Partial pressure of carbon dioxide in air. The partial pressure of oxygen in air at different altitudes from sea level. Partial gas pressure: concept and formula

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 saturating 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. the highest value of the partial pressure of carbon dioxide should be in the tissues, the smaller - 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.

1. Partial pressure of oxygen and carbon dioxide in the human body in terrestrial conditions

When changing the values ​​​​specified in the table
and
disrupted normal gas exchange in the lungs and throughout the human body.

I would like to summarize the information about the principles of diving in terms of breathing gases in the format of keynotes, i.e. when understanding a few principles eliminates the need to remember many facts.

So, breathing under water requires gas. As the simplest option - air supply, which is a mixture of oxygen (∼21%), nitrogen (∼78%) and other gases (∼1%).

The main factor is the pressure of the environment. Of all possible pressure units, we will use "absolute technical atmosphere" or ATA. The pressure on the surface is ∼1 ATA, every 10 meters of immersion in water add ∼1 ATA to it.

For further analysis, it is important to understand what partial pressure is, i.e. 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. Partial pressure and the dissolution of gases in liquids are described by Dalton's laws and are most directly related to diving, because a person is mostly liquid. Although the partial pressure is proportional to the molar ratio of the gases in the mixture, for air, the partial pressure can be read by volume or weight concentration, the error will be less than 10%.

When diving, the pressure affects us all-encompassing. The regulator maintains the air pressure in the breathing system, approximately equal to the ambient pressure, less than exactly as much as is necessary for "inhalation". So, at a depth of 10 meters, the air inhaled from the balloon has a pressure of about 2 ATA. A similar absolute pressure will be observed throughout our body. Thus, the partial pressure of oxygen at this depth will be ∼0.42 ATA, nitrogen ∼1.56 ATA

The impact of pressure on the body is the following key factors.

1. Mechanical impact on organs and systems

We will not consider it in detail, in short - the human body has a number of air-filled cavities and a sharp change in pressure in any direction causes a load on tissues, membranes and organs up to mechanical damage - barotrauma.

2. Saturation of tissues with gases

When diving (increasing pressure), the partial pressure of gases in the respiratory tract is higher than in the tissues. Thus, gases saturate the blood, and through the bloodstream, all tissues of the body are saturated. The saturation rate is different for different tissues and is characterized by a “half-saturation period”, i.e. the time during which, at a constant gas pressure, the difference between the partial pressures of the gas and tissues is halved. The reverse process is called "desaturation", it occurs during ascent (decrease in pressure). In this case, the partial pressure of gases in the tissues is higher than the pressure in the gases in the lungs, the reverse process takes place - gas is released from the blood in the lungs, blood with an already lower partial pressure circulates through the body, gases pass from the tissues into the blood and again in a circle. A gas always moves from a higher partial pressure to a lower one.

It is fundamentally important that different gases have different rates of saturation/desaturation due to their physical properties.

The solubility of gases in liquids is the greater, the higher the pressure. If the amount of dissolved gas is greater than the solubility limit at a given pressure, gas is released, including concentration in the form of bubbles. We see this every time we open a bottle of sparkling water. Since the rate of gas removal (tissue desaturation) is limited by physical laws and gas exchange through the blood, a too rapid pressure drop (rapid ascent) can lead to the formation of gas bubbles directly in the tissues, vessels and cavities of the body, disrupting its work up to death. If the pressure drops slowly, then the body has time to remove the "extra" gas due to the difference in partial pressures.

To calculate these processes, mathematical models of body tissues are used, the most popular is the Albert Buhlmann model, which takes into account 16 types of tissues (compartments) with a half-saturation / half-saturation time from 4 to 635 minutes.

The greatest danger is the inert gas, which has the highest absolute pressure, most often it is nitrogen, which forms the basis of air and does not participate in metabolism. For this reason, the main calculations in mass diving are carried out on nitrogen, since. the effect of oxygen in terms of saturation is orders of magnitude less, while the term “nitrogen load” is used, i.e. the residual amount of nitrogen dissolved in the tissues.

