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Physical properties of air



1. Atmospheric pressure. As can be seen from the previous exposition of the material, the layer of air above the earth's surface extends to a height of about 1000 km. This air is held at the surface of the earth by gravity, i.e. has a certain weight. On the surface of the earth and on all objects located at its surface, this air creates a pressure equal to 1033 g / cm. Therefore, on the whole surface of the human body having an area of ​​1.6-1.8 m, this air, respectively, exerts a pressure of about 16-18 tons. Usually we don’t feel this, because under the same pressure the gases are dissolved in body fluids and tissues and balance the external pressure on the surface of the body from the inside. However, when the external atmospheric pressure changes due to weather conditions, it takes some time to balance it from the inside, necessary to increase or decrease the amount of gases dissolved in the body. During this time, a person may feel some discomfort, because with a change in atmospheric pressure only a few mm. Hg. column total pressure on the surface of the body varies by tens of kilograms. These changes are especially clearly felt by people suffering from chronic diseases of the musculoskeletal system, cardiovascular system, etc.

In addition, a person may encounter a change in barometric pressure in the process of his activity: when climbing to a height, when diving, coffering, etc. Therefore, doctors need to know what effect both lowering and increasing atmospheric pressure have on the body.

The effect of reduced pressure

With reduced pressure, a person is found mainly when climbing to a height (when excursions to the mountains or when using aircraft). In this case, the main factor that affects a person is oxygen deficiency.

With increasing altitude, atmospheric pressure gradually decreases (by about 1 mmHg for every 10 m of height). At an altitude of 6 km, atmospheric pressure is already twice lower than at sea level, and at an altitude of 16 km - 10 times.

Although the percentage of oxygen in the atmospheric air, as we noted earlier, almost does not change with a rise to a height, however, due to a decrease in the total pressure, the partial pressure of oxygen in it also decreases, i.e. the proportion of pressure that is provided by oxygen in the total pressure.

It turns out that it is the partial pressure of oxygen that provides the transition (diffusion) of oxygen from the alveolar air into venous blood. Rather, this transition occurs due to the difference in the partial pressure of oxygen in the venous blood and in the alveolar air. This difference is called diffuse pressure. With low diffuse pressure, arterialization of blood in the lungs is difficult, hypoxemia occurs, which is the main factor in the development of high-altitude and mountain diseases. The symptomatology of these diseases is very similar to the symptoms of general oxygen deficiency described by us earlier: shortness of breath, palpitations, blanching of the skin and acrocyanosis, dizziness, weakness, fatigue, drowsiness, nausea, vomiting, loss of consciousness. The initial signs of altitude or mountain sickness begin to appear already from a height of 3-4 km.

Depending on the partial pressure of oxygen in the air at different heights, the following zones are distinguished (according to the degree of influence on the human body):

1. Indifferent zone up to 2 km

2. Full compensation zone 2-4 km

3. Zone of incomplete compensation 4-6 km

4. Critical zone 6-8 km

5. Fatal area above 8 km

Naturally, the division into such zones is conditional, since different people tolerate oxygen deficiency in different ways. An important role is played by the degree of fitness of the body. In trained people, the activity of compensatory mechanisms has been improved, the amount of circulating blood, hemoglobin and red blood cells has been increased, and tissue adaptation has been improved.

In addition to oxygen deficiency, a decrease in barometric pressure when rising to a height leads to other disturbances in the state of the body. First of all, these are decompression disorders, expressed in the expansion of gases located in the natural cavities of the body (paranasal sinuses, middle ear, poorly filled teeth, gas in the intestines, etc.). In this case, pain can occur, sometimes reaching significant strength. These phenomena are especially dangerous with a sharp decrease in pressure (for example, depressurization of aircraft cabins). In such cases, damage to the lungs, intestines, nosebleeds, etc. can occur. Pressure reduction up to 47 mm Hg. Art. and lower (at an altitude of 19 km) leads to the fact that fluids in the body boil at body temperature, since the pressure becomes lower than the pressure of water vapor at this temperature. This is expressed in the occurrence of so-called subcutaneous emphysema.

The effect of high blood pressure

A man is forced to perform diving and caisson operations at elevated pressure. Healthy people endure the transition to high blood pressure quite painlessly. Only occasionally there are short-term discomfort. In this case, the pressure is balanced in all internal cavities of the body with external pressure, as well as the dissolution of nitrogen in body fluids and tissues in accordance with its partial pressure in the inhaled air. For each additional atmosphere of pressure, approximately 1 liter of nitrogen is additionally dissolved in the body.

