, and this latter will be greater than the partial pressure of oxygen in the tissues of the body.

To remove carbon dioxide from the body, it is necessary to have the ratio of its partial pressures opposite to that described, i.e. highest value partial pressure of carbon dioxide should be in the tissues, less - in the venous blood and even less - in the alveolar air.

At sea level at R H= 760 mmHg Art. the partial pressure of oxygen is ≈150 mm Hg. Art. With such
normal saturation of human blood with oxygen in the process of breathing is ensured. With increasing flight altitude
decreases due to the decrease P H(Fig. 1).

Special physiological studies have established that the minimum partial pressure of oxygen in the inhaled air
This number is called the physiological limit of a person's stay in an open cabin in terms of size
.

The partial pressure of oxygen is 98 mm Hg. Art. corresponds height H= 3 km. At
< 98 mmHg Art. visual impairment, hearing impairment, slow reaction and loss of consciousness by a person are possible.

To prevent these phenomena on the aircraft, oxygen supply systems (OSS) are used, providing
> 98 mmHg Art. in the inhaled air in all flight modes and in emergency situations.

Practically in aviation, the height H = 4 km as a limit for flights without oxygen devices, i.e. aircraft with a service ceiling of less than 4 km may not have an SPC.