Thus, the saturation of tissues depends on the composition of the gas mixture, pressure and duration of its exposure. For the initial levels of diving, there are restrictions on the depth, duration of the dive and the minimum time between dives, which obviously do not allow under any conditions the saturation of tissues to dangerous levels, i.e. no decompression dives, and even then it is customary to perform "safety stops".

"Advanced" divers use dive computers that dynamically calculate saturation from models depending on gas and pressure, including calculating a "compression ceiling" - the depth above which it is potentially dangerous to ascend based on current saturation. During difficult dives, computers are duplicated, not to mention the fact that single dives are usually not practiced.

3. Biochemical effects of gases

Our body is maximally adapted to air at atmospheric pressure. With increasing pressure, gases that are not even involved in metabolism affect the body in a variety of ways, while the effect depends on the partial pressure of a particular gas. Each gas has its own safety limits.

Oxygen

As a key player in our metabolism, oxygen is the only gas that has not only an upper but also a lower safety limit.

The normal partial pressure of oxygen is ∼0.21 ATA. The need for oxygen strongly depends on the state of the body and physical activity, the theoretical minimum level required to maintain the vital activity of a healthy organism in a state of complete rest is estimated at ∼0.08 ATA, the practical one is ∼0.14 ATA. A decrease in oxygen levels from “nominal” first of all affects the ability to physical activity and can cause hypoxia, or oxygen starvation.

At the same time, a high partial pressure of oxygen causes a wide range of negative consequences - oxygen poisoning or hyperoxia. Of particular danger when diving is its convulsive form, which is expressed in damage to the nervous system, convulsions, which entails the risk of drowning.

For practical purposes, diving is considered to be a safety limit of ∼1.4 ATA, a moderate risk limit is ∼1.6 ATA. At a pressure above ∼2.4 ATA for a long time, the probability of oxygen poisoning tends to unity.

Thus, by simply dividing the limiting oxygen level of 1.4 ATA by the partial pressure of oxygen in the mixture, one can determine the maximum safe pressure of the environment and establish that it is absolutely safe to breathe pure oxygen (100%, 1 ATA) at depths up to ∼4 meters (!! !), compressed air (21%, 0.21 ATA) - up to ∼57 meters, standard "Nitrox-32" with an oxygen content of 32% (0.32 ATA) - up to ∼34 meters. Similarly, you can calculate the limits for moderate risk.

They say that it is this phenomenon that owes its name to "nitrox", since initially this word denoted respiratory gases with lowered oxygen content for working at great depths, "nitrogen enriched", and only then it began to be deciphered as "nitrogen-oxygen" and designate mixtures with elevated oxygen content.

It must be taken into account that an increased partial pressure of oxygen in any case affects the nervous system and lungs, and these are different types of effects. In addition, the effect tends to accumulate over a series of dives. To take into account the impact on the central nervous system, the concept of "oxygen limit" is used as a unit of account, with the help of which safe limits for single and daily exposure are determined. Detailed tables and calculations can be found.

In addition, increased oxygen pressure negatively affects the lungs, to account for this phenomenon, “oxygen endurance units” are used, which are calculated according to special tables correlating the partial pressure of oxygen and the number of “units per minute”. For example, 1.2 ATA gives us 1.32 OTU per minute. The recognized safety limit is 1425 units per day.

From the foregoing, in particular, it should be clear that a safe stay at great depths requires a mixture with a reduced oxygen content, which is unbreathable at a lower pressure. For example, at a depth of 100 meters (11 ATA), the concentration of oxygen in the mixture should not exceed 12%, and in practice it will be even lower. It is impossible to breathe such a mixture on the surface.

Nitrogen

Nitrogen is not metabolized by the body and has no lower limit. With increased pressure, nitrogen has a toxic effect on the nervous system, similar to drug or alcohol intoxication, known as "nitrogen narcosis".

The mechanisms of action are not exactly clarified, the boundaries of the effect are purely individual, and depend both on the characteristics of the organism and on its condition. So, it is known that it enhances the effect of fatigue, hangover, all kinds of depressed state of the body such as colds, etc.