The situation is much more serious in the transition from an atmosphere with increased pressure to normal (during decompression). In this case, nitrogen, dissolved in the blood and tissue fluids of the body, tends to stand out in the external atmosphere. If decompression occurs slowly, then nitrogen gradually diffuses through the lungs and desaturation occurs normally. However, in the case of acceleration of decompression, nitrogen does not have time to diffuse through the pulmonary alveoli and is released in tissue fluids and in the blood in a gaseous form (in the form of vesicles), and painful phenomena called the decompression sickness occur. The release of nitrogen occurs first from tissue fluids, since they have the lowest coefficient of nitrogen supersaturation, and then can occur in the bloodstream (from the blood). Caisson disease is expressed primarily in the occurrence of sharp breaking pains in the muscles, bones and joints. The people this disease is very aptly called "break." In the future, symptoms develop depending on the location of the vascular emboli (marbling of the skin, paresthesia, paresis, paralysis, etc.).

Decompression is a crucial moment in such work and it takes a significant amount of time. The operating schedule in the caisson at a pressure equal to three additional atmospheres (3 ATM) is as follows:

The duration of the entire half-shift is 5 hours 20 minutes.

The compression period is 20 minutes.

Work in the caisson - 2 h 48 min.

The decompression period is 2 hours 12 minutes.

Naturally, when working in caissons with higher pressure, the decompression period is significantly extended and, accordingly, reduced

period of work in the working chamber.

2. The movement of air. As a result of uneven heating of the earth's surface, places are created with high and low atmospheric pressure, which, in turn, leads to the movement of air masses.

The movement of air helps to maintain the constancy and relative uniformity of the air environment (equilibration of temperatures, mixing of gases, dilution of pollution), and also contributes to the transfer of heat by the body. Of particular importance in the planning of populated areas is the so-called "wind rose", which is a graphic representation of the repeatability of the direction of the winds in a given area for a certain period of time. When planning the territory of populated areas, the industrial zone should be located on the leeward side in relation to the residential zone. The speed of air movement in the atmosphere can range from calm to hurricanes (over 29 m / s). In residential and public buildings, the air velocity is normalized within 0.2-0.4 m / s. Too low air velocity indicates poor ventilation, large (more than 0.5 m / s) - creates an unpleasant feeling of draft.

3. Humidity. The air of the troposphere contains a significant amount of water vapor, which is formed as a result of evaporation from the surface of water, soil, vegetation, etc. These pairs pass from one state of aggregation to another, affecting the overall humidity dynamics of the atmosphere. The amount of moisture in the air rises to a height rapidly decreases. So, at an altitude of 8 km, air humidity is only about 1% of the amount of moisture that is determined at ground level.

For humans, the most important is the relative humidity, which indicates the degree of saturation of the air with water vapor.
It plays a large role in the implementation of thermoregulation of the body. The optimal value of relative humidity is 40-60%, permissible - 30-70%. With low air humidity (15-10%), more intense dehydration of the body occurs. In this case, subjectively there is an increased thirst, dryness of the mucous membranes of the respiratory tract, the appearance of cracks on them with subsequent inflammatory phenomena, etc. Especially painful are these sensations in patients with fever. Therefore, microclimatic conditions in the wards of such patients should be given special attention. High air humidity adversely affects the body’s thermoregulation, making it difficult or increasing heat transfer depending on the air temperature (see further issues of thermoregulation).

4. Air temperature. Man has adapted to existence within certain temperature values. At the surface of the earth, air temperature, depending on the latitude of the terrain and the season of the year, ranges from about 100 ° C. With a rise in height, the air temperature gradually decreases (by about 0.56 ° C for every 100 m of rise). This value is called the normal temperature gradient. However, due to special meteorological conditions (low cloudiness, fog), this temperature gradient is sometimes violated and the so-called temperature inversion occurs when the upper layers of the air become warmer than the lower. This is of particular importance in solving problems associated with air pollution.

The occurrence of temperature inversion reduces the ability to dilute the pollutants released into the air, and contributes to the creation of their high concentrations.

To consider the effects of air temperature on the human body, it is necessary to recall the basic mechanisms of thermoregulation.

Thermoregulation. One of the most important conditions for the normal functioning of the human body is to maintain a constant body temperature. Under normal conditions, an average person loses about 2400-2700 kcal per day. About 90% of this heat is transferred to the external environment through the skin, the remaining 10-15% is spent on heating food, drink and inhaled air, as well as on evaporation from the surface of the mucous membranes of the respiratory tract, etc. Therefore, the most important way of heat transfer is the surface of the body. Heat is transferred from the body surface in the form of radiation (infrared radiation), conduct (by direct contact with surrounding objects and a layer of air adjacent to the body surface) and evaporation (in the form of sweat or other liquids).