Minor manifestations in the form of a state comparable to mild intoxication are possible at any depth, the empirical “martini rule” applies, according to which nitrogen exposure is comparable to a glass of dry martini on an empty stomach for every 10 meters of depth, which is not dangerous and adds good mood. The nitrogen accumulated during regular diving also affects the psyche akin to soft drugs and alcohol, to which the author himself is a witness and participant. It manifests itself in vivid and "narcotic" dreams, in particular, it acts within a few hours. And yes, divers are a bit of drug addicts. Nitrogen.

The danger is represented by strong manifestations, which are characterized by a rapid increase up to a complete loss of adequacy, orientation in space and time, hallucinations, which can lead to death. A person can easily rush to the depths, because it’s cool there or he allegedly saw something there, forget that he is under water and “breathe deeply”, spit out the mouthpiece, etc. In itself, exposure to nitrogen is not lethal or even harmful, but the consequences under diving conditions can be tragic. It is characteristic that with a decrease in pressure, these manifestations pass just as quickly, sometimes it is enough to rise only 2..3 meters to “sober up sharply”.

The probability of a strong manifestation at depths accepted for entry-level recreational diving (up to 18 m, ∼2.2 ATA) is assessed as very low. According to available statistics, cases of severe poisoning become quite likely from 30 meters depth (∼3.2 ATA), and then the probability increases as pressure increases. At the same time, people with individual stability may not experience problems at much greater depths.

The only way to counteract is constant self-monitoring and control of a partner with an immediate decrease in depth in case of suspicion of nitrogen poisoning. The use of "nitrox" reduces the likelihood of nitrogen poisoning, of course, within the limits of depth due to oxygen.

Helium and other gases

In technical and professional diving, other gases are also used, in particular, helium. Examples of the use of hydrogen and even neon in deep mixtures are known. These gases are characterized by a high rate of saturation/desaturation, the poisoning effects of helium are observed at pressures above 12 ATA and can be, paradoxically, compensated by nitrogen. However, they are not widely used due to their high cost, so it is virtually impossible for an average diver to encounter them, and if the reader is really interested in such questions, then he already needs to use professional literature, and not this modest review.

When using any mixtures, the calculation logic remains the same as described above, only gas-specific limits and parameters are used, and for deep technical dives, several different compositions are usually used: for breathing on the way down, work at the bottom and a staged way up with decompression, the compositions of these gases are optimized based on the logic of their movement in the body described above.

Practical conclusion

Understanding these theses makes it possible to give meaning to many of the restrictions and rules given in the courses, which is absolutely necessary both for further development and for their correct violation.

Nitrox is recommended for use in normal diving because it reduces the nitrogen load on the body even if you stay completely within the limits of recreational diving, this is a better feeling, more fun, less consequences. However, if you are going to dive deep and often, you need to remember not only about its benefits, but also about possible oxygen intoxication. Always personally check oxygen levels and determine your limits.

Nitrogen poisoning is the most likely problem you may encounter, always be considerate of yourself and your partner.

Separately, I would like to draw attention to the fact that reading this text does not mean that the reader has mastered the full set of information for understanding the work with gases during difficult dives. For practical application, this is completely insufficient. This is just a starting point and a basic understanding, nothing more.

(The last column shows the O 2 content, from which the corresponding partial pressure at sea level can be reproduced (100 mm Hg = 13.3 kPa)

Rice. four. Zones of influence of oxygen deficiency when climbing to a height

3. Zone of incomplete compensation (zone of danger). It is implemented at altitudes from 4000 m to 7000 m. Unadapted people develop various disorders. When the safety limit (disturbance threshold) is exceeded, physical performance drops sharply, the ability to make decisions weakens, blood pressure decreases, consciousness gradually weakens; possible muscle twitches. These changes are reversible.

4. Critical zone. Starts from 7000 m and above. P A O 2 gets lower critical threshold - those. its lowest value, at which tissue respiration can still be carried out. According to various authors, the value of this indicator ranges between 27 and 33 mm Hg. Art. (V.B. Malkin, 1979). Potentially lethal disorders of the central nervous system occur in the form of inhibition of the respiratory and vasomotor centers, the development of an unconscious state and convulsions. In the critical zone, the duration of oxygen deficiency is of decisive importance for the preservation of life. A rapid increase in RO 2 in the inhaled air can prevent death.

Thus, the effect on the body of a reduced partial pressure of oxygen in the inhaled air under conditions of a drop in barometric pressure is not realized immediately, but upon reaching a certain reaction threshold corresponding to an altitude of about 2000 m. (Fig. 5).

Fig.5. Dissociation curves of oxyhemoglobin (Hb) and oxymyoglobin (Mb)

S-shaped the configuration of this curve, due to one hemoglobin molecule binds four oxygen molecules plays an important role in the transport of oxygen in the blood. In the process of oxygen absorption by the blood, PaO 2 approaches 90-95 mm Hg, at which hemoglobin saturation with oxygen is about 97%. At the same time, since the dissociation curve of oxyhemoglobin in its right part is almost horizontal, with a drop in PaO 2 in the range from 90 to 60 mm Hg. Art. saturation of hemoglobin with oxygen does not decrease much: from 97 to 90%. Thus, due to this feature, a drop in PaO 2 in the indicated range (90-60 mm Hg) will only slightly affect blood oxygen saturation, i.e. on the development of hypoxemia. The latter will increase after overcoming PaO 2 lower limit - 60 mm Hg. Art., when the oxyhemoglobin dissociation curve changes from a horizontal to a vertical position. At an altitude of 2000 m, PaO 2 is 76 mm Hg. Art. (10.1 kPa).

In addition, the drop in PaO 2 and the violation of hemoglobin saturation with oxygen will be partially compensated by increased ventilation, an increase in blood flow velocity, mobilization of deposited blood, and the use of the oxygen reserve of blood.

A feature of hypobaric hypoxic hypoxia, which develops when climbing in the mountains, is not only hypoxemia, but also hypocapnia (a consequence of compensatory hyperventilation of the alveoli). The latter determines the formation gas alkalosis with the corresponding shift of the oxyhemoglobin dissociation curve to the left . Those. there is an increase in the affinity of hemoglobin for oxygen, which reduces the flow of the latter into the tissues. In addition, respiratory alkalosis leads to ischemic hypoxia of the brain (spasm of cerebral vessels), as well as to an increase in intravascular capacity (dilatation of somatic arterioles). The result of such dilatation is pathological deposition of blood in the periphery, accompanied by a violation of the systemic (fall in BCC and cardiac output) and organ (impaired microcirculation) blood flow. In this way, exogenous mechanism of hypobaric hypoxic hypoxia, due to a decrease in the partial pressure of oxygen in the inhaled air, will be supplemented endogenous (hemic and circulatory) mechanisms of hypoxia, which will determine the subsequent development of metabolic acidosis(Fig. 6).

Under normal conditions, a person breathes ordinary air, which has a relatively constant composition (Table 1). Exhaled air always contains less oxygen and more carbon dioxide. The least oxygen and the most carbon dioxide in the alveolar air. The difference in the composition of alveolar and exhaled air is explained by the fact that the latter is a mixture of dead space air and alveolar air.

Alveolar air is the internal gas environment of the body. The gas composition of arterial blood depends on its composition. Regulatory mechanisms maintain the constancy of the composition of the alveolar air. The composition of the alveolar air during quiet breathing depends little on the phases of inhalation and exhalation. For example, the content of carbon dioxide at the end of inhalation is only 0.2-0.3% less than at the end of exhalation, since only 1/7 of the alveolar air is renewed with each breath. In addition, it flows continuously, during inhalation and exhalation, which helps to equalize the composition of the alveolar air. With deep breathing, the dependence of the composition of the alveolar air on inhalation and exhalation increases.

Table 1. Composition of air (in %)

Gas exchange in the lungs is carried out as a result of the diffusion of oxygen from the alveolar air into the blood (about 500 liters per day) and carbon dioxide from the blood into the alveolar air (about 430 liters per day). Diffusion occurs due to the difference in the partial pressure of these gases in the alveolar air and their tension in the blood.