In ordinary comfortable conditions (at room temperature in light clothing), the ratio of the degree of heat transfer by these methods is as follows:

1. Radiation - 45%

2. Carrying out - 30%

3. Evaporation - 25%

Using these mechanisms of heat transfer, the body can largely protect itself from the effects of high temperatures and prevent overheating. These thermoregulation mechanisms are called physical. In addition to them, there are also chemical mechanisms, which are that when exposed to low or high temperatures, the metabolic processes in the body change, resulting in an increase or decrease in heat production.

The complex effect of meteorological factors on the body. Overheating usually occurs at high ambient temperatures in combination with high humidity. In dry air, high temperature is much easier to carry, because at the same time a significant part of the heat is given off by evaporation. With the evaporation of 1 g of sweat, about 0.6 kcal is consumed. Heat transfer is especially good if accompanied by air movement. Then evaporation occurs most intensively. However, if high air temperature is accompanied by high humidity, then evaporation from the surface of the body will not occur intensively enough or will cease altogether (the air is saturated with moisture). In this case, heat transfer will not occur, and heat will begin to accumulate in the body - overheating will occur. There are two manifestations of overheating: hyperthermia and convulsive disease. With hyperthermia, three degrees are distinguished: a) mild, b) moderate, c) severe (heat stroke). Convulsive disease occurs due to a sharp decrease in the blood and body tissues of chlorides, which are lost with intense sweating.

Hypothermia. Low temperature combined with low relative humidity and low air velocity are well tolerated by humans. However, low temperature combined with high humidity and air velocity create the possibility of hypothermia. Due to the large thermal conductivity of water (28 times more air) and its high heat capacity under conditions of raw air, heat transfer sharply increases by the method of heat conduction. This is facilitated by the increased speed of air movement. Subcooling can be general and local. General hypothermia contributes to the occurrence of colds and infectious diseases due to a decrease in the overall resistance of the body. Local hypothermia can lead to chills and frostbite, and most of all, limbs ("trench foot") are affected. With local cooling, reflex reactions that occur in other organs and systems may also occur.

Thus, it becomes clear that high air humidity plays a negative role in thermoregulation issues at both high and low temperatures, and an increase in air velocity, as a rule, contributes to heat transfer. The exception is cases when the air temperature is higher than body temperature, and relative humidity reaches 100%.

In this case, an increase in the air velocity will not lead to an increase in heat transfer either by the method of evaporation (air is saturated with moisture) or by the method of conduct (air temperature is higher than body surface temperature).

Meteotropic reactions. Weather conditions have a significant impact on the course of many diseases. In the Moscow region, for example, in almost 70% of cardiovascular patients, the deterioration in time coincides with periods of significant change in meteorological conditions. A similar relationship was noted by many studies conducted in almost all climatic and geographical regions both in our country and abroad. Hypersensitivity to adverse weather also distinguishes people suffering from chronic non-specific lung diseases. Such patients do not tolerate weather with high humidity, sudden changes in temperature, strong winds. A very pronounced relationship with the weather course of the disease bronchial asthma. This is reflected even in the uneven geographical distribution of the disease, which is more common in areas with a humid climate and contrasting weather changes. So, for example, in the Northern regions, in the mountains and in the south of Central Asia, the incidence of asthma is 2-3 times lower than in the Baltic countries. Hypersensitivity to weather conditions and their changes in patients with rheumatic diseases is also well known. The occurrence of rheumatic pains in the joints, preceding or concomitant with a change in the weather, has become one of the classic examples of a meteopathic reaction. It is no coincidence that many patients with rheumatism are figuratively called "living barometers." Patients with diabetes, neuropsychic and other diseases often react to changing weather conditions. There is evidence of the effects of weather on surgical practice. In particular, it was noted that during adverse weather, the course and outcome of the postoperative period in cardiovascular and other patients worsens.

The starting point for the justification and implementation of preventive measures during meteotropic reactions is a medical assessment of the weather. There are several types of classification of weather types, the simplest of which is classification according to G.P. Fedorov. According to this classification, three types of weather are distinguished:

1) Optimal — day-to-day temperature fluctuations up to 2 ° C, speed

air movement up to 3 m / s, atmospheric pressure change up to 4 mbar.

2) Раздражающая— колебания температуры до 4°С, скорость движения воздуха до 9 м/сек, изменение атмосферного давления до 8 мбар.

3) Острая — колебания температуры более 4°С, скорость движения воздуха более 9 м/сек, изменение атмосферного давления более 8 мбар.

В медицинской практике желательно производить медицинский прогноз погоды на основании этой классификации и предпринимать соответствующие профилактические меры.

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