Partial gas pressure: concept and formula

Partial pressure gas in a gas mixture in proportion to the percentage of gas and the total pressure of the mixture:

For air: P atmospheric = 760 mm Hg. Art.; With oxygen = 20.95%.

It depends on the nature of the gas. The entire gas mixture of atmospheric air is taken as 100%, it has a pressure of 760 mm Hg. Art., and part of the gas (oxygen - 20.95%) is taken as X. Hence the partial pressure of oxygen in the air mixture is 159 mm Hg. Art. When calculating the partial pressure of gases in the alveolar air, it must be taken into account that it is saturated with water vapor, the pressure of which is 47 mm Hg. Art. Consequently, the share of the gas mixture that is part of the alveolar air has a pressure of not 760 mm Hg. Art., and 760 - 47 \u003d 713 mm Hg. Art. This pressure is taken as 100%. From here it is easy to calculate that the partial pressure of oxygen, which is contained in the alveolar air in the amount of 14.3%, will be equal to 102 mm Hg. Art.; accordingly, the calculation of the partial pressure of carbon dioxide shows that it is equal to 40 mm Hg. Art.

The partial pressure of oxygen and carbon dioxide in the alveolar air is the force with which the molecules of these gases tend to penetrate through the alveolar membrane into the blood.

Diffusion of gases through the barrier obeys Fick's law; since the membrane thickness and diffusion area are the same, diffusion depends on the diffusion coefficient and pressure gradient:

Q gas- the volume of gas passing through the tissue per unit time; S - tissue area; DK-diffusion coefficient of the gas; (P 1, - P 2) - gas partial pressure gradient; T is the thickness of the tissue barrier.

If we take into account that in the alveolar blood flowing to the lungs, the partial oxygen tension is 40 mm Hg. Art., and carbon dioxide - 46-48 mm Hg. Art., then the pressure gradient that determines the diffusion of gases in the lungs will be: for oxygen 102 - 40 = 62 mm Hg. Art.; for carbon dioxide 40 - 46 (48) \u003d minus 6 - minus 8 mm Hg. Art. Since the diffuse coefficient of carbon dioxide is 25 times greater than that of oxygen, carbon dioxide leaves the capillaries more actively into the alveoli than oxygen in the opposite direction.

In the blood, gases are in a dissolved (free) and chemically bound state. Diffusion involves only dissolved gas molecules. The amount of gas that dissolves in a liquid depends on:

• on the composition of the liquid;
• volume and pressure of gas in liquid;
• liquid temperature;
• the nature of the gas under study.

The higher the pressure of a given gas and the temperature, the more the gas dissolves in the liquid. At a pressure of 760 mm Hg. Art. and a temperature of 38 ° C, 2.2% oxygen and 5.1% carbon dioxide dissolve in 1 ml of blood.

The dissolution of a gas in a liquid continues until a dynamic equilibrium is reached between the number of gas molecules dissolving and escaping into the gaseous medium. The force with which the molecules of a dissolved gas tend to escape into a gaseous medium is called pressure of a gas in a liquid. Thus, at equilibrium, the gas pressure is equal to the partial pressure of the gas in the liquid.

If the partial pressure of a gas is higher than its voltage, then the gas will dissolve. If the partial pressure of the gas is below its voltage, then the gas will go out of solution into the gaseous medium.

The partial pressure and tension of oxygen and carbon dioxide in the lungs are given in Table. 2.

Table 2. Partial pressure and tension of oxygen and carbon dioxide in the lungs (in mmHg)

Diffusion of oxygen is provided by the difference in partial pressures in the alveoli and blood, which is equal to 62 mm Hg. Art., and for carbon dioxide - it is only about 6 mm Hg. Art. The time of blood flow through the capillaries of the small circle (an average of 0.7 s) is sufficient for almost complete equalization of partial pressure and gas tension: oxygen dissolves in the blood, and carbon dioxide passes into the alveolar air. The transition of carbon dioxide into alveolar air at a relatively small pressure difference is explained by the high diffusion capacity of the lungs for this gas.